Hill's Space

Making an Eclipse Timelapse, Two Years Later

hillexed@email.com (hillexed) — Sun, 26 Apr 2026 00:00:00 +0000

Click the preview above to see the timelapse!

This is a several hour long timelapse of the 2024 eclipse, made from all the photos I took on that day. These images was taken by a phone camera looking through an 80" finderscope with a solar filter on it. A raspberry pi pico connected to the phone and acted like a bluetooth mouse to “click” the shutter button and take photos.

Tracking mounts are expensive, so instead I moved the Dobsonian mount by hand
I sped up the rate at which images were being taken just before totality.
During totality, I planned to have the pico take a variety of images with different exposure times. Unfortunately, it skipped straight to the one-second exposures, the extra processing lag messed up the mouse position, and by the time I restarted the sequence totality was almost over. I got exposure times that were too short and too long and none just right :(
It was an incredibly cool experience. Not the pitch blackness I was expecting, but instead a dark blue sky with a ring of orange around us.

To make this exposure, I wrote some python to process each .raw file into a .tif (PIL doesn’t support 16-bit RGB images, it seems).

I attempted to stabilize the image by moving each image to an approximate center point. My first attempt selected the row and column with the highest brightness sum and cropped around it. That resulted in an incredibly shaky camera, and is not what you’re seeing here.

This image is also on my Favorite Astrophotos page!

Hacking an LED strip

hillexed@email.com (hillexed) — Fri, 24 Apr 2026 00:00:00 +0000

One day in the computer store I saw a 25 foot addressable LED strip on sale for $17. Less than $1/foot seemed like a good deal, I thought, and perhaps I could remove the LEDs and control them myself via an ESP32 microcontroller.

The model is the MLW7-1025-ICM smart LED strip. Interestingly, it isn’t visible on the manufacturer’s website but can still be found on Amazon or Walmart. Each addressable LED controls 3 physical LEDs that share the same color.

I took it home and unboxed it. Turns out they were one step ahead of me - the manufacturer already put an ESP32 in there. Specifically, they use a CDW6986850-00 module, which contains an ESP8266, a cheaper variant of the ESP32C3.

Could I reprogram it? I grabbed an FTDI cable and made the appropriate connections. I was able to obtain serial output! Unfortunately, the microcontroller refused to enter download boot mode upon being reset. Normally the ESP32 requires certain GPIO pins to be high or low to enter a boot mode where it will accept new programming. The various bits of the boot state encode the state of those pins, which helped me determine where on the CDK module the pins connected. (Lesson learned: the ESP32C3 has different strapping pins than the bare ESP32!)

Eventually I configured the pins properly, and yet the CDK module refused to boot. I believe this is because the manufacturers internally burned a fuse preventing the module from ever entering boot mode. Very rude.

Instead, I desoldered their ESP32 and replaced it with my own. The circuit board is very simple; most of it is simply regulating 9V power to 3V. It only uses two GPIO pins; one for the button on the back of the switch, and one output for the LEDs.

The most common RGB LED protocol is WS2811. Could this LED strip be using WS2811? I programmed my xiao with a simple WS2811 example code without thinking too hard; it worked on the first try.

Home Assistant

The dream would be to be able to control the LEDs from my phone. To do that, I installed ESPHome onto the ESP32. With a different device running a Home Assistant server, I gain the ability to program color changes at different times of day, including the ability to flash red at bedtime. ESPHome also impressed me with over-the-air firmware update capability.

Years ago, a friend upgraded his phone and donated me the old one. Could I use it as a Home Assistant hub? At first I thought I would need to install a custom OS, but I was unable to root the phone. This means I could not get mDNS to work. Thankfully, I am not the first person to try running Home Assistant on Termux. When I tried the HomeAssistant-Termux repo, the instructions there didn’t quite work - I got a no image found in manifest for platform android/arm64 home assistant error. What did work was Termux-UDocker.

The results

I can now use my phone to control the color of my lights, zero cloud servers required. What a fun feeling. At sunset, the lights turn on. At midnight, if the lights are still on, they flash a warning to head to bed. Fun!

12" polishing update: Oblate no longer?

hillexed@email.com (hillexed) — Sat, 10 May 2025 17:47:25 +0000

Five or so 30 minute long sessions of long parabolizing W strokes have finally turned my center hill into a center hole. The dip at around 80% to 90% diameter remains, while the 90% to 100% diameter edge zone is turned up.

I made two changes to get better pitch contact: I am filling a plastic container with hot water and pressing the pitch to the glass underwater, and I am only polishing for 30 minutes at a time instead of going for a full hour. I think it’s working!

Hopefully now that it’s no longer an oblate spheroid, the standard 1/3 strokes will work properly and allow me to fix that bumpy outer zone and its turned up edge.

Creating a mirror cell

hillexed@email.com (hillexed) — Thu, 08 May 2025 13:23:13 +0000

I have made a contraption called a “mirror cell”

it traps naughty glass so I can examine its sins

hillexed@email.com (hillexed) — Tue, 06 May 2025 02:10:51 +0000

I thought I was close to a sphere, but some 1/3 center over center strokes look like they just made my oblate mirror shape less spherical. I realized I have an oblate spheroid, with a center slightly higher than it should be.

My theory is: my mirror has a slight central hill. When I press the pitch lap on top of it, the pitch forms a ring shape, with not much contact in the center. Shorter strokes without much side to side motion wear down the edge more and not the center. So I think I just need to wear down that center hill with lots of side to side motion in my strokes, placing the center of my mirror over the tool’s ring of good contact, to wear down that outer ring. Hence: lots of side to side motion while grinding.

Capturing Galaxies with Cameras

hillexed@email.com (hillexed) — Thu, 24 Apr 2025 13:52:15 +0000

Today, I tried finding some galaxies. I couldn’t see M101, couldn’t see M105, but did see M81 and M82 by eye. It was tough, but I finally took a phone pic. I decided to give Pingu Camera a try… and the phone plus printed adapter plus phone remote shutter plus waiting a while for vibrations to die down worked! I got a picture of both M81 and M82!!

hillexed@email.com (hillexed) — Tue, 22 Apr 2025 02:31:02 +0000

More wide parabolizing strokes are bringing my 12" mirror closer and closer to a sphere…

I have a slight worn down edge, overall a tiny bit oblate, as well as a tiny central hill, but it’s all very small.

Here’s a Ronchi test picture of the mirror today compared to two weeks ago. As a reminder, straight up and down is spherical.

12" mirror making: take testing seriously

hillexed@email.com (hillexed) — Sun, 20 Apr 2025 13:11:25 +0000

I’m around 10 hours into polishing, and it looks like I’m very close to polished out! I can see barely any tiny pits under a microscope. I think I’m done.

Next up, figuring. I can test a mirror’s shape easily. I have been doing that already despite not quite being done with polishing.

The problem with a thin mirror is that glass (and anything else) will happily bend a few hundred nanometers under its own weight. Leaning my glass mirror against a wall means all the weight is placed on one point, causing visible distortion. I need to remove that to isolate any real astigmatism. I need to build a structure that holds the mirror while resting its weight across many different points so it doesn’t flex, called a “mirror cell”.

Telescope mirror cells are usually made of metal and involve triangles on triangles that can move and turn. This is a solved problem for big telescopes, but usually involves metal parts. I haven’t worked with metal before; usually just 3D printing. Designing this will take some thought.

Big Baffles

hillexed@email.com (hillexed) — Tue, 15 Apr 2025 18:55:12 +0000

My 8" telescope has been upgraded with some new printed shields to block unwanted light near the top.

Pics of said light blocking baffles. Also added a mount for my pifinder.

Continuing Polishing

hillexed@email.com (hillexed) — Fri, 11 Apr 2025 17:57:15 +0000

Polishing continues. I’ve noticed that my short center over center strokes seem to be creating a hill in the center of this mirror, so I’ve switched to longer 1/2-diameter strokes and 1/3 side to side motion in order to wear down the center of the mirror. It seems to be working, according to these ronchi test images before and after. I’m jumping the gun a little by figuring while polishing but there’s a long way to go so I should be fine.

2025 Total Lunar Eclipse

hillexed@email.com (hillexed) — Fri, 14 Mar 2025 08:13:51 +0000

Lunar eclipse! Funny enough the telescope made things brighter and therefore even during totality it looked less contrasting through the telescope than by eye

Polished out?

hillexed@email.com (hillexed) — Sun, 09 Mar 2025 13:35:11 +0000

My mirror is now reflective enough to see the entire surface and get my first Ronchi test image! But I’m not done with polishing - it may look reflective by eye, but the same region under a microscope reveals tiny pits left over from 5 micron grit are still there. I’m not polished out until all those are gone.

12" Mirror Making: Pitch Lap Problems

hillexed@email.com (hillexed) — Fri, 07 Mar 2025 00:03:50 +0000

My pitch lap polishing tool wasn’t making good contact with the center of my mirror. I can tell because I’m just beginning polishing and so I can see that the mirror’s outside is more reflective than the inside. Also, the tool looks unevenly worn.

I tried heating up the tool using a tub of hot water so the pitch would flow and then pressing the tool against the mirror some more (pictured) but it seems to have only helped slightly. In the end I placed both pitch lap and mirror in a tub of hot water and pressed while underwater - and that heat seems to have finally let the pitch flow and fully copy the mirror shape across the entire surface.

Starting to Show Reflections

hillexed@email.com (hillexed) — Sat, 15 Feb 2025 22:19:44 +0000

Another hour and a half of polishing and my piece of glass is looking much more reflective!

I was stroking with tool on top, but I noticed the center of the pitch lap wasn’t making good contact. I suspect the plaster backing tool that the mirror rests on isn’t evenly supporting the mirror; it picked up a slightly wavy surface I was hoping wouldn’t cause issues. When I poured the tool I used wax paper to ensure it wouldn’t stick to the mirror, and the wax paper crumpled slightly from being flat on a non-flat mirror surface. I think I will need to pour epoxy on top of the existing epoxy to get a nice smooth backing surface.

Total polishing time: 3 hours

12" Polishing Start!

hillexed@email.com (hillexed) — Sat, 08 Feb 2025 19:21:53 +0000

I finally started polishing this 12" mirror! It has taken so long and many failures to get here. I took the pitch lap tool that I made and carved some bonus channels into it, squirted on cerium oxide mixed with water, and got polishing. An hour and a half later, the outside of my mirror is polished and now is smooth enough to reflect light! It looks like my pitch lap wasn’t pressed for long rnough to make good contact with the mirror, though, because I can see that only a ring around the outside is being worn down, not the center.

12" Pitch Lap Created

hillexed@email.com (hillexed) — Mon, 03 Feb 2025 00:25:11 +0000

I created a pitch lap, a tool used for the last two steps of mirror making. I will use it for polishing and figuring. Pitch, also known as asphalt, is a viscous liquid like honey and will flow to match the shape of the mirror when pressed against it.

Oh my I can see features on mars

hillexed@email.com (hillexed) — Fri, 24 Jan 2025 03:42:12 +0000

Rough mars sketch! The atmosphere is not cooperating but there’s this big Africa-shaped gray region!

Mars Behind Moon

hillexed@email.com (hillexed) — Tue, 14 Jan 2025 04:15:36 +0000

Goodbye mars! (Zoom in!)

Taken using my 3D printed telescope, with a crowd of friends invited to watch the occultation of mars by the moon! The phone I was taking photos on ran out of battery, so I hastily threw a friend’s iphone into the holder just in time to capture these.

Goodbye Mars!

hillexed@email.com (hillexed) — Mon, 13 Jan 2025 00:00:00 +0000

(Zoom into these photos to see Mars!)

Taken using my Hadley 3D printed telescope, with a crowd of friends invited to watch the occultation of mars by the moon! The phone I was taking photos on ran out of battery, so I hastily threw a friend’s iphone into the holder just in time to capture these.

Leavitt's First Light

hillexed@email.com (hillexed) — Wed, 01 Jan 2025 00:00:00 +0000

I finally used my new 8" telescope with a working mirror cell! Report:

Jupiter is nice and bright!

M42 is so much brighter than in my first telescope! It’s green and I can the bat wings and texture in the trapezium along with four stars!

There are so many stars everywhere!

High power reveals flaws. High power stars look like ovals. After some investigation, my secondary mirror wasn’t centered in the eyepiece view. I don’t have enough room to fix it all the way before it hits the spider. Time to redesign.

Mars looks blurry. No detail. May be due to seeing. A hint of ice cap but not much else, and then my eyepiece fogged over.

Focuser is bad at holding eyepieces. They keep sliding out. Something with set screws is probably a better idea. I managed to take a few pictures but the weight of a camera meant it was no longer face-on and had distorted stars.

At least M44 looks great!

Leavitt Complete!

hillexed@email.com (hillexed) — Sat, 21 Dec 2024 21:16:46 +0000

8" telescope: COMPLETE!!

It has literally taken all year, but my second telescope ever is done! It’s made of 3D printed parts, metal tubes, nuts and bolts, and a mirror I hand-polished over the course of 8 months!
I’m very proud.

The Mirror Cell

hillexed@email.com (hillexed) — Fri, 20 Dec 2024 00:00:00 +0000

The part of the telescope that holds a mirror in place is called the “mirror cell”. The simplest way to hold a mirror in place is with three dabs of silicone glue. If glue is too hard, it can pull on the mirror and the stress will distort its shape on a scale of hundreds of nanometers, which is bad.

Small mirrors can usually get away with just three dabs of glue. Because my mirror was so accurate, I was advised to make a “flotation cell”, which holds the mirror in place without glue. I designed this modification of the stock Leavitt mirror cell in Onshape. The gray edge supports are also what stop the mirror from falling out if the telescope is turned upside down. These parts are printed separately and held tight (but not too tight that they press on the mirror and distort it) with a screw that screws into the base. Making the edge supports separate removable parts is important so that I can get the mirror in and out.

This mirror cell works… okay. I don’t see much distortion after the mirror has cooled down. The height of the edge supports means it sometimes scrapes against the inside of the telescope tube if pulled too much in one direction.

On to 5 micron, for the third time

hillexed@email.com (hillexed) — Fri, 13 Dec 2024 15:00:18 +0000

I ran out of leftover 12 micron, so I’m going to 5 micron. The longer scratch seems to be entirely gone, and the short scratch is now two tiny slices less than a millimeter long. Hopefully it’ll grind out.

Current cumulative grinding time: 46 hours.

Third time is the charm...

hillexed@email.com (hillexed) — Thu, 12 Dec 2024 23:12:33 +0000

Went back to 12 micron grinding. 1 hour and 45 minutes later, the scratches are now little dots spaced along a line. There aren’t any more lines trailing in from the edge, at least, so I hope that counts as a success for my beveling.

Scratches, again...

hillexed@email.com (hillexed) — Fri, 06 Dec 2024 02:32:43 +0000

Tragedy!

I thought beveling the edge would help, but… I see scratches. Again. And only after 20 minutes of grinding. (I drew pencil marks next to them for visibility). Long, too.

Maybe my sanding made particles of 400 grit fly into my tool? Aaaaargh

Rebeveled Edge to Stop Sleeks?

hillexed@email.com (hillexed) — Fri, 06 Dec 2024 01:14:33 +0000

I bought some 400 grit alox sandpaper and glued it to a failed 3D print to act as a bootleg grinding stone. Hopefully this will stop the bevel from introducing particles.

Sleeks Spotted

hillexed@email.com (hillexed) — Sat, 30 Nov 2024 14:00:26 +0000

Another hour of 5 micron grinding and, unlike the first time, no scratches!

But then I looked closer. By using my phone flashlight, I saw one very thin and not very deep scratch (a “sleek”). That’s fine. I also saw some thicker scratches around 1-2mm long intersecting the edge - see the photo.

Maybe those marks are from bits of glass that were torn from the beveled edge? This adds credence to my theory that my previous scratch problems were bevel problems, not a speck of coarse grit contaminating.

That said, a tiny sleek and a few tiny scratches at the edge? That’s only a tiny bit of light collecting area lost. I’ll happily move on to polishing if I can instead of spending more effort getting these out.

Grinding At 12 Micron, Again

hillexed@email.com (hillexed) — Mon, 25 Nov 2024 19:58:15 +0000

Mirror grinding with the newly refurbished tool is going extremely well! After two hours I ran out of 30 micron aluminum oxide grit and moved on to 12 micron. (You can move on when you’ve eliminated pits from the previous bigger grit size, but since previously I was at 5 micron, I didn’t have any pits and could move on whenever I wanted, but I also wanted to grind the tool as smooth as possible).

It took only an hour and a half for the 12 micron grit to eliminate all the pits from 30 micron. Last time it took over 3 hours! Wow.

Now I’m back to 5 micron, the last grit size of fine grinding. Last time I was here, I got scratches. Here’s hoping a few more hours of work with my refurbished tool will let me finally finish fine grinding. The picture shows how much of the tool is texture-free now and making good contact.

Oh and all these hours of grinding have moved my focal length from 37 inches to 39.5 inches, making this mirror a 12" f/3.3. Anything below f/3 is incredibly hard to parabolize, so this will make it easier for myself. This telescope is going to have such bright views.

Back to 30 Micron Grit With Refurbished Tool

hillexed@email.com (hillexed) — Sun, 24 Nov 2024 02:13:08 +0000

My refurbished tool is ready to go. However, I have to go back a few sizes to coarser grit to ensure the tool is ground down evenly to make good contact across its full area. From 5 micron (the last size of grit needed) back to 30 micron size grit I go…

Thankfully, after an hour of grinding, it looks like it’s wearing evenly. It’s sliding smoothly across the mirror. All tiles started out with some texture from the webbing they were originally attached to, so in the picture a smooth tile is a sign that the tile has been worn away (and making good contact). Some tiles still have the texture, so they must be lower. The worn tiles seem randomly distributed instead of clustered on one side of the tool, which is good.

For the most part in between the tiles is a 1 mm wide gap with a smooth floor from the hardened resin. Some tiles have crud filling the gap; it looks like the resin climbed up the gap between tiles through capillary action and hardened. Annoying. I spent a while with a knife removing some of that but there’s too much; I hope it doesn’t trap grit particles and cause scratches later.

Tool Refurbishing

hillexed@email.com (hillexed) — Sat, 23 Nov 2024 16:47:04 +0000

15 minutes of more grinding created even more scratches, starting from the edge and moving in. I think maybe I wasn’t using enough liquid and my tool was knocking off bits of the edge glass and dragging them inwards.

To avoid any possibility of my tool having coarse grit caught in the channels in between tiles, I remade my tool. I bought some “tabletop epoxy” which takes 3 days to cure and covered my existing tool with it, then put on a new layer of tiles. I kept the tiles on the webbing this time, and it looks like that has created even, easily cleanable gaps in between each tile.

Scratches Showing Up

hillexed@email.com (hillexed) — Sun, 17 Nov 2024 22:56:11 +0000

Another tragedy! As I move to 12 and then 5 micron aluminum oxide grits, the final stages of fine grinding, scratches are showing up. I keep losing tiles from my tile tool, too. A scratch means some particle bigger than the current abrasive size is being introduced - maybe grit from earlier stages of grinding was trapped in between narrow gaps gaps in my tool’s tiles? I probably have to go back to a coarser grit to get it out. Sighhhh

Slow Progress

hillexed@email.com (hillexed) — Thu, 14 Nov 2024 04:01:05 +0000

Update: at long last, my now aluminized mirror is finally being shipped back to me!

A timeline of recent events:

October 19: The mirror is aluminized. The aluminizer notes that he will wait a week to ensure the coating cures properly (reasonable and expected)
November 1: I am told that the mirror is “all set to send it out”
November 7: I ask for any shipping updates
November 10: I get a reply saying “I’ll do it tomorrow”
November 11: radio silence
November 13: I ask for updates, and am told “I shipped it yesterday”.

Sigh. A fitting end to the saga.

Edit: Update from the future! The saga didn’t end.

November 15: Canada’s postal union goes on strike.

November 30: My mirror continues to sit in an airport.

December 19: Canada’s government orders the striking laborers back to work, which is very unfair since they deserve to be paid more, but my package is officially back on the move.

Mars Observations

hillexed@email.com (hillexed) — Sat, 09 Nov 2024 04:54:16 +0000

I looked at mars with my 4.5" telescope. Previously it just looked like a big circle, but adding a 2x barlow lens to get to 150x zoom made it big enough I could make out a tiny line of slightly darker area on Mars! That’s the first time that I saw any detail on mars! That’s neat, but also wow my standards are so low.

Crack Coverup

hillexed@email.com (hillexed) — Wed, 06 Nov 2024 18:29:20 +0000

I’m slowly dealing with the crack’s aftermath. Slowly, I ground through the big hole I made when burnishing the crack. As I ground it down over the week, the crack wasn’t visible under the loupe unless I put a flashlight sideways and used a finger to cover the top of the mirror, illuminating from the side. After watching the hole get smaller and smaller, I took a look… and the crack was still barely visible. It’s there in the last image as a slight diagonal line below the dot.

After the last picture, I ground for another hour and I can’t find the crack anymore. I hope that means it’s gone and I can move from size 320 silicon carbide grit to 30 micron aluminum oxide grit!

Total time spent grinding out the crack: 7.4 hours

Crack Tragedy!

hillexed@email.com (hillexed) — Sun, 27 Oct 2024 04:12:33 +0000

Tragedy!

I dropped my tool onto the glass from an inch up. I thought I was fine, but close inspection revealed a crack a millimeter long. I was advised to try to burnish it out because the crack could travel very deep and spread.

I ended up putting a screw in a drill and using the head as a bootleg buffing wheel with my 30 micron water mix to try to dig out the crack and its subsurface damage. Now I have an ugly 4 square mm divot on my mirror, which is otherwise polished down to 30 micron abrasives. Sigh…

Not Real Numbers

hillexed@email.com (hillexed) — Fri, 25 Oct 2024 00:00:00 +0000

I rediscovered the xkcd up goer five text editor, which lets you type using only the 1000 most common english words. So here’s my attempt at writing a description of something:

Not Real Numbers

We know how to do many number problems. Number problems with adding are easy. Adding the same number over and over is also easy. If you take a number and add it many times, the same number of times as that number, you get a box number.

We can go forward from number to box number. Sometimes you want to go back from box number to smaller number. In fact, if you have a number problem with box numbers and adding over and over and adding, you learn in school a box problem answer form to do the problem. But part of the answer form says to go back from box numbers.

A box number is always above the nothing number, or is the nothing number. Five boxed is twenty five, and so is five under nothing boxed. Even numbers under nothing, if you box them by adding them that many times, are above the nothing number. But for some problems, the box problem answer form wants you to find a number where its box number is under nothing. Oh no!

So the number people made up a new number, to imagine its box number is one under nothing. Some people were mad and called these new numbers not real. They were surprised when these not real numbers were good for doing problems! We only need to make up one new number to do all number problems with adding and adding over and over! Also, we can draw not real numbers by imagining they go in a different direction than real numbers.

The Rest Is Logistics

hillexed@email.com (hillexed) — Tue, 22 Oct 2024 16:15:28 +0000

My 8" mirror that I polished by hand is officially a mirror! The person with a vacuum chamber, who I gave my mirror to two months ago, finally aluminized my mirror and sent me a picture! It’s shiny and reflective like a mirror should be!

Now it just has to survive being shipped back…

Comet Tsuchinshan-Atlas

hillexed@email.com (hillexed) — Thu, 17 Oct 2024 12:49:09 +0000

Cell phone pic of Comet Tsuchinshan-Atlas!

It was visible faintly in the sky by eye too!

Weridly it seemed brighter by eye than through the telescope. That could be because my telescope wasn’t shielded from a nearby streetlight and stray light created a brighter background?

Back End

hillexed@email.com (hillexed) — Wed, 09 Oct 2024 22:09:08 +0000

18 minutes later, I ran out of grit and the unground region is less than 0.5mm wide. I’m calling it: good enough! I’m done grinding the back!

Total back grinding time: 5 hours 57 minutes

Back Grinding Proceeding Nicely

hillexed@email.com (hillexed) — Wed, 09 Oct 2024 21:08:45 +0000

Grinding the back has gone smoothly. At first I didn’t get good contact between tool and mirror; one orientation would slide freely but after turning the mirror 90 degrees any attempts to slide would lock up. Eventually the tool wore down and I got good contact.

In these pictures, my goal is to spread the frosted area (where the mirror has been sanded down to a sphere) everywhere. Any smooth areas are where the grit hasn’t made good contact (lower areas). You can really see that one axis was flatter than the others. Now the back is basically spherical, aside from one small region!

Meniscus mirror: twyman time!

hillexed@email.com (hillexed) — Wed, 09 Oct 2024 20:05:25 +0000

When grinding thin mirrors, there is something called the “twyman effect”: grinding one side adds stress that can slightly curl the mirror’s other side. It can lead to slight astigmatism in thin mirrors that disappears once you grind the back side to relieve that stress. (See https://quinsightspectre.com/16-25-f-3-1-meniscus-mirror/ !)

To avoid any problems, I decided to follow the wisdom and grind my meniscus mirror’s convex back through #220 grit. Plus, it would get rid of any saddle shape, which I could see existed when beginning to grind the front. I needed a concave tool to match the convex side, and while I had one already to rest the mirror on, it didn’t work well as a grinding tool because it had no tiles. The grit seems to have destroyed the soft plaster more than the hard glass, leaving a slurry of sticky yellow mud that grabbed the mirror. Tiles are useful!

I mixed and poured a new tool with ceramic tiles using 1.2L of water and 2.49 kg of dental stone; my ratio had bit too much water, I think. Since dental stone hardens at 8 minutes I way overcompensated and poured at 3 minutes, giving a nice level top. The end result: a 1in thick (probably thicker than I needed) sturdy tool I could use for grinding the back and afterwards as a stand for grinding the front.

Aluminizing Summary

hillexed@email.com (hillexed) — Fri, 27 Sep 2024 13:02:28 +0000

My 8" mirror is done; now I need to aluminize it.

The final step of making a telescope mirror, after spending months grinding glass into a parabola, is to cover the perfectly shaped glass in a thin layer of shiny reflective metal.

The most common technique is melting aluminum in a vacuum chamber so the atoms gently float onto the glass and stick. (There’s also a newer spray silvering technique that involves $400 of chemicals but amortizes out to cheap for many mirrors. ). For me, my only option is to send it to someone else with a vacuum chamber.

One plan for an international smuggling relay race and several progress-halting third party injuries later (long story), my mirror has been sitting at the home of someone with a vacuum chamber. I just got word that after two months, my mirror should hopefully be aluminized this weekend! It’ll still take a few more weeks to get checked for oxidation and shipped back to me, though…

Divot Gone

hillexed@email.com (hillexed) — Mon, 23 Sep 2024 00:25:40 +0000

Looks like after 3 hours of grinding at #220 grit, the divot that has been in my glass since the start has been ground out! Yay! I’m glad I went back to coarser grit; this could have easily been six hours had I stuck with #320.

A mirror with a short focal length will make objects look brighter because they’re zoomed out, but is much harder to parabolize at the very end of the mirror making process. I was hoping my grinding would increase the focal length from 36" back to around 40". I measured and it’s… 37.5", around f/3.1. Oh well.

On to 220

hillexed@email.com (hillexed) — Sat, 21 Sep 2024 00:08:45 +0000

I’ve gone to 200 grit from the finer 320 grit. I’m glad I did; an hour of grinding later and my last tiny divot still has’t ground away. Had I stuck with 320 grit, I’d need to spend twice as long to get to where I am now. Hopefully another hour will get it out.

A Partial Lunar Eclipse

hillexed@email.com (hillexed) — Wed, 18 Sep 2024 04:40:01 +0000

Partialest of partial lunar eclipses

Focal length flattening

hillexed@email.com (hillexed) — Sun, 15 Sep 2024 13:05:10 +0000

A problem! After 6 hours of total grinding, I measured my mirror’s focal length and found it was 37". That’s weird because before it was 39" - maybe my hands were applying pressure in the center of the tool? Lower focal length means more zoomed out (and brighter) telescope views, but makes the final stage of parabolizing harder. If focal length / mirror diameter is less than 4, you also get “coma” that distorts the stars towards the edges of the view.

So… should I move forwards with my fine #320 grit, accepting my shorter focal length? Or should I lose progress and start using a coarser #220 grit to speed up glass removal and make the curve shallower, so there’s less coma and parabolizing is easier?

Good grinding progress

hillexed@email.com (hillexed) — Fri, 13 Sep 2024 17:19:52 +0000

Today’s hour of mirror grinding has completely gotten rid of the low zone! Now I just need to grind down through that one tiny divot.

I noticed it seemed like my tool was pushing the water I sprayed out off the mirror. I used a blade to try to carve notches in between my hexagonal tiles, and afterwards I could feel the tool sliding more smoothly, allowing water to flow between the tool and the mirror. Channels are important!

Total grinding time: 4.9 hours

Introducing Pingu Camera

hillexed@email.com (hillexed) — Thu, 12 Sep 2024 04:32:14 +0000

Introducing… Pingu Camera. I bought a used DSLR for astrophotography four months ago hoping it would take better photos than my phone. I 3D printed an adapter for it to fit into a standard eyepiece slot. Unfortunately it was a bit of a failure for two reasons: firstly, this camera isn’t mirrorless, so it had a shutter inside it, and when that shutter flips up to take photos it sends vibrations through the entire telescope. I couldn’t get any good photos of planets. Secondly, the sensor is so far back into the camera that I have to get out a screwdriver and slide the entire upper tube a few inches closer to the mirror to achieve proper focus.

Meniscus mirror after one and two hours of grinding

hillexed@email.com (hillexed) — Tue, 20 Aug 2024 16:50:24 +0000

The cloudy part is where the grit has sanded the glass. Clearly part of the glass is higher and making less contact. I bet it’s because the furnace cement mold was slightly curved because the wooden mold was slightly curved thanks to the grain of the wood

12" meniscus mirror: Cool Channels

hillexed@email.com (hillexed) — Mon, 19 Aug 2024 16:35:25 +0000

My grinding tool is making cool patterns!

You’re looking at water trapped under my 12" meniscus mirroe, and flowing between the channels in the home depot porcelain tiles on my grinding tool. Those tiles will grind flat eventually.

Meniscus Mirror: Grinding Stand

hillexed@email.com (hillexed) — Sun, 18 Aug 2024 23:18:04 +0000

I got a table for $5 at a yard sale. It looks perfect for a mirror grinding setup after putting some weights on the bottom rack.

Now that I’ve completed my 8" mirror polishing, it’s time to try my bigger telescope mirror project. Just making this blank took a lot of effort - now, to grind it.

Let the grinding start!

The planets... There's two of them

hillexed@email.com (hillexed) — Tue, 13 Aug 2024 14:46:32 +0000

Taken using my 4.5" telescope and phone! Both Mars and Jupiter fit into telescope view!

Very annoying observing session. Tried to take photos of the ring nebula but missed; planet conjunction was extremely slightly out of focus and I didn’t see until morning, and I looked for but still couldn’t see M33. Couldn’t see any detail on mars by eye either; that should change as we get closer to opposition. I think I need to finish my bigger telescope.

Those Lords really are Elemental

hillexed@email.com (hillexed) — Thu, 08 Aug 2024 04:53:23 +0000

@sylvie hi get fanarted for a fun game with neat movement mechanics

There’s something inherently funny about someone who only knows how to descend ladders by backflipping

8" Mirror: actually done!

hillexed@email.com (hillexed) — Sun, 04 Aug 2024 19:15:51 +0000

Someone brought an extremely accurate “double pass autocollimator” tester to an amateur telescope maker meetup… and it showed an uniform shade of gray across the whole mirror, meaning no defects (ignore the circular reflections from the test stand). That means my mirror is an extremely smooth parabola! I’m done mirror grinding!

🎉 First mirror: complete! 🎉

That’s so cool! I thought I was slightly overparabolized based on my Foucalt testing. I guess not!

I don’t know the exact numeric accuracy of my mirror without an interferometer. The actual smoothness is “no deviations visible on a double pass autocollimation test”. A professional optician who was helping with the test estimated that as 1/20th wave or better, and I’m inclined to take their word for it.

Start to finish: 8 months. Total time spent working on this mirror, not counting setup or lap-pressing or testing time): 32.9 hours.

8" Mirror: actually done!

hillexed@email.com (hillexed) — Sun, 04 Aug 2024 19:15:51 +0000

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🎉 First mirror: complete! 🎉

That’s so cool! I thought I was slightly overparabolized based on my Foucalt testing. I guess not! That’s so cool!

Start to finish: 8 months. Total time spent working on this mirror, not counting setup or lap-pressing or testing time): 32.9 hours.

8" Telescope Making: Remember to Measure your Owls

hillexed@email.com (hillexed) — Thu, 01 Aug 2024 16:47:32 +0000

8" mount v1 complete!

After several days of designing, 3D printing, and experiencing the joys of my favorite telescope related activity, hacksawing through metal, I managed to assemble a complete mount! Design-wise, I had to replace 0.5" screws with 0.75" screws in a few places because the redesigned EMT tubes were too close together. The telescope uses only 0.5" screws, so it would have been nice to match, but it’s fine for an one-off. I also removed the horizontal bar across the front of the telescope when breaking the top part into two parts; I might need to figure out how to add that back if they’re wobbly.

However, this mount has a few problems. Firstly, It’s slightly too short. Because this Leavitt telescope design was downloaded from the internet, I had to measure its width using a tape measure to load into my CAD program as a reference. I measured it around 1/4" too short - turns out it’s actually 29.8 cm wide, meaning my mount has a gap between where the scope should go. Also, I managed to forget a screw socket (which I can fix by using longer screws), and it looks like there’s a bit of wobble. Maybe I’ll do a version 2.

8" Telescope Making: Draw The Rest of the Owl

hillexed@email.com (hillexed) — Thu, 01 Aug 2024 14:37:48 +0000

Every telescope needs a mount, to hold it up and let it rotate. 6 months ago, I thought that I was close to the finish line, since all I had to do was make a mirror and a mount! Then the mirror took six months. Now I need to sit down and actually finish this mount. My 4.5" scope uses a truss of EMT steel tubes and the scope sits on a wide C-shaped part. I modified that design for this 8" telescope’s bigger bearings… only to realize before 3D printing that this 8" scope is 10" wide, too big for a support to fit onto my 3D printer. I sat down and modified the design so the telescope sits on two separate parts instead of sitting on one wide C-shaped part. I’ll probably need to add an extra reinforcing bar on top to compensate for that extra bar I removed. Hope it works and I don’t need more revisions.

8" Telescope Mirror Parabolizing: Done??

hillexed@email.com (hillexed) — Sat, 27 Jul 2024 23:40:39 +0000

I did a tiny bit more parabolizing, then re-measured my mirror. Last time, my spreadsheet and measurements told me my outer zone needed to focus tiny bit farther and my lower zone needed to be a tiny bit closer. To dig the inner zone out, I did some W strokes with lots of mirror movement on the tool edge, and then used long center over center strokes with a tiny offset to dig everywhere but a tiny strip of outside. For parabolizing, you imagine the edge of your moving circle does the cutting, so stroking back and forth one inch off center (while rotating around the mirror!) will dig everywhere except an area around one inch into the mirror. Compared to last time I only did a tiny bit of polishing: just three times around the circular table.

My mirror looks so close to perfect. Each zone is focusing light to within 0.006" of where it should. My goal was to turn a sphere into a parabola; I may have been a touch hasty and dug a slightly too deep hole and made an “overcorrected” mirror shape. But… wow. That’s pretty much perfect! I might be done!

8" Telescope Mirror Parabolizing: Done??

hillexed@email.com (hillexed) — Sat, 27 Jul 2024 23:40:39 +0000

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8" Telescope Mirror Parabolizing: So close...

hillexed@email.com (hillexed) — Sat, 20 Jul 2024 23:47:54 +0000

I’ve switched to using the Foucalt test and a Couder mask for parabolizing. The Foucalt test is more precise (but more annoying) than the Ronchi test, but my local astronomy club has a foucalt testing rig with a micrometer that lets me measure very precise details of where different regions of the mirror focus light, so it’s more accurately moveable than my handheld Ronchi tester. I built a cardboard “Couder mask” to block off regions of my mirror to see where the edge and center are focusing light to separately. I need to take so many measurements for this Foucalt test, which is time-consuming. I also built a spreadsheet to help me interpret the results. I’m so close to perfect parabola…

8" Telescope Mirror Parabolizing: So close...

hillexed@email.com (hillexed) — Sat, 20 Jul 2024 23:47:54 +0000

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Meniscus mirror: Grinding Tool Making, Attempt 2

hillexed@email.com (hillexed) — Wed, 17 Jul 2024 02:07:25 +0000

(Attempt 1)

This time I bought some new porcelain tiles from a local store for $5. Apparently glaze on tiles are slightly softer than porcelain, but per advice I just placed the tiles upside down. Mirror sat below, then plastic wrap on top, then tiles, and finally dental stone gets mixed and poured on top. Dental stone is a waterproof plaster that hardens fast; last time it hardened in my bucket after just 15 mins of mixing. This time I set a timer for 8 minutes to pour, and used a thinner ratio if around 45 mL water to 100 g powder. I accidentally lifted up some tiles with a sticky hand, but it poured successfully! Now I can use this tool for the first half of grinding that 12" meniscus blank I made into a mooth mirror!

Meniscus Mirror: Making a Grinding Tool, Attempt 1

hillexed@email.com (hillexed) — Thu, 04 Jul 2024 13:16:20 +0000

To grind my 12" blank into a mirror, I need a grinding tool. Since the glass is already curved, I’m going to make a grinding tool for my slumped telescope blank using these ceramic tiles I bought and dental stone, a waterproof plaster! The tiles are hard and will resist the grinding from grit pieces; they’re placed onto the curved glass so the tool takes its curved shape, and dental stone will be poured over this and harden into a convex tool for grinding with hard ceramic pieces sticking out. Ideally I want a 1" or thicker tool.

I added the water first, then slowly added more dental stone and kept mixing. That was a good idea compared to adding water second. However, dental stone hardens incredibly fast. There was a moment in the pouring before all of the powedered dental stone was added when I noted it was getting a bit thick… and I decided to keep adding more, and the mixture stopped being liquid and smooth. That was my downfall, and it happened only after around minute 12. Sigh.

Result: Failure. Most of my dental stone hardened into a giant block in the bottom of my bucket that I couldn’t get out. So much for having a 1" thick tool.

Meniscus mirror: draping success!

hillexed@email.com (hillexed) — Thu, 27 Jun 2024 20:41:38 +0000

I got the news from my local glass studio in an all-caps email that said nothing but “IT WORKED!”

Look at this! My glass is now clearly curved, around 1/4" taller at the sides than at the center! That’s very close to f/4, my goal! ~~SLUMPING~~ DRAPING: SUCCESS

Weirdly, even though the mirror is the right shape, the cement form seems to have warped. The top face is now flat, and the bottom is now curved - which is weird because originally the bottom was flat and the top was curved. The bottom side looks… cracked? Like the bottom expanded? Maybe the water I sprinkled on top to prolong crystal formation ended up causing something weird.

The mirror is pretty smooth except for two pits, each 1/4mm deep. I circled them in black. Not bad at all!

Next steps: grind it smooth to within nanometers!

8" Mirror Grinding: At the finish line for the third time

hillexed@email.com (hillexed) — Thu, 20 Jun 2024 13:46:20 +0000

I’m at the final step of polishing a telescope mirror: turning a sphere shaped piece of glass into a parabola shaped piece of glass, called “figuring” or “parabolizing”. I messed up, twice, and since then I have been regrinding my glass back to a sphere for figuring attempt #3. Today, after several months, I finally have a spherical enough surface to start figuring attempt #3!

Here’s both a Ronchi test and a Foucalt test. The Ronchi shows straight lines (yahoo!) and the foucalt shows slight ring shaped surface variations. I don’t know what in my stroke is causing those rings. Maybe I wasn’t varying the stroke length enough?

While pressing my pitch lap into my mirror, I placed the mesh from a bag of lemons in between the two to create regular grooves called microfacets. On a hunch, I stopped using the lemon skin for the last few presses, and the rings seemed to decrease in height (but they were around 1/10 of a wave tall, so not a big deal). Maybe the lemon skin was so tall it stopped the pitch lap from pressing into the surface of the mirror somehow? Or maybe it’s unrelated to lemon bag and the rising temperature made the pitch softer, and that conformed to the surface better. Anyway, my mirror is much smoother now, and it’s time to make a parabola once and for all.

8" Mirror Grinding: At the finish line for the third time

hillexed@email.com (hillexed) — Thu, 20 Jun 2024 13:46:20 +0000

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Meniscus mirror: The power of teamwork!

hillexed@email.com (hillexed) — Wed, 15 May 2024 14:31:55 +0000

My goal of making meniscus mirrors is in sight! Now that I have a properly shaped cement form and a circle of glass cut out of a used countertop, I just need a computer controlled kiln to heat them up in.

There happened to be an artist showcase near me, so I went and talked to a glass artist. She suggested I could find a kiln by… asking a studio if I could rent space in their kiln. I didn’t realize that was a thing I could ask an artist for.

It worked on the first try! A nice local glass studio offered to do it for cheap, and we had a great conversation about all the infinite ways to manipulate glass for art. They had three huge computerized kilns! The telescope makers are wrong: I learned what I’m doing, heating glass up over a convex mold, is actually called “draping”. It’s not called “slumping” unless it’s a concave mold.

Time to cradle, and then I finally have a lightweight blank!

Hey, an aurora!

hillexed@email.com (hillexed) — Sat, 11 May 2024 16:25:09 +0000

I saw the aurora! It looked like light pollution at first, and I only realized what it was when I noticed the underwhelming “clouds” were in a different place after ten seconds. An hour later they had slight tints of red and green and distinct ray-like shapes! Very cool. Phone camera picked up way more color and detail than I could see by eye, and long 8s exposures picked up even more.

Meniscus mirror mold attempt #5: success!

hillexed@email.com (hillexed) — Sat, 04 May 2024 14:45:50 +0000

Bought some new furnace cement. Reused the same mold as before. Gave it four coats of spray polyurethane, sanded with #320 sandpaper before the last coat, taped up paper to the edge, mixed and poured cement and hurt my hand when my 3D printed spatula cracked in half. Lots of water ended up leaking out of the sides, which I was worried would be a bad thing and dry it out, so I added more water on top after the first day.

But it looks like it worked! Getting it out of the wood mold was a nightmare, I went around the side with a blade and it didnt help. Then I wised up and brought it outside, turned it upside down, gave it some thumps and flexed the wood, and it fell out.

It looks so smooth! Hopefully now I can move on to actually slumping glass!

Meniscus mirror mold attempt #4

hillexed@email.com (hillexed) — Sun, 28 Apr 2024 23:27:02 +0000

I carved a new wood mold and used the last of my furnace cement on attempt #4… and because I was working with 1/3 a bucket of cement, I ended up with something very thin that cracked when I tried to lift it off the wood mold. Sigh

I only waited one day before trying to remove it, and I patted it dry with a paper towel. I think both of these were mistakes - the longer it sits wet, the harder it gets as crystals grow.

In this picture, attempt #4 is on the right, with the crack on the left. On top is attempt #1, which I set on fire. Attempt #2 on the left, with the wrong kind of cement. At least I’m improving…

8" Mirror Grinding: Seeing Flaws

hillexed@email.com (hillexed) — Wed, 17 Apr 2024 16:50:10 +0000

It’s been a while but I gave the 8" mirror some grinding. I’m on my way back to sphere, undoing parabolizing attempt #2. 30 minutes of grinding seems to have created some turned down edge and a hole in the middle. Great.

I put the mirror into the telescope, leaned it against a big crate again, and I was able to use it to see the moon! It’s amazing that even though I’m not done figuring, it’s smooth enough I can see things in space. It was noticeably dimmer and slightly less sharp than in my hadley, which makes sense because the 8" mirror isn’t aluminized.

Now to keep at it, and then start parabolizing try #3.

8" Mirror Grinding: Seeing Flaws

hillexed@email.com (hillexed) — Wed, 17 Apr 2024 16:50:10 +0000

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Now to keep at it, and then start parabolizing try #3.

Free Mirror??

hillexed@email.com (hillexed) — Wed, 17 Apr 2024 00:15:17 +0000

So I was talking to new local friends and telling them about telescopes and someone says “I have a mirror sitting on my table, is that something that could be useful?” and so after picking it up I now have a ginormous 12.25" mirror that’s 2.25" thick!!

…now what do I do with it? Honestly I’m tempted to make a scope out of it… but that would take money and I’d need to learn new skills like woodworking. And I still need to finish figuring my 8", which already has an entire telescope built waiting for the mirror to be done, and I have the 12" meniscus mirror project. So I guess I’m leaning towards selling it?

The rock displeased me so I set it on fire

hillexed@email.com (hillexed) — Thu, 28 Mar 2024 16:03:45 +0000

I finally saw my friend’s kiln! It’s a big kiln.

I took the cement mold (the wrongly shaped one) and cast it into the flames for displeasing me.

Looks like the kiln can successfully get up to 1200 degrees F! The problem is, the kiln controller can only go up to one target temperature. For the actual glass slumping, I will need a ramp/soak controller, which can let me program it to hold at a certain temperature for some amount of time then move onto another temperature. Time to scour ebay and see if I can find one…

Meniscus mirror attempt #3: OH NO

hillexed@email.com (hillexed) — Tue, 26 Mar 2024 03:10:41 +0000

…the cement form’s curvature isn’t right. I measured the sagitta (how deep the curve is in the middle compared to the edge) and it’s 1/16". it should be around 1/4". This would make a telescope that’s 12 feet long.

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAARGH

I CNCed mold’s shape wrong. I have two choices: I can either grind/sand down the edges of my cement form into a deeper curve (probably releasing tons of bad to breathe in dust), or start the process all over again with new wood.

Meniscus mirror attempt #3: I'M A GENIUS

hillexed@email.com (hillexed) — Tue, 26 Mar 2024 03:08:49 +0000

I GOT IT OUT!!!

FINALLYYYYYY AFTER THREE TRIES AND SO MUCH POURING I HAVE A KILN RESISTANT THING THAT CAN FIT MY FURNACE CEMENT!!!

12" Meniscus mirror mold attempt #3

hillexed@email.com (hillexed) — Fri, 22 Mar 2024 15:02:42 +0000

In addition to grinding an 8" mirror, I’m trying a technique to make big thin 12" mirrors. See the #meniscusmirror tag for more. Previously, setting things on fire didn’t improve the problem.

Now I have a $35 tub of castable furnace cement (amazon had it for $14 cheaper than home depot). It’s dry sand that needs water and throws up clouds of dust, glad I have a respirator from covid. Let’s try this

Meniscus mirror mold-making attempt #3: begin!

8" Parabolizing attempt #2: Abort...

hillexed@email.com (hillexed) — Sun, 17 Mar 2024 18:05:44 +0000

My local astronomy club has a foucalt tester, and today I tested that mirror after 30 seconds of parabolizing. It revealed a big turned down edge (yahoo…) and some slight zones in the center of the mirror. I knew those zones existed - heck, you can even see the slight non-sphericalness in the second image of my “close enough to spherical!” celebration - but the radius of curvature changes rapidly where one zone meets another instead of smoothly blending. I was willing to call that close enough, but according to my local astronomy club’s resident professional optician, that non-smooth curvature would throw light all over the place and was bad enough I should return to a sphere and smooth that out. AAAAARGH

For reference, a mirror is usually considered good enough if its shape is within 1/4 of a wavelength of light (usually green light, wavelength 500 nanometers) - of an idealized parabola. This optician is a perfectionist thinks there’s no reason an amateur telescope mirror can’t get to within 1/10 of a wavelength. I appreciate his optimism but man I don’t want to return to sphere a second time.

…I’m going to return to sphere a second time. I saw the zones using my current testing setup - I just ignored them. This is possible. I can do this.

Total grinding time: 26 hours

8" Parabolizing attempt #2: Abort...

hillexed@email.com (hillexed) — Sun, 17 Mar 2024 18:05:44 +0000

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…I’m going to return to sphere a second time. I saw the zones using my current testing setup - I just ignored them. This is possible. I can do this.

Total grinding time: 26 hours

8" Mirror Parabolizing: Oh No Not Again

hillexed@email.com (hillexed) — Sat, 16 Mar 2024 14:38:47 +0000

I’m parabolizing my telescope mirror - I’ve ground it to a sphere shape, and now I’m grinding a tiny bit more to change its shape precisely into a parabola.

I’ve done 3 grinding sessions for a total of 34 minutes of grinding. Here’s a Ronchi picture of my current progress. Mel’s online Ronchi calculator has overlaid some semi-transparent white lines which show the ideal Ronchi grid of a somewhat parabolized mirror. Looks like the center is fine, but the edge is focusing light too close - similar to what happened in parabolizing attempt #1. I’ll need to think to figure out how to solve this.

8" Mirror Parabolizing: Oh No Not Again

hillexed@email.com (hillexed) — Sat, 16 Mar 2024 14:38:47 +0000

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I’m parabolizing my telescope mirror - I’ve ground it to a sphere shape, and now I’m grinding a tiny bit more to change its shape precisely into a parabola.

8" Mirror grinding: Parabolization #1

hillexed@email.com (hillexed) — Sat, 16 Mar 2024 00:43:46 +0000

I’m in the home stretch - my goal is to turn this sphere shaped mirror into a parabola shaped mirror.

The final stage of mirror grinding is called either parabolizing, because you’re making a parabola, or figuring, because you have to measure carefully and that involves numbers which are also called figures. You use a stroke that goes up and down fast and side to side slowly in a big zigzag, called a W stroke, to remove a tiny bit of glass from both the center and the edge, as seen in the first picture (from Mel Bartels’ site).

This stage involves removing very small amounts of glass, so parabolizing is very quick. After 15 minutes of a W stroke - which is crazy fast compared to hours and hours it took to get to spherical - my center was slightly deepened, enough to match a parabola! My mirror edge, however, still had very straight bands. The Ronchi test calculator said my edge wasn’t where it should be.

Then I made a mistake: the amateur astronomers in the discord said I had a turned edge, and I needed to return to spherical. A mirror edge that’s too low can only be fixed by grinding the rest of the rest of the mirror down to that height. So I spent anouther two hours undoing all my hard parabolizing work.

But in retrospect, I didn’t need to do that at all - my edge was focusing light too close, so if I wore down the edge it would have focused light at a longer radius of curvature, which is what I wanted! I could have done that! I mistook the error for “turned down edge” when actually the edge wasn’t turned down enough!

Anyway, I’m back at a sphere. Parabolization attempt #2: begin!

8" Mirror grinding: Parabolization #1

hillexed@email.com (hillexed) — Sat, 16 Mar 2024 00:43:46 +0000

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I’m in the home stretch - my goal is to turn this sphere shaped mirror into a parabola shaped mirror.

Anyway, I’m back at a sphere. Parabolization attempt #2: begin!

8" mirror grinding: SPHERICAL ENOUGH TO CELEBRATE

hillexed@email.com (hillexed) — Sun, 10 Mar 2024 18:11:13 +0000

This site is my secret weapon: https://www.bbastrodesigns.com/ronchi.html

It’s a slightly janky javascript app made by some old telescope maker in Oregon, and it lets you get quantitative measurements from all those Ronchi pictures. Very important.

Looks like my outer zone focuses light at around 0.066 in from my desired radius of curvature, and inner zone focuses light at around 0.1 in. (There’s probably big error bars on both those numbers). Only the relative number of “inner zone focuses 0.1-0.66in = 0.03 in closer than outer zone” really matters.

Since the focal length is half the radius of convergence, that means my mirror focuses space light to 0.015" of where a spherical mirror would. Not bad!

CLOSE ENOUGH! SPHERE-ISH ACHIEVED!!

But my goal is to focus space light. A sphere will bounce light from center point back to itself, but to focus parallel light rays from space to a single point, I need a parabola.

Jean Texereau’s book says the formula for “parabolic abberation”, how different a parabola is from a sphere, is that the zone with radius h from the mirror should focus a distance Δp = h^2 / R further from the radius of curvature. Tape measure and eyeball says as I move my tester backwards, the ronchi switches from bands get wider to bands get smaller at 89 + 1/16", so that’s the radius of curvature of my mirror (which is twice the focal length). By that formula, my goal is to have the very edge focus h^2/R = (1*8/2)^2/89 + 1/16" = 0.1796" further from the front.

Above I calculated that the edge focuses 0.03in closer than the front. Wow, I went so far past parabolic in my quest to get to spherical. I think I’m ready to parabolize.

To parabolize, I have to deepen this curve around six times its current amount. That’s doable! The open astronomy discord recommended I start by smoothing out the division between the two zones with long strokes, and then begin using a different stroke to deepen the center. Let’s do this

Total grinding time: 21 hours

8" mirror grinding: SPHERICAL ENOUGH TO CELEBRATE

hillexed@email.com (hillexed) — Sun, 10 Mar 2024 18:11:13 +0000

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This site is my secret weapon: https://www.bbastrodesigns.com/ronchi.html

It’s a slightly janky javascript app made by some old telescope maker in Oregon, and it lets you get quantitative measurements from all those Ronchi pictures. Very important.

Since the focal length is half the radius of convergence, that means my mirror focuses space light to 0.015" of where a spherical mirror would. Not bad!

CLOSE ENOUGH! SPHERE-ISH ACHIEVED!!

But my goal is to focus space light. A sphere will bounce light from center point back to itself, but to focus parallel light rays from space to a single point, I need a parabola.

Above I calculated that the edge focuses 0.03in closer than the front. Wow, I went so far past parabolic in my quest to get to spherical. I think I’m ready to parabolize.

Total grinding time: 21 hours

8" Mirror Grinding Update: ALMOST THERE

hillexed@email.com (hillexed) — Sat, 09 Mar 2024 21:30:19 +0000

I’M SO CLOSE TO A SPHERE

(You know you’re a sphere when the lines are completely straight. See this infographic for more)

total mirror grinding time: TWENTY GODDAMN HOURS

8" Mirror Grinding Update: ALMOST THERE

hillexed@email.com (hillexed) — Sat, 09 Mar 2024 21:30:19 +0000

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I’M SO CLOSE TO A SPHERE

(You know you’re a sphere when the lines are completely straight. See this infographic for more)

total mirror grinding time: TWENTY GODDAMN HOURS

Mirror grinding update

hillexed@email.com (hillexed) — Fri, 08 Mar 2024 23:53:47 +0000

I did 20 minutes without my center paper cutout and managed to dig a tiny zone in the center. Yaaaaay. Then I spent two hours ignoring it and trying to get the outside zone down.

On the plus side that outer zone at 70%+ diameter looks spherical now.

Now I have a choice: I think I’m close to a parabola. With that deeper center, if I can flatten the inner edge of that transition I’m very close. Do I keep going all the way to a completely flat sphere, or try to stop halfway through and thread the tricky needle?

Mirror grinding update

hillexed@email.com (hillexed) — Fri, 08 Mar 2024 23:53:47 +0000

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I did 20 minutes without my center paper cutout and managed to dig a tiny zone in the center. Yaaaaay. Then I spent two hours ignoring it and trying to get the outside zone down.

On the plus side that outer zone at 70%+ diameter looks spherical now.

8" Mirror grinding: Flower Power

hillexed@email.com (hillexed) — Wed, 06 Mar 2024 21:33:57 +0000

I’m using an advanced technique: cutting out pieces of paper and putting them under the tool while pressing to avoid certain areas of the tool touching the mirror. I know I have a hole in the middle, so by blocking the middle from wearing down I can concentrate my wearing on the outer zones without making the center even deeper.

Looks like after a few hours I’ve managed to get the outer zone almost completely spherical! There’s a bit of TDE left to fight and the center is a bit messed up but I think I’m very close.

From here I could either get everything to a sphere and grind out that TDE and then wear down the center again to make a parabola, or see if I can go to a parabola from where I am now. I don’t quite know how to decide.

8" Mirror grinding: Flower Power

hillexed@email.com (hillexed) — Wed, 06 Mar 2024 21:33:57 +0000

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Looks like after a few hours I’ve managed to get the outer zone almost completely spherical! There’s a bit of TDE left to fight and the center is a bit messed up but I think I’m very close.

A Ronchi Test Infographic: How to read my funny red stripe pictures

hillexed@email.com (hillexed) — Mon, 04 Mar 2024 23:05:04 +0000

Check out this infographic of how to read the Ronchi tests of my telescope mirror! I’ve been posting lots of pictures like this as I grind, and this tells you how to interpret what the pictures say about a mirror’s shape. The Ronchi test can be used to roughly measure a mirror’s shape and see any turned down edge (“TDE”), and with a computer program to analyze them, even give some quantitative measurements.

(I drew the light hitting the mirrors on the left as parallel lines for clarity, but in the ronchi test the light is actually coming from a single point close to the focus. don’t tell anyone)

A Ronchi Test Infographic: How to read my funny red stripe pictures

hillexed@email.com (hillexed) — Mon, 04 Mar 2024 23:05:04 +0000

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(I drew the light hitting the mirrors on the left as parallel lines for clarity, but in the ronchi test the light is actually coming from a single point close to the focus. don’t tell anyone)

8" mirror grinding: yahoo!

hillexed@email.com (hillexed) — Mon, 04 Mar 2024 21:00:51 +0000

Two hours of mirror grinding brought me from the first pic to the second! The center zone got so big! It’s so much straighter! I’m much closer to a spherical mirror!

8" mirror grinding: yahoo!

hillexed@email.com (hillexed) — Mon, 04 Mar 2024 21:00:51 +0000

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Two hours of mirror grinding brought me from the first pic to the second! The center zone got so big! It’s so much straighter! I’m much closer to a spherical mirror!

Current mirror progress: March 1 edition

hillexed@email.com (hillexed) — Sat, 02 Mar 2024 16:36:41 +0000

The center zone, visible in the first pic as the inner area with red on the left and black on the right, is a bit more smooth and has grown from around 50% diameter to around 80% diameter. The outer zone is still a gradual slope but it focuses light to around a centimeter or so further than the inner zone. Previously that outer zone was extremely too tall, so my goal was to reduce it, and compared to a few weeks ago it looks reduced but not gone. There’s also still 1/6" of turned down edge, yaaaaay

My pitch lap has developed regions around the edge where the pitch has fallen down and lost contact. I think the dental stone released a gas and created a bubble, which made the pitch above the bubble slowly fall down once the gas could escape.

I tried a new technique: modifying my tool by putting a small cut portion of paper over the center while pressing, so the center of the tool doesn’t make contact and wears down the center of the mirror less. It seems to be working.

Total grinding time so far: 12 hours

Current mirror progress: March 1 edition

hillexed@email.com (hillexed) — Sat, 02 Mar 2024 16:36:41 +0000

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Total grinding time so far: 12 hours

Setting things on fire

hillexed@email.com (hillexed) — Mon, 26 Feb 2024 17:10:05 +0000

After a few weeks of drying, the wood was very warped and I couldn’t get the furnace cement out; perhaps it had bonded to the wood. What’s the difference between wood and furnace cement? Cement is heat resistant, and wood is very flammable.

So I started a fire. I grabbed some sticks and some newspaper as kindling and piled it on, then lit it on fire inside a barbecue. Turns out I suck at starting a fire - I didn’t have enough small sticks to sustain a fire for a while and catch the large wooden mold on fire. It burned decently, but didn’t get hot enough to catch the wooden mold on fire.

After all that… I ended up with lots of ash and some blackened wood that STILL wasn’t separated from the furnace cement.

Also, the cement itself, which was supposed to resist the heat, didn’t resist the heat! It sprouted a giant pocket in the middle like naan. I thought I got rid of the moisture by drying for weeks… what happened?

Turns out… I was using the wrong type of furnace cement all along. I need “castable” furnace cement - otherwise it won’t let the water out. I was using “furnace cement” designed to seal over cracks, and in the fine print it said “spread a thin layer”. Thin meant “less than 1/4” I suppose. Water and air couldn’t get out of my 3/4" blockt, and when I set it on fire it expanded and caused those bubbles.

So I need to buy some castable furnace cement. I did some searching and it’s only available in 50lb boxes for $70. Aaaaargh

Meniscus mirror mold attempt #2

hillexed@email.com (hillexed) — Wed, 21 Feb 2024 16:43:26 +0000

To make a meniscus mirror I need a precisely-shaped thing that will stay the same shape even at 600C. On Jan 31, I machined out a new piece of wood, sprayed it with polyurethane, and then poured in furnace cement!

…but several days of drying later, I couldn’t get the cement out of the mold. It stuck to the wood too well. In attempt #1 I used 3 coats of polyurethane, but in this I only used one. Eventually the wood started warping, so I figured I’d try using a hacksaw to try to cut out the wood, but even that didn’t help. Maybe the edges were holding it in place? Nope. The wood was practically bonded to the cement.

So… how can I remove this furnace cement (which cures at high temperatures) from the wood?

I decided to set it on fire.

Current mirror progress

hillexed@email.com (hillexed) — Mon, 19 Feb 2024 17:46:10 +0000

Here’s my mirror in a Ronchi test and a Foucault test. I’ve managed to reduce the turned down edge to a very small area but it’s still there, visible in the ronchi as hooks at the edges and in the Foucault as a slight black zone at the bottom left of the image. I’ve also managed to dig a huge hole in the center of the mirror trying to fix that edge…

Current mirror progress

hillexed@email.com (hillexed) — Mon, 19 Feb 2024 17:46:10 +0000

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It's M42!

hillexed@email.com (hillexed) — Sat, 17 Feb 2024 00:15:58 +0000

Since Orion was so high in the sky compared to my last photo I was curious if photos would look better… and they do!

This is one 1/2s phone pic, scaled to hell and back to bring out the “bat wings”: big dark edges that go from bottom left to top right. Now I know there’s many ways to make this better, such as motorizing or a bigger telescope or stacking multiple pictures - but for just one phone pic this isn’t that bad!

Grinding continues

hillexed@email.com (hillexed) — Fri, 16 Feb 2024 02:39:57 +0000

It looks like there were pockets of air under the pitch. Leaving it alone for a week meant the bubbles seem to have popped and some areas sank downwards.

This turned down edge is slowly going down but it still feels like an endless quest with no end in sight.

Total grinding time so far: 8 hours

Grinding continues

hillexed@email.com (hillexed) — Fri, 16 Feb 2024 02:39:57 +0000

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It looks like there were pockets of air under the pitch. Leaving it alone for a week meant the bubbles seem to have popped and some areas sank downwards.

This turned down edge is slowly going down but it still feels like an endless quest with no end in sight.

Total grinding time so far: 8 hours

The Art Of Figuring

hillexed@email.com (hillexed) — Thu, 08 Feb 2024 18:00:57 +0000

This pretty art piece is actually a precise measurement of my progress in telescope mirror grinding. It looked fun enough out of context I decided to post it!

You’re looking at multiple overlaid square images. Each one is a Ronchi test, which tells me the shape of my mirror on a nanometer scale in the shape of the red stripes. As I polish the mirror, it changes shape, so after 20 minutes of polishing I want to measure the new shape. Each time I test, I precisely align my handheld tester, and then take a photo with my phone. Since the tester is in a different place at a time, I have to place my phone on its corner and lean it to one side to put the camera at the right height - but that rotates the image, so afterwards I load up the Ronchi test images in Krita and de-rotate them so the stripes go up and down, so I can compare the images as I grind. You’re looking at a Krita file with many Ronchi images, each rotated and scaled so the mirror is the same size and in the same place. It makes a nice flower shape!

The Art Of Figuring

hillexed@email.com (hillexed) — Thu, 08 Feb 2024 18:00:57 +0000

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This pretty art piece is actually a precise measurement of my progress in telescope mirror grinding. It looked fun enough out of context I decided to post it!

hillexed@email.com (hillexed) — Mon, 05 Feb 2024 23:26:38 +0000

you can mirror grind in two ways: mirror on top or tool on top. Apparently if you have a turned down edge to fix it you do center over center strokes with amplitude 1/3 of the length, according to a video by Gordon Waite. I did it for two hours yesterday and made little progress. Today I took another look at the video and noticed he was doing it tool on top. I wasn’t. 30 minutes of work later it looks so much better. sighhh

hillexed@email.com (hillexed) — Mon, 05 Feb 2024 23:26:38 +0000

hillexed@email.com (hillexed) — Mon, 05 Feb 2024 00:38:26 +0000

Figuring sucks.

My mirror should be a sphere. It isn’t a sphere. I’ve introduced a “turned down edge”, where the edge is ground lower than the rest of the mirror and you have to remove all the glass in the center to fix it. You can see it in these ronchi test pictures, each taken after a few sessions of 30 minutes of total polishing. The straight lines show that part of the mirror is spherical. There’s an annulus shaped around each one where it isn’t straight lines. I hope these pics show that it’s getting better over time…

hillexed@email.com (hillexed) — Mon, 05 Feb 2024 00:38:26 +0000

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Figuring sucks.

8" mirror Polishing, part 4: Seeing by Standing on the Shoulders of Plastic

hillexed@email.com (hillexed) — Fri, 02 Feb 2024 18:12:05 +0000

To test the shape of a mirror, amateur telescope makers have a few tests which involve bouncing light off the mirror to see its shape. One cheap one is the Ronchi test, which sends light through a grating of fine lines, bounces off the mirror, and then you place the grating at the radius of convergence of your mirror so it blocks part of the light and reveals the mirror shape. Nowadays you can make a grating by using a high DPI laser printer! I didn’t have one but someone in the Observational Astronomy discord named chantepierre did, so he mailed me a Ronchi grating in an envelope from France two months ago. Thank you, chantepierre!

I set up a Ronchi tester using a 3D printed part, LED, resistor, and battery - I just taped the Ronchi grating to the front. But I couldn’t see anything when I looked, because the mirror was tilted wrong. So I printed a mirror stand! The mirror stand sort of helped, but the print warped, so it wasn’t as helpful. Finally I realized I needed my tester to be higher, so once tester v2 stood proudly on tester v1, I was able to see the LED’s reflection through the grating and take a photo!

These stripes tell me… that a fridge nearby is vibrating. Sigh. But at least I’m seeing stripes!

Total polishing time so far: 70 minutes

8" mirror Polishing, part 4: Seeing by Standing on the Shoulders of Plastic

hillexed@email.com (hillexed) — Fri, 02 Feb 2024 18:12:05 +0000

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These stripes tell me… that a fridge nearby is vibrating. Sigh. But at least I’m seeing stripes!

Total polishing time so far: 70 minutes

Meniscus mirror mold, attempt #2

hillexed@email.com (hillexed) — Wed, 31 Jan 2024 22:34:45 +0000

To give a circle of glass (approximately) the right curve for a telescope mirro,, I want to make a precisely shaped mold that can withstand kiln temperatures. This technique makes what’s called a meniscus mirror.

Last time I tried this, the wood delaminated and came apart at the boundary where one plank was glued to another. This time, I’m spraying the wood with polyurethane spray on both the front AND back side, to avoid any warping before I put the furnace cement in. It seems to have warped a tiny bit, but only a fraction of an inch off the ground at one end. Hopefully it’ll be OK.

Printed a new trowel (I love that I can say that), sanded the wood, wiped off dust with alcohol soaked paper towel, bought a different type of furnace cement, and spread furnace cement. Hopefully this will give me a nice curved furnace cement mold!

Meniscus mirror mold, attempt #2

hillexed@email.com (hillexed) — Wed, 31 Jan 2024 22:34:45 +0000

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8" Mirror Polishing, Part 3: First Strokes

hillexed@email.com (hillexed) — Tue, 30 Jan 2024 17:11:15 +0000

After all that setup, actually polishing a mirror is surprisingly simple. First you put your mirror on your tool and apply pressure so the pitch flows and takes the shape of your mirror (which happens faster if it’s hot, so you can leave the pitch lap in hot water to heat it up and speed up pressing). Then you take your mirror, put it on your tool, and push it back and forth without applying any downwards pressure. Pressure is an advanced technique that makes the polishing go faster, but too much pressure can grind one part of a mirror slightly lower.

After a few minutes every time I rotated the mirror there was a ringing squeak noise like dragging your finger along the rim of a wine glass. Allegedly that’s a good thing, but it makes me nervous when I hear it.

It turns out grinding a mirror involves grinding. My arms got tired after 30 minutes. Well well well, if it isn’t the consequences of my own actions.

8" Mirror Polishing, Part 3: First Strokes

hillexed@email.com (hillexed) — Tue, 30 Jan 2024 17:11:15 +0000

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It turns out grinding a mirror involves grinding. My arms got tired after 30 minutes. Well well well, if it isn’t the consequences of my own actions.

8" Mirror Polishing, Part 2: Curveball Pitch

hillexed@email.com (hillexed) — Fri, 26 Jan 2024 17:16:15 +0000

I’m making a tool called a pitch lap to grind an 8" mirror. Previously, I discovered the best way to find dental stone is a dentist, and made a yellow plaster disk.

Pitch lap step 2: Pour the pitch!

Pitch is a weird material. It’s a liquid so viscous it looks like a solid. At high temperature it’ll pour like honey, at low temperature it’ll act solid but very very slowly flow. When grinding a mirror, we use pitch so it’ll keep flowing by a few nanometers and keep touching and grinding the mirror even as the mirror’s shape changes.

Hot pitch sticks to anything. Cold pitch cracks and sprays tiny dust crumbs everywhere. It’s annoying to work with. I bought a sacrificial pan from goodwill for $5 to hold the pitch, and I’m glad I did because that pan is never getting the pitch out.

The individual steps themselves were pretty simple: first heat up the pitch (this took hours to heat it evenly up to 150F), then pour it on top of your tool, then press the mirror onto your pitch so the top surface becomes shaped like your mirror.

Before that I had to cut away the edges of my dental stone tool with a razor blade, which I’d never done before and my slowness made the local astronomy club impatient.

The very very final step is to cut lines into the plaster so it can flow easier. Normally this is done right after pouring… but I had to leave for something time sensitive, and that meant the 99% complete pitch lap sat in a box for a month.

A month later, I cut some channels using a razor blade (the others made it look so easy!), warmed up the pitch by putting it in warm water, and then pressed the mirror face onto it. A bag of onions helped create tiny channels called “microfacets” so the pitch can flow easier.

Pitch lap complete! All I need is some cerium oxide polishing agent and a way to test my mirror, and I’m ready to start polishing.

8" Mirror Polishing, Part 2: Curveball Pitch

hillexed@email.com (hillexed) — Fri, 26 Jan 2024 17:16:15 +0000

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I’m making a tool called a pitch lap to grind an 8" mirror. Previously, I discovered the best way to find dental stone is a dentist, and made a yellow plaster disk.

Pitch lap step 2: Pour the pitch!

Before that I had to cut away the edges of my dental stone tool with a razor blade, which I’d never done before and my slowness made the local astronomy club impatient.

Pitch lap complete! All I need is some cerium oxide polishing agent and a way to test my mirror, and I’m ready to start polishing.

8" Mirror Polishing, Part 1: The Power of Teeth

hillexed@email.com (hillexed) — Fri, 26 Jan 2024 05:20:59 +0000

Previously, @Beasmeeply generously donated me an 8" mirror blank, kick-starting an attempt to finish grinding it and make an 8" telescope. There are four stages of mirror grinding, so to figure out which step I needed to start with, I put the mirror into a Foucault mirror tester, and it gave a smooth-ish image, telling me BeasMeeply had gotten through the first two stages of mirror making, rough and fine grinding, and it was ready to polish.

The next step is polishing, and to do that, I need a pitch lap! A pitch lap is a disk the same size as a mirror with a layer of pitch poured onto it and used as a grinding surface.

Step 1: Make a disk.

Nowadays the cheapest way to do it is with waterproof plaster called dental stone. A local astronomy club had some and was willing to mix up some so I could use it… but they only had a paper bowl to mix it up in, we used too little, and the result was a thin pancake that cracked easily. I needed more stone and a thicker tool.

I looked online for where to buy dental stone. Assuming you don’t want to buy a 25 lb bulk box ( I only need 1lb), it’s very hard to find anywhere but the slightly expensive firsthanddiscovery.com .

And then: by chance, I had a pre-scheduled dentist appointment. During the appointment, I asked nicely, and they sent me home with two ziplock bags full of dental stone!

A trip to Goodwill got me a big plastic jug for mixing lots of dental stone in, and which I was okay ruining with bits of hardened stone. (The flexible plastic walls ended up being convenient for cleanup later - bending them meant large bits of hardened-on plaster cracked off!)

I got help online with the water-to-powder ratio 30 ml water/100 g powder, grabbed one of my failed wooden carvings from the meniscus mirror project to contain any spills, and got mixing. Dental stone hardened so quickly - within two minutes it was noticably hard to mix. I thought I had messed up the water-to-powder ratio because it felt too dry, so I added more water and then it was suddenly very runny. When I dipped a plastic fork in it, it didn’t drip off the fork, however.

Worried that if I waited any longer it would harden, I poured the plaster (using my mirror blank as the mold with a layer of plastic wrap on top). The dental stone was already so viscous it didn’t smooth itself out to a level surface, giving me an uneven back.

Then I had to wait a week for it to dry before I could turn the stone disk into a proper grinding tool!

8" Mirror Polishing, Part 1: The Power of Teeth

hillexed@email.com (hillexed) — Fri, 26 Jan 2024 05:20:59 +0000

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The next step is polishing, and to do that, I need a pitch lap! A pitch lap is a disk the same size as a mirror with a layer of pitch poured onto it and used as a grinding surface.

Step 1: Make a disk.

And then: by chance, I had a pre-scheduled dentist appointment. During the appointment, I asked nicely, and they sent me home with two ziplock bags full of dental stone!

Then I had to wait a week for it to dry before I could turn the stone disk into a proper grinding tool!

It's mirror time

hillexed@email.com (hillexed) — Sun, 21 Jan 2024 05:10:38 +0000

Finally finished making a pitch lap for @BeasMeeply’s mirror! Yahoo! Now I can start polishing… as soon as I get some cerium oxide to polish with.

It's mirror time

hillexed@email.com (hillexed) — Sun, 21 Jan 2024 05:10:38 +0000

Finally finished making a pitch lap for @BeasMeeply’s mirror! Yahoo! Now I can start polishing… as soon as I get some cerium oxide to polish with.

Half Moon

hillexed@email.com (hillexed) — Thu, 18 Jan 2024 06:08:43 +0000

A quick moon pic from my 3D printed telescope and phone cam. Some out of focus tree branches snuck into the top and traced some faint dark lines!

My gearbox finally has gears in it

hillexed@email.com (hillexed) — Sun, 14 Jan 2024 23:09:28 +0000

This is going to turn the turntable that my telescope will sit on

Gearbox has some decent progress

hillexed@email.com (hillexed) — Sat, 13 Jan 2024 05:54:02 +0000

I’ll print out a prototype and see if it holds everything the way I want it to.

Question I still need to answer: how do I stop the axles from fall out

Gearbox design going... okay

hillexed@email.com (hillexed) — Fri, 12 Jan 2024 23:19:36 +0000

I don’t know how to design a sketch in one document (for say a stepper motor mount plate) then import it into a different document in a different location (for, say, mounting that stepper in a different place)

First gearbox test

hillexed@email.com (hillexed) — Fri, 12 Jan 2024 19:00:14 +0000

Looks like the belts and pulleys I got fit! Now to design the gearbox and 3D print it so my telescope can move very small amounts accurately

The Jan 2024 Telescope Shenanigan List

hillexed@email.com (hillexed) — Thu, 04 Jan 2024 19:18:07 +0000

I have many projects in mind for telescope upgrades. Since the last list I’ve completed one and added one. Here’s what I want to do this year telescope wise:

#1: Sliced Pifinder (complete!)

Complete! I built a Pifinder for 1/5 of the list price by using a different cheaper camera, secondhand older pi and battery pack, and printing and soldering parts myself. It’s been very helpful when it works, and let me take pictures of M33 even without seeing it! It’s crazy that I have a computer on my telescope at all times.

#2: Motorizing

The Earth’s rotation is my enemy. The stars smear into lines if I take long exposures, where long means “one second”. To fix that, I want motors to move my telescope as the sky moves!

I scrounged some free stepper motors, and bought $40 of gears and axles and bearings from China. One person in the discord for Hadley 3D printed telescopes has already motorized a Hadley, so I’m stealing their overall design: two axes, alt/az, each with GT2 belts and pulleys to gear down the motor for very small rotations, held in place by 3D printed housing and using ball bearings to hold the axes. Unlike that one, which uses 8mm and 5mm axles, I’m using only 5mm axles to make ordering simpler.

There’s lots of work to do, including actually designing the thing. I still need to design the gearboxes, buy wood for the central turntable, and figure out how they’ll fit on it and spin the scope. But if it works it’ll boost the exposures I can take up to ten seconds or so!

#3: Make Bigger Scope

@BeasMeeply donated me an 8" glass blank that was partially ground, and this year I’m going to finish polishing it and make a telescope! I’ve printed all the parts for a “Leavitt”, the only 8" printed scope whose design is public. I’ve confirmed the blank is going to make an 8" f/5.5. I’ve inspected it to figure out it was ready for polishing, poured dental stone, and nearly finished a pitch lap. Now I just need to build a ronchi testing setup and start polishing.

#3.5: meniscus mirrors

Meniscus mirrors are a way to build very big mirrors that are thin so they cool down fast and aren’t heavy. I got a chipped glass countertop for $20 but need to find a way to cut a circle out of it. I also tried making a precision shaped heat resistant mold by pouring refractory plaster into some CNCed wood, but it cracked and failed. I need to try a different way. Once I have my circle of glass and a precision mold, I can put my mirror in a local kiln to shape it, and then grind it into a 12" or 13" mirror. Then, of course, I have to build a telescope around it!

Observing amongst the salt thieves

hillexed@email.com (hillexed) — Wed, 03 Jan 2024 04:30:22 +0000

I drove out to what Google maps said was a park and then saw a sign saying “state property no trespassing” and a second sign saying “don’t steal road salt or gravel from here, if we catch you you’ll be prosecuted”. I guess that’s what some silly rural fellows get up to in their free time? So I parked right in front of the entrance off the side of the road, since it was pretty dark. Eventually someone did drive up in a truck and ask if I was lost before driving in and then out a few minutes later (to I imagine check for salt thieves)!

Astronomy: it was so dark. I could see mountains and trees silhouetted against the sky once my dark vision adapted. I could see a naked eye slight fuzzy patch, point my scope, and it was M44! Lots of stars in one place. Everything had a slight bloom to it, like the sky some haze, and eventually a high cloud came in and reduced visibility to “only” a bortle 6 or so. Wow.

I also saw a dim fuzzy near Auriga - maybe M37? Pleiades looked as good as ever. Jupiter had four nice moons.

The pifinder didn’t get a GPS lock, and then shut down from lack of battery pretty quickly. I think the battery pack might be having more issues from when the scope fell off the mount a week ago?

The galaxies are aligned!!

hillexed@email.com (hillexed) — Tue, 02 Jan 2024 00:11:06 +0000

This is the best photo I’ve made of the Andromeda galaxy!

It’s my second time stacking pictures to bring out faint details. You’re looking at 20 phone pics taken through my 3D printed telescope, each 1/2s exposure so the Earth didn’t rotate as much during the photo and smear the stars. I tried the program deepskystacker, but it failed to stack my pictures (maybe my phone camera’s pics had too much noise?) so over 3 days I wrote some custom python code using astropy and astroalign to stack them myself!

Look at all those dim stars! That’s so many stars! In the bottom right that slightly fuzzy star is actually the core of M31’s satellite galaxy M110! I was hoping to see some dust lines or spiral arms, but I guess I’ll need more data for that.

THE STARS ARE ALIGNED

hillexed@email.com (hillexed) — Fri, 29 Dec 2023 21:18:14 +0000

BEHOLD, MY FIRST STACKED ASTROPHOTOGRAPHY IMAGE!

This combines 20 1/2sec exposures taken with my phone through my 3D printed 4.5" telescope. You can see stars down to magnitude 12!! This is so cool.

Turns out I didn’t see a galaxy in my photos because… I was looking at photos of a non-galaxy. This is the ring nebula, M57. Oops.

The stars are not aligned

hillexed@email.com (hillexed) — Fri, 29 Dec 2023 17:50:51 +0000

I took many 1/2s exposures of the galaxy M33 but it wasn’t visible. I’m trying to combine them in python to see if together it can simulate a long exposure. It’s not going too well

(The lines are from stars moving as the earth rotates!)

Cracked a Black Hole

hillexed@email.com (hillexed) — Sat, 16 Dec 2023 00:40:11 +0000

Poured some cement into my CNCed mold! Very fitting that my experimental telescope making technique involves a black hole

I noticed my mold had warped before putting cement on it - maybe from the polyurethane spray I used, or from waiting a month in between cncing and pouring? I put some weights on it because of the warping, walked away… And the next day I found my wood split down the middle of a boundary between planks. Oh well

hillexed@email.com (hillexed) — Thu, 14 Dec 2023 22:37:55 +0000

One of my telescope shenanigans is trying to make a meniscus mirror by slumping glass in a kiln. To do that, I need to make a precisely shaped form that can withstand kiln temperatures. I CNCed a wood mold, and next I need to put some furnace cement into that wooden mold so it becomes the right cement shape. I’ve never used furnace cement before. Wish me luck

Those galaxies are just too darn faint

hillexed@email.com (hillexed) — Wed, 13 Dec 2023 05:35:50 +0000

There’s a galaxy (named M33) in most of this picture

but it’s so faint I couldn’t see it by eye or by camera :(

The pifinder helped me find this place, so I took like 30 exposures in the hopes that I can stack them and see it

Grabbing Gears

hillexed@email.com (hillexed) — Tue, 12 Dec 2023 19:22:32 +0000

I found a design by d.revan to motorize a telescope using GT2 timing belts and it’s cool. He uses:

GT2 belts and pulleys instead of gears
608-2RS skateboard bearings to let the shafts spin (smart!)
a giant mega printed gear attached to wood for the final gear stage
4:1, 4:1, then 15:1 gear stages

…but I tried seeing how much the parts would be on AliExpress, and it was surprisingly hard to find gears with the right 8mm bore and wider than normal 10mm belt width. And the shipping added up - you’d see a $1 part plus $4 shipping. And buying two different parts from the same seller was twice the shipping! Just the tiny metal rods were $3 with $15 shipping, let alone gears!

So instead I asked around the synthesizer community, and found robotdigg.com, which is a janky 3D printer china company with a slightly hanky website but sells GT2 belts and pulleys for very cheap. And thankfully it all ships from one place, so only one shipping fee! I was able to buy the parts for one gearbox for $25!!!

So $50 later, I bought enough for two gearboxes, one per axle.

Now to CAD it up! D.revan’s design is slightly different from mine because I’m using smaller pulleys, but the idea seems similar enough that I think it’ll work. If I can motorize my telescope for $50 that’ll be incredible.

Oops, pifinder actually wasn't done

hillexed@email.com (hillexed) — Sun, 10 Dec 2023 17:07:33 +0000

Oops. NOW my Sliced PiFinder is done.

The original PiFinder uses a $50 USB GPS module. To avoid spending $50, I wrote some code to fake a GPS. Eventually the dev brickbots switched PiFinders to a $10 solderable GPS module that uses UART instead of USB, and $10 felt reasonable, so I bought one and soldered it in. I thought I was done and that all I had to do was edit the software to remove my fake GPS code and use the regular GPS code! I was wrong - the GPS didn’t connect.

My Sliced PiFinder uses a raspberry pi 3 but the PiFinder was designed for a raspberry pi 4. I thought the only difference was CPU speed, but there’s more: the instructions asked me to solder the GPS to two pins which couldn’t speak UART on a pi 3 but could on a pi 4. Oops.

The Pi 3 has only one pair of GPIO pins that can speak UART: 14 and 15. By default it’s used for a “linux serial console”, but you can configure it for custom use (like listening to GPS) by adding enable_uart=1 in /boot/config.txt.

So I desoldered my GPS, attached it to pins 14 and 15 with some truly janky soldering (pic above), fixed a “NMEA unknown msg” problem by turning off echo according to comments here, edited gpsd.conf, and now GPS is finally working.

PIFINDER: COMPLETE!
...for real this time

Pifinder: complete!

hillexed@email.com (hillexed) — Sat, 09 Dec 2023 20:13:06 +0000

I’m building the Sliced PiFinder, a device to help my telescope find things! Previously, I tried using a cheaper IMU but gave up and bought a $30 fancy chip.

Originally I didn’t want to buy a $50 GPS USB stick but the PiFinder creator found a $10 solderable GPS unit for a v2, so I bought one and soldered it in.

After soldering in the GPS chip, dubiously electrical taping it in place, dropping it and cracking my 3D printed parts, printing new parts, using a soldering iron to remove heat inserts out of the old parts so I can place them into the new parts, printing the case, discovering the case wasn’t designed for my battery and blocks access to the on/off switch and USB ports, melting holes in case with soldering iron, putting on a cover plate over the screen and LEDs… I finally put it on my telescope!

PIFINDER: COMPLETE!

Testing on the telescope went well. The printed dovetail mount was slightly too thin and sagged slightly under gravity, but the field of view was wide enough it wasn’t a problem. And even when I couldn’t see any stars through my finderscope, the pifinder’s camera still got a lock.

Eventually I tried using the find-objects feature, and it successfully pointed me towards M31! I tried pifinding to M33, but I didn’t see it by eye. Maybe the streetlight 30 feet away was just too bright? Perhaps it’ll show up on a long exposure phone cam.

Total cost, including the pi and battery pack I scavenged for free: $118. Much better than $550!

hillexed@email.com (hillexed) — Thu, 07 Dec 2023 04:59:55 +0000

Perseus double cluster

Equipment used: my 3D printed 4.5" telescope, custom mount, Pico pic-taker button, my phone

I have a raw version of this where some stars are noticeably blue or orange but it has more camera noise than this

hillexed@email.com (hillexed) — Wed, 06 Dec 2023 05:50:35 +0000

hey rememer that pinfinder I made a longpost about? I dropped it. 3d printed parts cracked and have to reprint em

Pifinder Perils

hillexed@email.com (hillexed) — Mon, 04 Dec 2023 15:29:08 +0000

I’m building a PiFinder! It uses a camera to take pictures of the sky, connected to a raspberry pi which uses a database of stars to tell you where in the sky your telescope is pointing. But a PiFinder is $550 new. A stock pifinder uses the newest and most expensive options for pis and cameras, and when I looked at the parts list, I thought: I can build something similar for a fifth of the price! And so began the quest to create the Sliced PiFinder: a DIY PiFinder made from an used raspberry pi for a small slice of the cost. Previously, I bought a $33 camera and 3D printed a custom enclosure for it.

On the one hand, if I can successfully modify this project so I don’t need expensive parts, I save $400! On the other hand, my hubris has led to consequences.

This is a longpost about those consequences and the shenanigans I found to work around them.

(None of this would be possible if the pifinder wasn’t open source, so thanks to the creator brickbots, who has given me tons of help!)

Problem 1: Camera FoV

We live in a world where I use software written by a space agency across the ocean for free. What a world we live in.

Once the PiFinder takes a picture of the sky, it needs to use a database of stars to figure out where in the sky the camera was looking when the picture was taken. That process is called “plate solving” because we used to take star pictures on photographic plates.

How do you plate solve? Amazingly, that’s the easy part! The European Space Agency released an open source python library called “tetra3” that does it all for you! All you need to know is… hmmmm… the exact position of every single star in the sky your camera can see.

Amazingly, you can just download a database of every single bright star in the sky! Astronomers have been making star catalogs for thousands of years, including launching space telescopes like Gaia devoted to doing nothing but star cataloging. The PiFinder software downloads a database of stars automatically!

However, for tetra3 to plate solve, it needs to know your camera’s field of view (FoV), so it knows how far apart various stars will look to the camera. Different lenses change FoV. The PiFinder software expects you to have a Raspberry pi HQ Camera ($50) and 25mm lens ($20), which has a FoV of 10.2. I’m using an IMX462 camera ($33) and 12mm lens from a different camera ($10), so how do I figure out my FoV?

One option is math. I tried pointing the camera at Orion, taking a picture of the screen, measuring the distance in pixels between stars in orion, and comparing that to the known angular distance. That gave me an estimate of 15 degrees of FoV. Unfortunately, I didn’t know at the time that the the picture was wider than the screen.

Then I realized I was using a raspberry pi and could access the raw hardware. I SSHed in, used the command line to take a picture, then downloaded the raw .png. Now I knew what the software was looking at. I uploaded it to nova.astrometry.net, which plate solved my image and told me not just the exact coordinates of where I was looking (and the constellation!) but also the image’s FoV.

(Also, just for fun, I used it to take my first ever long exposure shot of 30s, and was blown away by the number of stars visible. Image above!)

Another complication: images are 2D. Does tetra3’s FoV input mean horizontal FoV or vertical FoV or diagonal FoV? I didn’t see it anywhere in the documentation. (It was horizontal)

In the end, my new IMX462 camera with 12mm lens had about 26deg of horizontal FoV. I could buy a new lens to reduce that down to the PiFinder software’s expected 10.2 degrees… but instead brickbots helped me write software to crop the image down to 10.2 degrees. Now plate solving works!

Problem 2: Alt/Az

Let’s say you want to see the andromeda galaxy. How do you know if it’s is visible in your night sky right now or not? Should you look north or east or south to find it?

Since the earth is a sphere, “down” changes from place to place. Since north is perpendicular to down, that means north/south/east/west look at different parts of the sky based on your place on earth.

Thankfully, if you know your location on Earth and the time, since Earth rotates once per 24 hours you can do some math to figure out where to look relative to your local north and down directions. Those coordinates are called “altitude” and “azimuth”.

The PiFinder gets your location on Earth and the time from a $50 GPS USB dongle. But that’s expensive and my phone already has a GPS. If I can modify the software so I can enter my coordinates and the time, I save $50.

It turns out parts of the software don’t work unless it knows your coordinates, but instead of showing an error message some features simply do nothing. I edited the software’s config file to add my GPS coordinates… but coordinates in the config file are never used, since all previous pifinders have had GPSes and got coordinates from there. So I had to modify the existing “GPS_fake.py” (which previously did nothing) to actually send fake GPS and time messages.

And because computing altitude and azimuth isn’t complicated enough: the raspberry pi has no internal clock, so every time it starts up the time is wrong (unless it can connect to wifi and download the time using NTP). This works for me at home, but if I want to take this to dark places, I eventually need to program a screen that lets you enter the coordinates and time on the device itself!

Problem 3: Gyroscope Fusion Failures

If you move a telescope with a PiFinder left or right, changing your azimuth, its screen should move the star display left or right quickly. That’s hard for a few reasons.

First, the Pifinder needs long exposures to capture dim stars. If it moves during a long exposure, the image it captures will have stars that look like smeared lines instead of dots and it can’t plate solve. That means if you bump the device, there’s a few seconds of delay before the camera can lock on to the sky again.

To get quicker feedback during unsuccessful pictures, the PiFinder uses an intertial measurement unit (IMU) chip, which combines a gyroscope (measures changes in angle), an accelerometer (measures acceleration, including gravity), and a magnetometer (measures Earth’s magnetic field). The PiFinder uses a fancy $30 IMU chip called the BNO055, which has a tiny processor that computes the chip’s current altitude and azimuth 100+ times a second.

However, I scavenged an LSM6DS3TR-C + LIS3MDL IMU from a different project with the exact same sensors: magnetometer, gyroscope, accelerometer. Surely, I thought, I could write some code to compute altitude/azimuth from those sensors and save $30!

Combining data from the 3 sensors is called “sensor fusion”. It’s incredibly hard. Thankfully, I’m not the first to study sensor fusion (drone builders want it too) and there’s two main sensor fusion algorithms which already exist, named mahony and madgwick. Adafruit has an AHRS library which implements both… in C++, but I was using python. Eventually I found an implementation of both and downloaded it, loaded in my sensor readings… and spinning the device 90 degrees didn’t change the output by 90 degrees. Why?

Problem #1: Axis remapping!

In my PiFinder, the IMU is oriented so the sensor’s +Y direction is the one the camera is pointing towards, but I had a hard time figuring that out because the chip was buried inside a circuit board and the code has many options to switch coordinates based on how a PiFinder is mounted on a telescope.

Problem #2: Units

The Madgwick filter expected the gyro’s inputs to be in degrees/second when they were in radians per second. No problem, I can just multiply them by 180/pi. Then I took a look at the Madgwick code, and it requests degrees because it multiplies the numbers by pi/180 to turn them back into radians. Aaargh.

Problem #3: Calibration!

Magnetometers and gyroscopes will drift - if they output a range, say, 4 units wide, instead of getting sensor readings from -2 to 2, your actual readings might be shifted so you read values from 0.5 to 3.5. Magnetometer readings shift in a similar way for a different reason: Earth’s magnetic field is different everywhere. You can take many many readings and average them to find the true zero point, then subtract that from all future readings to make zero the middle point.

For a gyroscope, those readings need to be taken while the gyroscope isn’t moving. I realized I could use the accelerometer for that - if the accelerometer’s gravity direction isn’t changing, I can use that to know I’m not moving and grab some gyroscope readings. Once you integrate the gyroscope over time, it looked pretty stable, with only around 0.3 degrees of error.

For a magnetometer, spin it around as much as possible, and the unchanging 3D vector of earth’s magnetic field, as measured by the magnetometer’s 3 axes, should trace out a sphere! Then you can use the center of that sphere as your reference zero point.

Engineers did what engineers do in overly specific fields and made up magnetometer calibration number jargon. The values for center of the sphere (which should be at (0,0,0) but usually isn’t) are called the “hard-iron offsets”. But you can get fancier: if you have a magnet (or some other electrical device) near the magnetometer, it might make its own magnetic field and skew one axis at a time, and that skew is called the “soft-iron offset”. They’re just coefficients in a calibration matrix!

The simplest way to calibrate a magnetometer is to ignore soft-iron entirely, keep track of maximum and minimum values in all 3 magnetometer axes as you spin the device around every which way, assume the hard-iron offset is the average of the max and min, and subtract that calibrated value every time you read from the magnetometer in the future.

I had to do all that calibration myself. The output still didn’t work very well. The sensor fusion algorithm’s output reported altitude and azimuth axes which didn’t measure down correctly. Why wasn’t it working?

Problem #4: EVERY SINGLE MAGDWICK FILTER EVER WAS WRONG 3 YEARS AGO

According to this research by someone named Mark Uckermann from a British bike GPS startup,, apparently almost every Magdwick library has a subtle bug in the code compared to the original paper, invisible if you use a small enough “beta” parameter. Oops. That was 3 years ago, and I think my library has fixed it? But I can’t tell since the variable names are slightly different.

Problem #5: Speed

The sensor fusion algorithms involve lots of math. Doing these calculations myself in python (along with the camera and plate solving and everything else the pifinder was doing) took up so much time that on my pi 3, the IMU code was only able to read values from the sensors around 12 times a second. The sensors were updating 104 times a second. That meant I was losing tons of info, including updates of “how much the angle shifted since the last time you checked” data from the gyroscope. I could configure the sensors to slower speed… But the gyroscope+accelerometer could only go to discrete values like 12.5 Hz while the magnetometer could only go to different discrete values, like 10 Hz. Aargh.

After all that… turning my device 90 degrees still wouldn’t make the azimuth output change by 90 degrees. Aaaaargh.

But wait. If my sensor fusion algorithm wasn’t working… maybe it was a problem in the sensor fusion algorithm code? The accelerometer tells me the direction of gravity - maybe I can compute azimuth. The IMU can tell me where magnetic north is. If I measure both… maybe there’s a way to compute altitude and azimuth directly? Maybe, just maybe, if I sat down and did a ton of galaxy brain math, I’d be able to use my own scavenged IMU instead of buying the pifinder’s recommended BNO055 for $30!

Oh yeah. The BNO055 chip is $30. It does calibration for you.

I gave up, desoldered my old IMU, bought a BNO055, and soldered it in.

It better work.

Pifinder is almost pifinding!

hillexed@email.com (hillexed) — Sun, 26 Nov 2023 01:17:31 +0000

The pifinder is a tool to help you aim a telescope. It uses a camera to take pictures of the sky, connected to a raspberry pi which uses a database of stars to tell you where in the sky your telescope is pointing. Then, if you want to find a specific object, it tells you what direction to move your telescope in.

I’m building a janky pifinder with some nonstandard parts: instead of a $60 raspi 4 and $50 HQ camera and $25 lens and $30 IMU, I’m using a $10 lens, a secondhand raspi 3, and a cheap “pi camera module v1” from 2013 that was attached to the raspi, and a $20 IMU with unpronounceable name I found lying around in a drawer of sensors. Previously, it didn’t see any stars, so I bought a better camera, an IMX462.

Turns out the two cameras are different sizes, so the enclosure for my first camera didn’t fit the second one. The IMX462 has some holes in it that fit M2 screws, but my local hardware store only sells metric screws in specific lengths like 12mm, while the pifinder wants 8mm screws.

So I designed a 3D printed enclosure for the new camera, and printed it in orange! It was too big; the screws I bought weren’t long enough.

So I designed another 3D printed enclosure for the new camera, and printed it in blue! It did fit!

Then I put it together, and took it outside… and the display showed that the camera is successfully seeing stars! This camera is definitely much better and more sensitive than the old one. Plus, because it’s connected to a raspberry pi, I can just SSH in and ask it to take pictures to see the raw camera output.

The pifinder still isn’t plate solving properly - it should be trying to compute where in the sky it’s looking and it isn’t. It could be the lens/camera combo means this image is a different FoV than it expects, meaning it’s seeing more or less of the sky and the stars are closer or farther apart than it expects. We’ll see.

Maybe this camera will make the pifinder see stars

hillexed@email.com (hillexed) — Thu, 23 Nov 2023 17:42:35 +0000

Pifinder update: the new $33 camera arrived! But it’s slightly bigger than my previous camera, so I had to design a new part to hold it. While the pifinder was designed with M2.5 screws, this camera only accepts smaller M2 screws. I had to go to the hardware store yet again.

The pifinder is designed for a raspberry pi HQ camera ($50). Originally instead I wanted to use a pi camera module V1 ($10, from 2013) because I recycled it from another project, but it didn’t work too well. I designed a custom printed mount for that. This innomaker IMX462 camera is $33 and has a big pixel size, which means it’s more sensitive to light (good for astronomy!). This means I’m on round 2 of designing custom holders for cameras the pifinder wasn’t designed for!

Circle cutting: Failure

hillexed@email.com (hillexed) — Tue, 21 Nov 2023 19:26:27 +0000

The scoring tool cut a circle fine, but then the next step was supposed to be “press it from the rear side and the crack will deepen”, and it just… Didn’t. Maybe this glass is just too thick? I still have half my countertop left, I need a new way to cut 1/2" glass

hillexed@email.com (hillexed) — Thu, 16 Nov 2023 05:04:39 +0000

I built enough of a pifinder to test it! This pifinder is using a pi camera v1 because I salvaged an old secondhand pi project… and it was able to see one or two stars, but not much more because of light pollution. Not enough to see the stars and plate solve to see where in the sky it was aimed at. I ordered a better $33 camera, maybe that’ll work

My first Orion nebula!

hillexed@email.com (hillexed) — Sat, 11 Nov 2023 04:00:21 +0000

The first picture is at 36x zoom, the second one at 150x zoom. The more zoomed in one is fainter because the same light is spread across more area, but you can see the four trapezium stars as four separate streaks!

This was a very frustrating night for photography because finding things in a big sky is hard. Light pollution made it hard to see M31, I couldn’t find M33, and then finally clouds rolled in and it became a race against time to photograph the Orion nebula M42. I really need a motorized mount or other device to help me find things.

hillexed@email.com (hillexed) — Mon, 06 Nov 2023 04:59:02 +0000

Today in observing I:

Tried to get a photo of the ring nebula through the 6mm lens. Realized my finderscope was misaligned, couldn’t get it aligned in time before it sank below a tree.
Tried to get a photo of Jupiter, succeeded! Tried out a new phone app named skeyecam that lets you take many many photos with the same settings, for some reason it made Jupiter green.
Then I tried taking a photo of m42 through the 6mm lens, but I couldn’t get anything on camera. Then I messed with my setup to see if it was aligned, only saw a star and not a nebula, tried to take off, then dropped and broke my openocular, and only after all that discovered I was aiming at the wrong group of 3 stars in Orion and was actually aiming at alnilan instead. Oops

hillexed@email.com (hillexed) — Fri, 03 Nov 2023 00:29:16 +0000

I found an used glass sheet 1/2" thick at a used furniture store! Now I can cut glass circles to make mirror blanks out of it!

A friendly older guy helped me use his shopbot

hillexed@email.com (hillexed) — Tue, 24 Oct 2023 17:57:56 +0000

I’m so excited. Making really big really thin meniscus mirrors has officially gone from cool idea to possible

A friendly older guy helped me use his shopbot

hillexed@email.com (hillexed) — Tue, 24 Oct 2023 17:57:56 +0000

Unable to find image IMG_20231024_132057346_HDR_1.jpg

Unable to find image IMG_20231024_134541085_HDR_1.jpg

I’m so excited. Making really big really thin meniscus mirrors has officially gone from cool idea to possible

Adjusting for mirror size

hillexed@email.com (hillexed) — Mon, 23 Oct 2023 14:55:28 +0000

I got a mirror that’s 2.3 inches wide. However, several weeks ago, I printed a mirror holder that was designed to fit a mirror 2.46 inches wide. Sure, I could just use the bigger holder, but the bigger the secondary holder the more light it blocks from reaching your mirror. Is it worth a smaller secondary mirror holder that will block half a square inch less light? Yes, I decided. So I opened up the model and slightly scaled it down. It was a bit tricky because I didn’t want to scale down the nut and screw holes, but I counted the sizes and scaled down the outside vertices in blender. I feel bad about wasting my previous print, but plastic is so cheap I’ll only feel a little bad about it.

Secondary mirror get!

hillexed@email.com (hillexed) — Thu, 19 Oct 2023 20:19:03 +0000

Visiting family in another city, and turns out there’s an astronomy store named La Maison De L’Astronomie (The Astronomy House)! I called them up to see if they had some secondary mirrors in stock… And they did! They had so many big telescopes, way more small refractors than I was expecting, a huge cabinet of binoculars, and some absolutely massive tripod mounts.

The bbastro calculator says the most optimal uniform illumination secondary mirror size is 62.5mm, which is $60 online (before tax and shipping!) but anything above 50mm will be better the bigger you go. I got this 58mm from them for $40 after tax and I am the shipping. An absolute steal. Just like the space shuttle and canadarm, my 8" Leavitt telescope will be mostly American with one vital Canadian part.

M13, the great hercules cluster

hillexed@email.com (hillexed) — Wed, 18 Oct 2023 14:08:06 +0000

I found this with the help of an amateur with a huge telescope and laser pointer so bright it looked like a line pointing into the sky. Looked like a dim circular smudge to the eye. Picture taken with the 25mm eyepiece, so I bet I could get an even better picture through the more zoomed in 6mm

Annular eclipse today!

hillexed@email.com (hillexed) — Sat, 14 Oct 2023 14:21:02 +0000

Reminder to North and South Americans, there’s an eclipse today! https://www.timeanddate.com/eclipse/in/usa?iso=20231014 DON’T LOOK AT THE SUN WITHOUT SPECIAL ECLIPSE GLASSES, EVEN DURING A PARTIAL ECLIPSE! If you don’t have one, take a piece of paper, punch a hole in it, and look at the shadow. Trees or colanders with many holes will make very cool shadows!

hillexed@email.com (hillexed) — Fri, 13 Oct 2023 17:14:11 +0000

I was cleaning out my boxes when I discovered @Beasmeeply not only sent me a primary mirror, but also a 46mm secondary mirror! Wow! Thank you!

Secondary mirror sizes are weird; apparently having a secondary mirror too small means the outer part of your view is slightly dimmer. I did lots of research into what kind of secondary mirror to use. This calculator says a 62.5mm will give me the most even illumination across the field, but that’s for super huge eyepieces so a slightly smaller one should be fine. I’d love to find a 60mm or 55mm one but I guess now I don’t have to look nearly as hard

I GOT THE RING NEBULA

hillexed@email.com (hillexed) — Wed, 11 Oct 2023 15:22:43 +0000

YEAHHHHH FINALLY

IT’S SO SMALL

FINDERSCOPE HELPED

hillexed@email.com (hillexed) — Wed, 11 Oct 2023 14:30:55 +0000

Got this used finderscope from the local astro club, designed and printed a mount for it. Took two revisions

Telescope update:

hillexed@email.com (hillexed) — Mon, 09 Oct 2023 18:13:12 +0000

I put a telescope on my telescope

I tested my donated mirror

hillexed@email.com (hillexed) — Sun, 08 Oct 2023 15:06:35 +0000

WOW! That’s a good mirror!

Previously, @BeasMeeply graciously donated me an 8" mirror. The problem: I don’t know what stage of mirror making I have to do.

So I talked to a local astronomy club. I got to use a spherometer to measure the curve - same curvature all around, accurate to within 0.0002 inches. I put it in a foucalt tester, and after lots of help and fiddling I got some pictures of the result! The fact that it’s transparent enough that I can see through the glass and get a result in the foucalt test tells me @BeasMeeply got through grinding and polishing, and now I can skip to making a pitch lap.

How do I interpret this? I don’t know yet. But someday I will

I tested my donated mirror

hillexed@email.com (hillexed) — Sun, 08 Oct 2023 15:06:35 +0000

Unable to find image IMG_20231007_152202849_smol.png

WOW! That’s a good mirror!

Previously, @BeasMeeply graciously donated me an 8" mirror. The problem: I don’t know what stage of mirror making I have to do.

How do I interpret this? I don’t know yet. But someday I will

Printing for the 8\" telescope has begun

hillexed@email.com (hillexed) — Sun, 01 Oct 2023 20:51:59 +0000

I’m not the first person to build a Hadley 3D printed telescope and then want something bigger. There are two 3D printed 8" telescope designs: the “Bradley” is only available by DMing someone on a discord server, and it’s designed for a printer big enough to fit an 8" circle. I don’t have a printer that big! Thankfully, there’s also the “Leavitt”, a design which splits 8" circles into three pieces so they can fit on a normal Ender-sized printer.

I’ve begun printing all the pieces! It’s a lot of printing; at least 100 print hours. I bought one roll of blue and one roll of black filament. In a telescope, you want to make inside surfaces black so they don’t reflect stray light and brighten the view. The tube parts are so tall and thick, and there’s so many near the path of light, that I used up all my black filament halfway through all the tube sections. I’ll need to print the rest in blue but spray paint them black.

Interestingly, there’s also a remix called the Leavitt Lite which edits the Leavitt to use metric screws/bolts and makes many parts thinner and lighter. One interesting change the Lite made is using a bigger secondary mirror than the original for better illumination; I’ve downloaded the file from the Leavitt Lite and edited it to use #10 nuts so I can use that change too. Now I need to find or buy a 62.5mm secondary mirror.

One other thing to think about: metal rods. I have three 36in thin-walled rods in my wobbly first mount. I could reuse those for my Leavitt for free. My donated mirror’s focal length is 43in right now, meaning I’d need to place the eyepiece 43in away from the mirror, and that won’t fit on those 36in rods.

One option is I could grind the mirror and reduce the focal length to 36in, giving me an 8" f/4.5 mirror, and then use my existing rods. Same focal length means same magnification… but I like the idea of having a longer focal length and therefore more magnification than my previous telescope. Another option: I could either leave the focal length alone (or increase it), but then I’d need to buy 3 new longer 1/2" tubes for an extra $40 or so.

Paint, rods, and secondary mirrors. Many things to think about.

Printing for the 8\" telescope has begun

hillexed@email.com (hillexed) — Sun, 01 Oct 2023 20:51:59 +0000

Paint, rods, and secondary mirrors. Many things to think about.

An incredibly generous donation

hillexed@email.com (hillexed) — Thu, 28 Sep 2023 15:12:15 +0000

A while ago I was musing about ways to get better space pics, and @BeasMeeply incredibly generously offered to donate an 8" blank sitting in his closet for 10 years if I paid for shipping! Thank you so much! I’ve got it now and it looks incredibly clear. He also sent a metal 1.25" focuser, which was an extra addition I wasn’t expecting.

It’s like I was standing on the edge of an abyss deciding whether not to jump in, and then someone threw a mirror at my back. I guess I’m falling in! Now I need to figure out how to measure its curvature, polish it using a pitch lap, and then aluminize it. It’s a lot of work… but it’ll be worth it. I’m excited!

An incredibly generous donation

hillexed@email.com (hillexed) — Thu, 28 Sep 2023 15:12:15 +0000

Unable to find image IMG_20230919_223643192_1.jpg

hillexed@email.com (hillexed) — Mon, 25 Sep 2023 04:59:14 +0000

I took out a relative’s 8x42 binoculars today. Saw M7 as a faint sparkling of stars, but Andromeda was a much dimmer smudge compared to my telescope that if I hadn’t seen it before I wouldn’t have been able to spot. It I was able to follow a satellite though! And I got to show off Mizar and Alcor to others!

Sadly the moon was out and it was so bright it was casting shadows, stopping me from seeing any nebulae like I was hoping. Grr, how dare the moon be out there.

ANDROMEDA GALAXY CORE

hillexed@email.com (hillexed) — Wed, 20 Sep 2023 14:36:37 +0000

I got to see it with my own eyes for the first time!! It looked like a gray oval-ish but slightly pointy smudge. But it’s a cool smudge in the sky!!

This is a phone pic with a 2s exposure and ISO cranked up to it’s max, 3200. The rainbow snow is random noise from my sensor magnified by turning the sensitivity up very high. You can even see M32 as a slightly fuzzy star almost directly above Andromeda’s core!

I’m really glad I found it. Aiming the telescope was annoying, but rubber bands on the mount helped it keep its aim. I guess that means when I tried and failed to find the ring nebula, it’s just too faint to see in my light polluted sky.

I took many 1s exposures, too. I need to figure out how to stack them and use astronomy software but that’s a process for another time

Dumb telescope idea: liquid mirrors

hillexed@email.com (hillexed) — Mon, 18 Sep 2023 17:27:56 +0000

I’ve been thinking about https://www.space.com/liquid-telescope-construction-in-space-ax-1 . I wonder if you could take a circular baking tray, spin it on a pottery wheel, and then pour SLA 3D printing resin into it so the resin would form a liquid mirror, then shine UV light on it to cure it and make a telescope mirror

I think I figured out why the pifinder recommends you use a beefy raspberry pi 3.

hillexed@email.com (hillexed) — Mon, 11 Sep 2023 04:28:35 +0000

The code’s open source, so I looked through it. Turns out every time it tries to draw a map of the sky, it loops through every single star in the sky down to magnitude 7.5. Every frame. Sigh

At least it’s open source, so I can probably add some caching and massively speed it up.

oh yeah, I'm making a pifinder

hillexed@email.com (hillexed) — Sun, 10 Sep 2023 01:01:27 +0000

The sky is too big. I went out telescoping looking for M13 and there were too many stars and it’s too big and I don’t know where I’m aimed. When you’re so zoomed in, there’s so many stars you don’t know where you are or where to look.

Enter the pifinder! I’m going to make one.

I went to my local makerspace, known for having too many donated bits and bobs they actively try to get rid of (what a wonderful problem to have). I found an old project involving a pi, a decade old pi camera v1, and a battery pack that I was allowed to take and cannibalize on the condition that I show off the final product. Thank you!

Pifinders are normally sold for $600 fully assembled because that’s what similar units cost, because the astronomy market is nuts. The creator sells cheaper DIY kits, thankfully. I’ll be able to save even more DIYing because unlike a normal pifinder, my janky secondhand pifinder will have:

A different battery No GPS module (those are $50!!!), instead I’ll mod the code to input my GPS coordinates from the GPS in my phone A slightly different IMU chip A much smaller and older camera, compared to the $50 raspberry pi HD camera twice the size with a convenient screw-on lens interface

Surely none of this will hassle me and I can save money without any consequences!

Progress so far

First, I needed to buy a lens for my camera!

Looking at what’s written on the ribbon cable of the camera, I have a pi camera v1. According to https://www.raspberrypi.com/documentation/accessories/camera.html the sensor is 3.76 × 2.74 mm big, so I can use FoV = 2 * math.atan((2.74/2)/22) * 180/math.pi

Turns out lenses are tricky. The official pifinder uses a 25mm lens. Why? It uses the “tetra3” library by the European Space Agency to go from pictures of stars to figure out where in space you are. You also need to pass in the FoV of the image. Pifinder expects a 10.2 degree FoV, and it has a database that works with FoVs from 10 to 20 degrees. I looked up the formula, and my smaller camera’s smaller sensor size would give me a TINY FoV with the same 25mm lens. Oops! I ended up buying a 12mm lens for a 13deg focal length.

Next, getting it on the board. I lost one of two screws holding it on. Following this video, I printed a M12 lens adapter, which had screw holes too small but otherwise fit perfectly. However, I also needed to remove the existing lens. There are printed tools for that, but many are a + shape for the pi camera v2, and the pi camera v1’s lens is a different shape with 3 spokes. Eventually I found one, printed it, unscrewed it, finished the job with pliers, and put on the lens. It does seem more zoomed!

Configuring the raspberry pi was a bit annoying. Running “libcamera-vid” shows a video, but for 5 seconds. You need to pass “-t 0” to get the preview to keep running. And after all that… when I turned off the lights, the video was pitch black. I need to figure out how to increase the exposure time if I want it to see stars.

I’ve spent $30 on electronics and the lens so far. I joined the pifinder discord and the creator was incredibly nice, willing to bundle multiple DIY components if I bought them at once instead of charging double shipping from the webstore price. PCBs and some hardware and LEDs will be arriving soon for another $15. The case is 3D printed, which will cost $little in filament. I have a lot of code work ahead of me to save that money, but it’ll be very cool if I can pull this off, with the help of that amazing makerspace.

Possible telescope upgrades

hillexed@email.com (hillexed) — Wed, 06 Sep 2023 20:39:46 +0000

My telescope is pretty good, but now that I have one I’ve been thinking about how to get even better. Here’s what I’ve been thinking about:

#1: Bigger telescope.

Bigger telescopes with bigger mirrors both capture more light and allow you to resolve tinier details. I saw someone made a “Leavitt” telescope, also 3D printed, designed to fit an 8" mirror! My telescope has a 4.5" mirror right now. It cost $30.

An 8" mirror, double the diameter, costs…

$360. Oof.

Another way to do it is to grind the mirror yourself. There’s a long tradition of amateur telescope makers doing that, but it takes lots of time grinding and polishing. The raw materials are still expensive-ish - an 8" circle of glass 1" thick is around $55, and there’s $50 extra of grits and powders you need. For making ultra-big mirrors, there’s some insane people who have started using kilns to take glass circles of normal plate glass and soften them over a mold, so they fall down and make a meniscus shape. It’s a rabbit hole I’ve already gone very very far down researching.

#2: Motorize the mount.

Stepper motors are decently cheap. With some gearboxes to reduce their speed, you could give your telescope the ability to automatically point itself at a target.

There are two kinds of mounts: Alt/Az and Equatorial. Alt/az involve one axis which rotates left/right along the ground, changing “azimuth”, and one axis which rotates up/down to change “altitude”, like a cannon. They’re the simplest to design, and someone has already made one for my Hadley telescope! But I’d need wood-cutting tools and $100 or so for just the wood alone, not to mention electronics.

Equatorial mounts are angled so one axis points towards the north pole, and rotates along the same axis that planet earth does to counteract the rotation of the planets. These are the ideal kind of mount. They’re much harder to design because they have to carry all the weight on those axes and geraboxes, and earth’s spin axis isn’t at all aligned with gravity.

Once you’ve set up motors, you can add “goto” functionality, which lets you press a button and watch as the telescope automatically figures out how to point at something. There’s software called “onstep” for that. I’d love goto someday!

Either type of motorized mount would also let me start taking long exposures to photograph very dim things out in space. An alt/az mount would let me take long exposures of up to a minute. Expose for too long with an alt/az mount and you’ll notice the thing you’re staring at starts rotating in your field of view, called “field rotation”. An equatorial mount doesn’t have that problem, but they’re expensive: a commercial entry-level one is around $700.

Surprisingly, there are people 3D printing decent equatorial mounts! Not many, though - it’s hard to design gearboxes without “backlash” to avoid tiny wobbles. Most are designed to hold DSLR cameras, not heavier telescopes.

#3: Pi-finder

https://www.pifinder.io/ is a neat raspberry pi powered device that looks at the sky and tells you where you’re pointed. It uses a high quality camera to take pictures of the sky, does some magic to figure out where you are, and then uses an accelerometer to track slight nudges. You still need to point and aim the telescope, but it would let me know where I’m aiming.

The most expensive components are a $50 high quality raspberry pi camera, a $20 lens, a $50 GPS USB stick, and an out-of-stock-everywhere raspberry pi 4. Ouch. But I do have a pi 1 lying around… maybe it’ll work? Plus, if I mod the software so I can input my coordinates from the GPS I carry around in my phone, I can save $100 or so.

Will I do any of these? I don’t know!

JUPITER BELTS JUPITER BELTS

hillexed@email.com (hillexed) — Thu, 31 Aug 2023 14:52:05 +0000

THE NEW TELESCOPE MIRROR IS GREAT

BEST $20 I’VE SPENT

Uploaded my telescope mount to Printables

hillexed@email.com (hillexed) — Sun, 27 Aug 2023 18:43:33 +0000

Now anyone can have a sturdy 3D printed telescope! https://www.printables.com/model/565412-hill-mount-for-hadley-telescope

On the Night of the True Enemy, I tried to find the ring nebula!

hillexed@email.com (hillexed) — Thu, 24 Aug 2023 23:28:28 +0000

For the record, past me was wrong. That’s not the ring nebula. There aren’t two bright stars aligned like that near the ring nebula. But that’s the prettiest Jupiter I’ve ever taken!

(Why is there a double saturn? I know why now!)

hillexed@email.com (hillexed) — Wed, 23 Aug 2023 03:24:01 +0000

Huh. I installed the front truss addon, I’m observing now and finally noticed the direction of the wobble. After a transient big vibration period of 3s or so it settles down… and stars look like two points right next to each other instead of a big glob of light. Progress! I looked at a low-altitude object… And the direction of the two stars was up and down. I think I’ve finally defeated side to side wobble only to find my true opponent was altitude wobble

Attacking vibration in Hill Mount v3

hillexed@email.com (hillexed) — Tue, 22 Aug 2023 16:56:49 +0000

Telescope mount v3 is pretty good, but it still wobbles slightly. If I touch or move the telescope, it wobbles a bit before settling down in 2-3 seconds. Before it settles down, Saturn turns into two images of Saturn next to each other.

I can use that picture and do the math to see how much it's wobbling:

https://www.timeanddate.com/astronomy/planets/distance says Saturn is 18.96" right now. That's a second of arc, 1/60 of a minute of arc, which is 1/60 of a degree. That means it wobbles 2 saturns * 18.96 arcseconds/saturn * 1/60 * 1/60 * 2π/360 radians/arcsecond = .0001838 radians. My telescope is about 1.75 feet long from the center point, so since sin θ = o/h, the tip of the telescope is moving side-to-side a distance of h * sin θ ≈ 1.5ft * .00009192 = 0.08403 mm in that picture. Wow, that's about one printed layer line.

How do I stop that?

Right now my telescope mount looks like this. It has a C-shaped top, so the telescope can rotate in and out.

It’s not a full truss - I can’t put a bar between the two tips of the C because that’s where the telescope goes. As a result, the tips of the C are only supported at one side, so they can move in and outwards. Experimentally, if I put pressure on the top part I can make the top part shift slightly in the xy plane.

I tried designing a tiny truss add-on to the front to help resist sideways force for mount v2:

I tried remaking it for mount v3! This time, here’s what mount v3 looks like. The rectangle in the center marks the telescope Bonk Zone, where I can’t put anything without hitting the telescope.

I printed it out, put it together, and noticed it was a very tight fit. Looks like I forgot to add tolerance to the hole sizes. I tried forcing it in anyways to see if it would work, and…

oh well. let’s see if it works once I reprint with bigger holes.

Oh my god there's so many stars in this galaxy

hillexed@email.com (hillexed) — Sun, 20 Aug 2023 06:27:52 +0000

I drove a few hours to visit friends and a bortle 4 sky. wow there were so many stars. You could barely make out an arc slightly lighter in the sky than the rest of it! The Milky Way! Through a telescope, there were much more stars than before and they just kept going and going if you moved the telescope. Saw the plediades too wow many bright stars near each other

No photos of nebulae, my goal, because my openocular malfunctioned and didn’t hold the phone to the camera well. No eye sightings of nebulae like m57 either because oh god the sky is so big and aiming is so hard and you look into the telescope and it’s full of stars and you’re lost

This photo is a 30 second timelapse at ISO 3200, the strongest it can go, from my phone camera placed on a concrete block. Edited to increase contrast and saturation. THAT’S A GALAXY

hillexed@email.com (hillexed) — Sat, 19 Aug 2023 22:26:14 +0000

Jupiter, four Galilean moons, and a blue star!

It could be improved if I was better collimated, but I’m happy I got all 4 of them with the high power eyepiece!

Brightnesses are cursed

hillexed@email.com (hillexed) — Tue, 15 Aug 2023 20:46:47 +0000

Today I learned that astronomers measure star brightnesses in the modern AB system, using (logarithmic) units of Janskys, where one Jansky is 10⁻²⁶ W⋅m⁻²⋅Hz⁻¹. Why is there the extremely cursed Hz⁻¹? Hz is already s⁻¹. Why is it like that. Astronomers, why is it like that. s^−1⁻¹ is just seconds

If you cancel all the units, you get… 1 Jy = 10⁻²⁶ kg m² s⁻³m⁻² s^−1⁻¹ = 10^-26 kg s⁻². …so we measure starlight in the same units as surface tension. If you really wanted to, you could talk about how bright a star is by how much its light would stretch a soap bubble.

Which 3D model takes less material to print?

hillexed@email.com (hillexed) — Sat, 12 Aug 2023 19:02:19 +0000

turns out it’s…

click to reveal

the first one?? 3D printers fill the inside of objects with really lightweight infill, and it's so light compared to 100% filled walls that it takes both less time and ends up being less weight than the fancy truss on the right. Which sucks because I spent such a long time designing that fancy truss only to be outdone by a brick.

I saw a meteor!

hillexed@email.com (hillexed) — Sat, 12 Aug 2023 04:30:41 +0000

I heard there was a meteor shower tomorrow! So naturally I went out today to see it. First I treated it like any other target and tried to find a good gap in the trees to see Perseus, but then I read online that meteors appear in the full sky, not just at the radiant.

So I lay down on the sidewalk, and 30 minutes later… zoop!

The meteor was super underwhelming compared to the time I spent waiting. A blink-and-you’ll miss it streak of a little bit of light. I wish they had a sound like in fiction.

Only bad space today, said the sky

hillexed@email.com (hillexed) — Mon, 07 Aug 2023 14:17:42 +0000

Everything was very blurry today and I kept wondering why focusing didn’t seem to fix it before realizing it was cloudy.

My custom Pico Bluetooth picture taking button worked great though!

A rabbit hole of bluetooth

hillexed@email.com (hillexed) — Fri, 04 Aug 2023 02:25:25 +0000

It all started with an idea for better telescope images.

I take photos using my phone. My telescope shakes when I press the capture button. I have a raspberry pi pico W, which recently added bluetooth. I wonder if I can connect the pico to my phone as a bluetooth device so I could press a button on the pico and remotely take a snapshot.

Bluetooth is a nightmare

the C++ standard is a mess of inscrutable acronyms. Apparently there’s multiple types of bluetooth services, keyboards and mice are called “HID” for human input device, I think bluetooth audio is called HSP for HeadSet Profile. Will that do what I want? Hope so, because I don’t know how to test yet! I found https://github.com/raspberrypi/pico-examples#pico-w-bluetooth, but clicking the links just takes you to some inscrutable C examples from the BTstack library. On the plus side, maybe that means things made for the BTstack library will simply work.

I found a micropython UART example, but my phone wouldn’t connect to it, possibly because it only supports specific services? sigh

I start to get it

okay, I’m starting to get this. bluetooth is many protcols in one, GATT is the meta-protocol that tells you what protocol you should use, each protocol has a number assigned to it like 0x180F for “telling you how much battery I have”, you can only find the numbers by searching a giant inscrutable PDF which doesn’t tell you what anything means.

Oh gosh which service will let me press volume down

Do I want… 0x1812 Human Interface Device (which I know is for keyboards and mice) 0x1843 Audio Input Control 0x1849 Generic Media Control 0x1848 Media Control 0x1855 Telephony and Media Audio 0x1844 Volume Control

Good question! Apparently the list of magic service numbers on https://www.bluetooth.com/specifications/assigned-numbers/ doesn’t actually say what each service does, and you need to go to https://www.bluetooth.com/specifications/specs/ and read a giant specification to find out! Yay!!

Unable to find image voldown.png

This image is all you get for “how to reduce the volume”. It doesn’t even say what Opcode or Change_Counter stand for. To do that I need to go to another paragraph which says “Change_Counter is read from Volume State”??? aargh

A page from the bluetooth HeadSet Protocol dscribing cases where this newfangled bluetooth technology might be used

Unable to find image bluetoothexamples.png

every single one of these 3 pictures is glorious

Pain

Unable to find image volcontrol.png

Ah, of course, the Volume Control Service is 0x1844 in the Assigned Numbers document, but you need to read the Volume Control Service document to learn that you also need Volume State and Volume Control Point characteristics. Naturally now that you know what characteristic names you need, their magic numbers (which are the true things you need) aren’t kept in this document, and you need to go back to the first one into this table. I wish it was sorted by “numbers you need for this service” instead of ALPHABETICALLY

A computer update

My computer can see what bluetooth services my phone supports.

It doesn’t support any of the 18xx fancy Bluetooth Low Energy profiles. NONE of them! There goes my work understanding Volume Control Service.

I gave up trying to use micropython and now I’ll just use C++

turns out, the official bluetooth examples https://github.com/raspberrypi/pico-examples are missing the actual bluetooth examples. you need to build these examples using make to generate the actual example code. of course they also give no instructions for building those examples.

I tried: cmake pico_sdk_import.cmake? Nope, CMake Error: The source directory is a file, not a directory.

cmake .? CMake Error at pico_sdk_import.cmake:55 (message): SDK location was not specified. Please set PICO_SDK_PATH or set PICO_SDK_FETCH_FROM_GIT to on to fetch from git.`

cmake . PICO_SDK_FETCH_FROM_GIT=1? CMake Warning: Ignoring extra path from command line:

“PICO_SDK_FETCH_FROM_GIT=1”

Turns out PARTIAL instructions are in https://github.com/raspberrypi/pico-sdk, and the implication is: they want me to edit the file CMakeLists.txt to add the line set(PICO_SDK_FETCH_FROM_GIT on). WHY WOULD YOU NOT WRITE THIS DOWN

Not all keyboards are made equal

I got an example working! Turns out the unofficial code that lets you code a Pico from the Arduino IDE has its own example classes for acting as a bluetooth keyboard or mouse. You can’t actually see their sample code without going here and looking for a library with BLE at the end of its name, but they work!

I tried sending the HID_KEY_VOLUME_DOWN key and it… did nothing.

After many hours of banging head against wall: I think not all keyboards are made equal, and as part of the bluetooth protocol a device send a “HID report descriptor” which says what buttons it has on it. Of course, HID_KEY_VOLUME_DOWN naturally isn’t part of the keyboard, it’s a completely separate thing called CONSUMER_CONTROL and the arduino-pico’s keyboard class doesn’t include the arcane hex constants that say “I can do consumer control device things like volume down” in its report descriptor. aaaaargh

Protocols all the way down

HID (Human Interface Device, the protocol for Bluetooth keyboards and mice) is a nightmare. It’s ALSO a protocol made of protocols. In fact, at the start of a HID connection the device needs to define its protocol by sending some arcane constants called the device descriptor which says what format all the future messages (called “reports”) will be in. And, of course, there can be multiple types of reports in the same device, with different formats. Turns out you can’t push Volume Down unless the device descriptor says “I have both a keyboard and a consumer control device”. Then you have two types of reports, and you send a report ID with each report to say whether you’re sending a keyboard type report or a consumer control type report. So you define your own custom protocol on top of HID on top of GATT on top of BLE! I’m four protocols down!

Remember, I want to take a picture on my phone. I thought the best way to do that is to press the volume down key.

The Arduino IDE for the Pico has a BLE keyboard library hidden in the examples. No BLE Consumer Control example library. Other people have Consumer Control libraries, so I spent a long time trying to modify the device description from “I am a keyboard” to “I am both a keyboard and a consumer device” and trying to figure out how to send the right report format.

Remember that report ID? That’s how normal Bluetooth works. Turns out all my effort was for naught because the BLE functions physically didn’t have a place to put that report ID. Turns out HID with BLE handles multiple report formats in a slightly different way compared to HID over normal bluetooth. And that’s when I realized something.

The end

I embarked on this quest to press the volume down button. But I don’t need to press the volume down button. I realized I could just click the take picture button with a mouse instead!

So I looked at the Bluetooth mouse library, and successfully used it to make the mouse move!

Sure, the “move to absolute position” didn’t work, and packets kept dropping if I sent them too quickly, and you can only move 128 pixels in a single move, and I can’t know where I am on the screen without moving to the bottom left repeatedly to reset the mouse position…

But I can finally press a button on my Pico, and watch the mouse move to the corner, then move to and click the “take picture” button! I declare: ABYSS SUCCESSFULLY GAZED

An idea for better images

hillexed@email.com (hillexed) — Thu, 03 Aug 2023 22:59:32 +0000

Welcome! Welcome, new galaxy!

hillexed@email.com (hillexed) — Wed, 02 Aug 2023 18:43:00 +0000

I got the Andromeda galaxy!!!

Aiming the telescope was a nightmare. “I’ll use a star app to aim!” I thought. I tried downloading Stellarium, it complained about a Google Play error. I downloaded SkEye, and it had some weird gyro issues because it expected the phone to be pointing in the same direction as the telescope, but I use my phone camera to take pics so it was mounted sideways looking at the telescope’s eyepiece. I didn’t really know my constellations, so it was hard to figure out what stars I could see in the light polluted sky were which stars on the app, and the part of the sky where it was seemed perfectly dark with no landmarks to aim at. I ended up pointing my telescope at a bright star, aiming upwards to what I thought was the right altitude using my mount v3’s degree markers, then rotating left/right at random patches of dark sky, shooting some four second long exposures, and hoping it would appear out of the gloom.

And somehow, looking through my pictures, I got it.

And then I immediately lost the telescope’s aim trying to see Andromeda with my eye and despite like 10 tries never managed to find it again.

I tried stacking my 3 exposures in Krita in Addition mode, but it didn’t seem to work well, so you’re looking at the best 4s exposure lightly edited to increase contrast.

hillexed@email.com (hillexed) — Sat, 29 Jul 2023 19:13:51 +0000

oh my god I think I can see multiple moons of saturn in my pictures if you turn the brightness up

Pictures from My 3D Printed Telescope, with Mount v3

hillexed@email.com (hillexed) — Sat, 29 Jul 2023 18:12:59 +0000

I finally finished Hill Mount v3, so I put it to the test. I got the clearest phone-cam Saturn I’ve ever seen! Wobbles still exist if you touch the telescope, but the wobbles are much smaller now and go away if I wait a few seconds or use a short exposure time.

Before the wobbles settle down, if you take a picture with a big exposure time, the wobbles cause the image to have two Saturns.

I think that confirms my theory that the long stretched out Saturn I saw with Mount V2 was caused by a tiny wobble. Overall, it's not wobble-free and could use some damping, but I declare Mount v3 a success!

I also got Jupiter again, and I could see two Galilean moons by eye but I couldn’t capture any using camera. I couldn’t see any of the stripes, though. Jupiter is way more featureless than I expected; I wonder if there are any cheap filters I can use to see more of the stripes.

In the last picture, I took a very long 4s exposure of Saturn. The earth’s rotation meant, Saturn blurred into a streak, but take a close look - there’s a smaller streak right next to it following Saturn! I think that’s one of its moons, possibly Titan! It’s so cool that I can see it from my backyard.

Things to improve:

Make the mount even stiffer?
How do I add damping, to make vibrations decay over time?
Just as I was writing this, I discovered some smudges on my eyepiece. Oops! Maybe that’s adding some glare. Hopefully once I clean that I can get even sharper images!
Try image stacking?

Telescope mount v3: Complete!

hillexed@email.com (hillexed) — Sat, 29 Jul 2023 01:19:05 +0000

This mount is my stiffest one yet!

Changes from last time:

uses steel EMT instead of wooden dowels, for extra stiffness
flat sides so you can see the degree markings
deeper and more visible degree markings

It feels pretty darn stiff! Unfortunately, tapping the side still makes the telescope wobble. It’s definitely much less wobble than before, though. Maybe it’ll be enough for clear pictures!

Maybe it wasn't the cheap filament's fault

hillexed@email.com (hillexed) — Fri, 28 Jul 2023 17:22:10 +0000

maybe this orange filament just likes to warp. Anyway, all parts printed!

Telescope mount v3 is half printed

hillexed@email.com (hillexed) — Thu, 27 Jul 2023 18:35:50 +0000

Just need one more big part to print!

Telescope mount v3 is printing!

hillexed@email.com (hillexed) — Wed, 26 Jul 2023 18:47:58 +0000

comment “safe printing mx mount” to lend it your energy

Here's a cool cutaway of how I'm holding the rods in place

hillexed@email.com (hillexed) — Wed, 26 Jul 2023 18:21:33 +0000

Here’s a cutaway view of my telescope’s 3D printed bases to hold rods in place. The rods go into the giant cylinders, but how do you stop them from slipping out? Insert a nut in on one side of the hole, and then screw it in from the other side. The cut in the top means the entire piece bends to hold the rod in tightly! This is so annoying to design because my rods are so close together you need to make sure that the cutaway for the nuts and screws don’t accidentally cut a hole in the wall of the places where the rods go.

Hope it works.

Gas giants, wooden wobbles

hillexed@email.com (hillexed) — Thu, 20 Jul 2023 17:07:03 +0000

I took my new wooden-dowel telescope mount for a spin, and got pictures of Jupiter and Saturn with my high-power eyepiece!

I took many pictures, and a few videos. I also figured out how to control the shutter speed on my phone’s camera, and got many photos at 1/40s shutter speed. Using a smaller shutter speed takes the planets from blinding white blurs to circular colored blurs, I assume because it lets in less light. It can make a planet go from blindingly white to almost completely disappearing into the night sky. However, my phone doesn’t let me control those settings when taking a video. The second and third images show the best raw pictures I got.

Saturn through my telescope had this weird diagonal smear to it. Since I’ve been fighting vibration issues with my wooden-dowel mount, I think it’s a very small high-frequency vibration from left to right causing that problem?

Then I tried using image processing software, Pipp plus autostakkert plus registrax, following this video tutorial. There were some shenanigans because Pipp’s site got taken down and I had to find a backup, but eventually I made the heavily processed Jupiter in the first image. Sadly, I haven’t figured out how to stack my photos because the software complained my images were different sizes (from before and after I pressed the “save as raw” button), but I was able to take a video of Jupiter without the high shutter speed setting and process that. It was a bit underwhelming - I went from dim blurry sphere to SLIGHTLY STRIPED dim blurry sphere! Wow! If there’s one thing I learned I suppose it’s that my telescope was out of focus.

I hope the pictures will be improved if I make a new mount using metal EMT pipes.

Built part of it

hillexed@email.com (hillexed) — Sat, 15 Jul 2023 20:42:24 +0000

It’s still sturdy, but wobbles when I hit it. I think it might be the springiness of the wooden dowels that’s causing the motion. I could try adding two more struts and seeing if it works but I don’t really want to. It’s also very colorful now.

Time for EMT, I guess!

Another Way Forward

hillexed@email.com (hillexed) — Sat, 15 Jul 2023 15:31:44 +0000

Last telescope update I finished mount v2 and was stuck because it wobbled. One way to try again is to make the rods out of metal EMT instead of wooden dowels. I bought a hacksaw and spent a while hacksawing my EMT to the right length before learning there were specific pipe cutting tools that were far easier to use. Oops! I’ll also need to redesign my mount to use the larger 0.706in EMT pipes.

Recently I went to my local astronomy club for the first time! It was filled with old retirees who could recognize messier objects by sight like I recognize pokemon. Incredible! As I talked about my telescope, eventually I nerd-sniped someone into helping me brainstorm how to resist the shear from the top twisting. They suggested turning the bottom part of the mount into a shear-resistant two-level truss, so that the vertical rods extend upwards to support the top part.

Apparently a truss is stable because of tension lengthwise along the rods - a theoretical truss can freely rotate at every vertex. These theoretical rods have no bending resistance. In practice of course rods do have bending resistance, and that’s what would resist my twisting. It’s worth a shot.

I designed some parts that include at least part of those truss structures to see if it would help. I have some wood left over, so I’ll test if this makes the mount any more rigid before designing an EMT mount. Hope it works.

A new rod challenger

hillexed@email.com (hillexed) — Tue, 11 Jul 2023 15:49:23 +0000

Update: I’ve learned about the existence of EMT conduit, which is steel so it’s much stiffer, cuttable with a handsaw, and most importantly way cheaper than the aluminum tubes I bought before!

The downside is it’s sized in nominal diameter, which means lying. The size “1/2 in” has an outer diameter of 0.706 in. But I’ve faced nominal diameter before and this time I’m prepared. Let’s try this.

Telescope mount v2: complete!

hillexed@email.com (hillexed) — Wed, 05 Jul 2023 21:50:20 +0000

It didn’t work. The main type of wiggling I was trying to stop is the top part twisting around the z axis compared to the bottom part. This new mount stood up well, but if I placed the telescope on the mount and gave it a sideways nudge, the telescope would wiggle from side to side at a period of about 1/4 s. It does seem less wobbly than my previous square mount, but it’s still pretty wobbly.

On the plus side, I added little degree markers so you could see what elevation your telescope is pointed at. I like them.

I’m stuck as to where to go from here. How do I change this design to make it more rigid? I’m out of ideas. Maybe the wooden dowels are too flexible and I need to buy aluminum rods? But I can’t cut aluminum to the right size. Damping exists as a theoretical idea, but how do I add some in practice?

Telescope mount V2 construction begins!

hillexed@email.com (hillexed) — Wed, 05 Jul 2023 17:22:59 +0000

Starting to build telescope mount v2! I’m using 0.5in rods, and v1 had holes 0.52in wide. This one has holes 0.51in wide, and that was a mistake, it made getting the rods in there so much more annoying.

hillexed@email.com (hillexed) — Tue, 04 Jul 2023 18:33:15 +0000

I was unsure about how to model this part to make it non-wobbly so I looked up and watched a full lecture from MIT OCW about vibration reduction. it didn’t really help but now I understand that when you bend a stick, the outer part expands and the inner part shrinks and so by making bigger areas farther from the bending (weird) you can resist shrinking and growing a bit more. how do I apply that to a complicated multi-beam twisting telescope part? unsure

Triforce of Space

hillexed@email.com (hillexed) — Tue, 04 Jul 2023 01:55:19 +0000

Telescope mount v2 has 3/4 pieces printed! The pretty colors are because I ran out of filament twice

hillexed@email.com (hillexed) — Sun, 02 Jul 2023 16:40:01 +0000

This redesigned telescope mount part is a good shape

It didn't work

hillexed@email.com (hillexed) — Sat, 01 Jul 2023 20:41:07 +0000

I put everything together for version 1 of my telescope truss mount. It stands! It was also an inch too wide for the telescope to properly rest on it. I had to rotate it the wrong way so it could fit on the stand for the picture. Aargh.

Some of the clamps did grip the rods well, some of the clamps didn’t. The base did feel pretty solid, but the top part is still pretty wobbly after full assembly. I’ve learned a few lessons:

Longer holes for more grab surface area
Add some holes to screw my mount into the floor, which will stop flexing in addition to being a requested feature
Right now I have 0.5in rods, and they’re fitting in 0.52in holes. If my mount is wobbly, maybe I should decrease the hole tolerance to 0.51in? It’s a tradeoff but may be worth it
Redesign rear base back grabber, it doesn’t grab properly
It’s 6.5 inches wide from outside to outside, not 7.5 inches. :(

Now, to design version 2!

hillexed@email.com (hillexed) — Wed, 28 Jun 2023 18:38:04 +0000

time to assemble

My telescope mount prototype is finished printing!

hillexed@email.com (hillexed) — Wed, 28 Jun 2023 18:23:59 +0000

Came out with some blobs and very stringy, which isn’t good, but they came off with some pliers. Time to go cut my dowels to size and see if it all fits

Designing a new mount

hillexed@email.com (hillexed) — Sun, 25 Jun 2023 20:53:32 +0000

My 3D printed telescope has a problem: it’s very wobbly. That means it’s hard to point the telescope at a planet and have it stay pointed at the planet. It also means I can’t focus it well, or take good pictures through my phone.

The telescope itself is fine, but the mount is the problem. There’s a 3D printed mount included in the files with the telescope, and I built that 3D printed mount (after trial and error and discovering pipe sizes are a lie ). However, the creator of the telescope doesn’t actually use the 3D printed mount - he has a mount made of solid wood. I don’t have a wood saw, so I can’t build that.

There’s another mount named Marci’s Mount, made by Marci, which involves using a vinyl record as a turntable so you can rotate the telescope along azimuth (very cool). It uses a sort of truss design with cross-braces to be even more stable, but you need a drill and a saw to use the turntable part. I don’t have either, which is a shame because it seems like a very cool mount.

I decided to try making my own mount. Halfway through studying that one, I discovered a third mount which is very clever and uses more rods to make a truss shape for extra stability. It also uses a clever screws-squeeze-the-plastic design to have more surface area grabbing the rods for more friction.

My plan now is to make my own mount, copying Ivan’s truss design but increasing the size of the bottom part for added stability. Hopefully that’ll make a more stable telescope!

hillexed@email.com (hillexed) — Fri, 23 Jun 2023 19:12:05 +0000

I woke up early and saw Jupiter and Saturn!

Thoughts on the observing: I have two eyepieces, a high power 6mm and a low power 25mm. I couldn’t get a picture of the planets with my high power lens because my mount is too wobbly and I couldn’t find the planets once I mounted my phone on the telescope. I did see Jupiter through it amidst the wobbling, and it looked like a featureless white blob. I could barely make out some stripes, whichwas cool!

To get at least some pictures before the sun rose, I switched to my low power lens. Using that, I found Saturn! And as the sun began to rise, I switched back to Jupiter, and I could see it as just a big round bright circle… but also with some of the Galilean moons next to it! And then morning clouds rolled in and covered everything and I went home.

Instead of taking pictures, now I’m taking videos so I can extract the good frames afterwards. I took a video of Saturn while trying to focus the telescope, hoping I’d get a good level of focus somewhere. Turns out my phone’s autofocus didn’t like Saturn and kept trying to focus incorrectly.

What that means is: as cool as these photos are I need to aim my telescope better, I need to focus better (I did find the button to turn off phone autofocus!), and if I do that, I can get even better photos with the higher powered zoom lens! There’s a ton of potential for improvement.

hillexed@email.com (hillexed) — Tue, 20 Jun 2023 08:34:18 +0000

I woke up early to go see Jupiter and Saturn and turns out it’s still really really hard to align this telescope! I took videos in the hopes I’d catch at least one good frame, now to review them and see if that’s true. I got a decent video of Saturn in my low power lens, hope it shows up well as rings instead of fuzzy blob

Moon Nyoom

hillexed@email.com (hillexed) — Sat, 03 Jun 2023 16:42:31 +0000

With all those upgrades, I was able to mount my phone to my telescope. Recently, the moon finally reached the right phase for it to be visible, so I got great shots of the moon!

With my 6mm high power lens, the moon was super duper detailed. And I realized something amazing: the moon was moving. I was so zoomed in I could see the rotation of the earth! That’s incredible!

Now that my phone is mounted, I don’t have to manually hold it up to take a picture, meaning video became possible. To my amazement, 3 minutes was enough for the moon to enter and exit the telescope’s view. I took a video and made a timelapse:

It’s one thing to know the earth is rotating. It’s another thing to see it with your own eyes. The universe is so cool.

Telescope upgrade 2: Electrical tape and openocular phone mount

hillexed@email.com (hillexed) — Thu, 01 Jun 2023 15:05:23 +0000

(Part 3 of my adventures with my 3D printed telescope! Previous part)

I tried adding 3 upgrades, but only two of them worked. Check out an amazing moon photo below!

Upgrades

My mount is very wobbly. To reduce the wobble I had two ideas: first, place wooden dowels along the diagonals of the sides to make the mount more rigid, and second, print an openocular.com phone holder so I didn’t have to touch the telescope to take pictures with my phone and wobble it.

I bought three more wooden dowels, then I designed two versions of 3D printed pieces to hold the diagonals in place. After printing, they didn’t fit my base. I forgot to account for the fact that there were plastic parts at the top and bottom of the pipes that would stop me from attaching them there. In the end, I just taped the dowels to the mount using electrical tape. I think it worked pretty well to make the telescope rigid, but it didn’t stop the wobbling.
The openocular.com phone mount is a 3D printed piece that lets you place a phone up to an eyepiece. That works for me, because that means I can place my phone in there then step back and the telescope’s wobbles will settle down! It still wobbled, but the wobbles died down after a while. Nice.

The mount itself is very fiddly, and it doesn’t help that if I see black through my phone, I can’t tell if the mount is aligned wrong and my phone isn’t pointed in the eyepiece, or if the mount is aligned correctly and the telescope is just pointed at dark sky. It’s also hard to focus with the mount on because the phone will also try to autofocus.

The open ocular and eyepieces are heavy, and plastic on plastic is slippery. If I put the open ocular on, the entire telescope rotates down because of the weight. To fix that, I unscrewed and adjusted the position of the center piece on the rods until it balanced. But I also added some electrical tape to the curved circular rockers, to make it less slippery, and that was a massive improvement. Now the telescope stays in place much better. I wholeheartedly recommend doing this.

Using all these upgrades, I took this picture of the moon using the phone mount.

Wow! You can see so many craters!

Telescope upgrade 1: basket

hillexed@email.com (hillexed) — Fri, 26 May 2023 21:59:57 +0000

(Part 2 of my adventures with my 3D printed telescope!)

I printed a collimation helper and a basket! The collimation helper helped me find out that I thought I was collimated, but was actually misaligned. With proper collimation, I went out again and took this new picture of Venus.

To the eye, collimation didn’t seem like it did anything, because my telescope and its mount was just so wobbly it looked the same to the eye as before. But my phone pic definitely seems better - now you can see the waxing gibbous shape instead of bright dot. Don’t know why.

I also found out “open ocular” is a 3D printable mount to hook a phone up to a telescope. I’m making one now. Hopefully that’ll help make it less wobbly?

One imposter remains

hillexed@email.com (hillexed) — Sat, 13 May 2023 05:21:48 +0000

this entire journey started because I could only find 3-packs of aluminum pipes and needed a fourth. What a story this frame has

I'M MAD AT AMERICAN PIPES

hillexed@email.com (hillexed) — Wed, 10 May 2023 18:39:09 +0000

I have a 1/2" diameter hole. I want to put a pipe inside the hole. What size PVC pipe fits into a 1/2" hole?

Did you guess 1/2" pipe? WRONG. “1/2 inch pipe” isn’t half an inch big on the outside. It’s closer to 7/8". You might think “ah so the 1/2” refers to the inner diameter". Wrong. The inner diameter of 1/2" PVC pipe is legally required to be 0.602 inches, according to the Schedule 40 standard. Want a pipe with outer diameter 1/2"? The closest size of PVC pipe is of course 1/4" PVC pipe. But even that won’t quite work, because, 1/4" pipe has outer diameter 0.54", which is more than 0.5" and way more than the 0.25" in the name of the pipe. PIPE SIZES ARE ALL LIES

Guess how I found out? I’m building a telescope, and as part of that I’m building a mount. I bought a cheap collection of 3 pipes off the internet, but I needed a fourth, so I went to the hardware store to buy some pipe. Guess who learned a $6.99 lesson in nonsensical standards…

Chamomile comments:

Wow, this is way more infuriating than 2x4’s actually being 1.5x3.5 inches.

Porglezomp addition:

Your comment made me think “oh well i know why 2x4s are like that at least that makes sense” and then I realized that there was probably a reason for this one too, so I went and figured that out:

You see, a 2x4 is 1.5"x3.5". And I thought “well, at least that has a good reason from a century ago,” but then I realized that this probably has one of those too. So let’s look into it.

Dimensional Lumber

A 2x4 is 1.5"x3.5". So why is it named like that? Well, a century ago a 2x4 referred to a 2x4 of green lumber—it was commonly cut and shipped green. And then it would shrink as it dried, and be planed to a precise size on-site. Eventually, we got better at making and transporting dry lumber, but stuck to those post-drying actual sizes for the same nominal sizes. For compatibility. So it’s the fault of compatibility with how things were manufactured over a century ago. It made sense at the time. Nominal Pipe Size

You might think “ah so the 1/2” refers to the inner diameter". Wrong. The inner diameter of 1/2" PVC pipe is legally required to be 0.602 inches, according to the Schedule 40 standard.

Well it turns out this is also the same thing. When nominal pipe sizes were standardized, they referred to the inner diameter of the pipe, but those pipes had thicker walls. Now we build stronger pipes that have thinner walls, but the dimensions were still standardized and named for those older inner diameters, and now they’re all nonsense again.

The reason for the discrepancy for NPS 1⁄8 to 12 inches is that these NPS values were originally set to give the same inside diameter (ID) based on wall thicknesses standard at the time. However, as the set of available wall thicknesses evolved, the ID changed and NPS became only indirectly related to ID and OD.

Source: https://en.wikipedia.org/wiki/Nominal_Pipe_Size#Application

This standard is from 1927 / 1939 / 1949 ?? so it is more annoying than the lumber standard, to me. Diamètre Nominal

I’M MAD AT AMERICAN PIPES

Good news! European pipes are no better! A 15mm pipe has a 21.3mm outer diameter, and the inner diameter also completely depends on the material.

Source: http://www.piping-engineering.com/nominal-pipe-size-nps-nominal-bore-nb-outside-diameter-od.html

Ionic Liquids

hillexed@email.com (hillexed) — Tue, 25 Apr 2023 14:38:14 +0000

Today I learned liquids don’t exist in space. It’s either gas or solid because apparently liquids only exist when you have a ton of force squishing atoms together and forcing them to intersct and normally that’s air smashing into things at high speed. Crazy. Space whales won’t know what liquids are

Later that day…

I found https://www.nasa.gov/ames/flute ! They want to use reflective liquids to make perfect parabola shapes for mirrors! I was hoping they found a liquid which stays liquid in space… but their liquids are kept inside pressurized space stations. Their plan so far is to use a liquid then harden the liquid into a solid by polymerizing it before exposing it into space. Nasa hasn’t found liquids that exist in space either :(

Even later…

A friend told me about “ionic liquids”. Like how table salt is made of sodium ions and chlorine ions, an ionic liquid is made of negatively and positively charged ions which attract - except the ions are intentionally so big and ugly they can’t fit right next to one another to make a symmetric solid structure, so they stay liquid. And they might survive in space!

So I researched them! I followed a citation on its wikipedia page to https://www.nature.com/articles/439797a, there’s a paragraph there which mentions “Ionic liquids have also been used in ultra-high vacuum conditions of 10−9 millibar for X-ray photoelectron spectroscopy analysis without decomposing or evaporating”, which cites https://pubs.rsc.org/en/content/articlelanding/2005/cc/b512311a#, which says 1-ethyl-3-methyl imidazolium ethylsulfate is an ionic liquid.

So I searched for that and found this chapter from a book on ionic liquids https://www.intechopen.com/chapters/40002 which cites that paper, all about putting ionic liquids into scientific tools which require vacuums (and they mention ionic liquids used for making nanoparticles and coating things so you can see them in scanning electron microscopes, whoa), and that chapter cites a paper https://pubmed.ncbi.nlm.nih.gov/20437507/ which suggests ionic liquids could be used as lubricants in outer space! That paper is wild, with examples of using ionic liquids to make gold nanoparticles, ionic liquids to help take scanning electron microscope images of things which normally can’t survive vacuums like seaweed without being dehydrated, using “electrowetting” to make little flagellae which move based on the voltage you apply (they use the word “electrocapillary”, which is made of two words that shouldn’t be next to each other what are you doing there)

I asked in a different community if anyone knew what they were and one friend said they had a friend who works on ionic liquids who can’t reveal what they’re up to, but pointed me to https://pubmed.ncbi.nlm.nih.gov/35956982/ a paper on trying to make insulin into a pill you can take by mouth. Someone else found a bacterium that could survive some ionic liquid along with its water: https://www.pnas.org/doi/full/10.1073/pnas.1112750109

In summary: ionic liquids are crazy things which CAN survive vacuums, and other ionic liquids can survive temperatures of -4 to 400C without freezing… but is there an ionic liquid which can stay liquid in both a vacuum and -250C, so it can stay liquid in space? No clue. The hunt continues.

this intro to synths guide tells you how to build circuit boards WITH A PAINTBRUSH

hillexed@email.com (hillexed) — Mon, 12 Dec 2022 22:10:23 +0000

This is the 1985 “electronotes builder’s guide and preferred circuits collection” and Bernie Hutchins is advising first time synth makers go buy chemicals at radioshack to etch copper and use a paintbrush to trace out wires manually. It’s so weirdly analog and hands-on compared to the way circuit boards are made today I’m used to, “pay someone else online to do it”

Today I learned about Living Newspapers

hillexed@email.com (hillexed) — Thu, 08 Dec 2022 20:36:09 +0000

As a tiny part of the New Deal, Hallie Flanagan helped create the Federal Theatre Project in 1935, where the US government funded unemployed actors to make free plays available to all. A big part of those were Living Newspapers: researchers became playwrights and wrote about current events lifted from the headlines to inform people.

The very first Living Newspaper was a hilarious tragedy. “Ethiopia” by Arthur Arent was all about how being invaded during the Second Italo-Ethiopian War was ravaging the country. Hilariously, it got exactly one dress rehearsal (the press was invited, it got good reviews), and then the US State Department cancelled it before its first showing, arguing that it was bad to show other world leaders, specifically this guy named Mussolini, in a bad light. And they only found out because someone in the production team tried to make the play more realistic by requesting a recording of one of President Roosevelt’s speeches, tipping off Washington…

Thankfully, other Living Newspapers about domestic issues became huge successes. Eventually the Federal Theatre Project fell into a fate that seems eerily similar to today: wealthy people got mad that it gave money to those who were seen as not deserving it despite being a tiny fraction of the budget, it was accused of being too communist after things such as plays which revolved around housing inequality and farmers, and the program was cancelled in one of the first victims of McCarthyism :(

(Source: https://doi.org/10.2307/3204873 , but I only found out about this from seeing the book “Furious improvisation : how the WPA and a cast of thousands made high art out of desperate times” by Susan Quinn in a library search)

i played myself

hillexed@email.com (hillexed) — Tue, 06 Dec 2022 01:12:58 +0000

i bought a t-shirt and the store offered a “$3 mystery t-shirt” so i figured cheap t-shirt why not. presumably that was so they could get rid of t-shirts that didn’t sell well

it finally arrived… and it’s extremely impressively the exact opposite of my style

a wqalk in the forwst

hillexed@email.com (hillexed) — Wed, 30 Nov 2022 20:12:39 +0000

ME: hi drabgon i see your home has many trees

DRABGON: contrary to poplar belief we don’t breathe fire

NEARBY TREE: don’t believe that liar! I bet it’s just waiting for the right moment

rocket today

hillexed@email.com (hillexed) — Wed, 16 Nov 2022 03:20:08 +0000

i hope the rocket goes away properly

(note from 2025: the JWST space telescope launched flawlessly!)

How Microprocessors Make Music

hillexed@email.com (hillexed) — Sat, 12 Nov 2022 18:11:52 +0000

So I’ve been working on trying to make a little Arduino powered synthesizer recently! Turns out it’s hard and sound is complicated and now I’m cursed with knowledge. The only way to get rid of curses is to dilute them, so here’s everything I’ve learned about how to make sound on an Arduino.

Speakers

A speaker takes some electricity and moves a little flat thing either outwards or inwards depending on the voltage, which pushes on air and makes sound. They convert changing electricity to changing sound waves. For a microprocessor to make sound, it needs to change voltage really fast so the speaker makes the right sound wave.

The Simplest Square Sound

Computers use zeroes and ones. Those are usually voltages: a zero is a low voltage, and an one is a high voltage. Any Arduino pin can easily output a zero or one, in the form of either 0 volts (“0V”) or 5 volts (“5V”). However, not all pins can output the values in between. Thankfully, there’s one easy type of sound wave you can output using only 0V and 5V: a square wave at maximum volume, which switches between 0V and 5V at a high frequency. Arduinos even have a built-in tone() function to do that, which uses a timer (timer1, which I’ll get to later) to manually switch whatever pin you want between 0V and 5V really fast in software. Unfortunately, to output other types of waves (like sine waves), or even just control a wave’s volume so your wave can fade out over time, you need to output values between 5V and 0V. For that you need PWM.

All the other sounds: PWM

PWM works like this. Let’s say you want to output 1 volt. Unfortunately, an Arduino physically cannot output 1 volt - it can only output 100% of 5V or 0% of 5V, with no in between. If only you could just output 20% of 5V. The arduino can’t output 20% of 5V, but it can output 5V, 20% of the time. If you switch between 5V and 0V really fast, outputting 0V the other 80% of the time, it averages out to 1V! That’s Pulse Width Modulation (PWM): switching between 5V and 0V really fast in order to output signals which are, on average, any voltage you want between 5V and 0V. (To do the averaging, usually a resistor and capacitor are placed in between the output and the speaker to act as an “RC filter”.)

Timing is everything

An Arduino has hardware built into the chip to do PWM. But that hardware is only built into certain pins of the chip. That’s because in order to switch between 5V and 0V fast enough to average out, you need a very fast and accurate timer to tell you when to switch. The ATMega328 chip which most Arduinos use has 3 timers built into it, so you can run 3 PWM outputs at different speeds at once if you really wanted to.

To control PWM, you’re really controlling the timers, and so you need to set specific bits in specific arcanely-named all-caps acronym variables like OCR1A which you can only understand by looking up “ATMega328 datasheet” and banging your head against a wall.

Timer 0 is used for the system functions millis() and micros(), so it’s encouraged not to use it if you have the option. Timer1 is a 16-bit timer, which means its PWM pins can output any multiple of 5/(2^16) volts at once. However, it can only output to digital pin 9 or pin 10 (hardware-chip-footprint-numbers 15 and 16). Timer2 is an 8-bit timer, which means it has less resolution than Timer1 and can only output 2^8 different values between 5V and 0V. It outputs on digital pin 11 or digital pin 5 (hardware-chip-footprint-numbers 17 and 11). That’s worse for audio, because you want smoother waves. Use timer 1.

PWM has some rough edges

Let’s say I want to output a sine wave at a frequency a. Mathematically, I want to output the function sin(at), where t increases over time. Mathematical sine waves are smooth, with infinite resolution. Unfortunately, sine waves created by computers are really just numbers, and those numbers have a fixed resolution.

Let’s say we’re using an 8-bit PWM. In order to use PWM and output your wave, you must tell the PWM an integer between 0 and 2^8 = 255, and it will interpret 0 means 0V and 255 means 5V, and anything in between is treated as a fraction with 255 on the bottom: 20 means 5 * 20/255 volts. The inputs are converted to integers. That means even if your wave is continuous, any values in between any two integers such as 4 and 5 will be rounded to either 4 or 5. Even if the math says your wave should output the value sin(1) * 255 = 4.45, that number will be rounded down to 4 and output 0.078V instead of 0.087 V.

(Math note: sin(at) is centered at y=0 and ranges from y=-1 to y=1. To properly fit sin(at) into the range 0 to 255, you really want to output sin(at) * 128 + 127 so that the center point is 127, the center of the 0-255 output range.)

The 8-bit timer forces us to round our numbers to 8-bit integers (0-255), and that causes some effects you can hear. Specifically, the rounding might sound like the wave is “tinny”, or has some extra buzzing. Some people like that sound because it reminds them of the old days of 8-bit computers and will add that effect into music on purpose, which is called “bitcrushing”.

In general, squeezing a wave into k bits will distort the sound less and less the bigger k is. 8-bit is pretty noticeable. 16-bit lets the values range from 0 to 2^16, and there’s basically no distortion. 16-bit audio is CD quality. Avoid 8-bit PWM for audio output and use something with more bits if you care about audio quality.

Multiple notes at once? That’s not in the budget

A piano can play more than one note at the same time. I want my microprocessor to be able to do that too!

Let’s say I’m stuck with my 8-bit timer, and I want to output two notes at once. Thankfully, it’s not too bad: to hear two things at once, just add up the waves. Then, output sin(at) + sin(bt). That’s just a number, so I can output it fine using PWM, right?

Unfortunately, sin(at) and sin(bt) both range from -1 to 1, so adding them together could give me a number anywhere from -2 to 2. That means if I used the same formula as before to convert the waves into the 0-255 range, wave(t) * 128 + 127, I’d get a value from −129 to 383, which is outside the valid 8-bit timer range 0 to 255. Whoops.

To fix this, instead of outputting sin(at) + sin(bt), shrink the wave vertically so it ranges from -1 to 1 by dividing by 2. Output (sin(at) + sin(bt))/2. Perfect.

You can even generalize this: to play eight notes at once, add up the 8 individual waves and divide by 8, and your signal will stay between -1 and 1. If you want your Arduino to be able to play up to 8 waves at once, you’ll want to output (wave1 + wave2 + wave3 + wave4 + wave5 + wave6 + wave 7 + wave8)/8. If you convert that into the range 0-255, it’ll be (wave1 + wave2 + wave3 + wave4 + wave5 + wave6 + wave 7 + wave8)/8 * 255 + 127. If you’re not playing all 8 notes at once, some of those waves might be 0 the entire time and not some form of sin(t).

However, if you program that into your microprocessor, then even just one note might sound very distorted! Why?

Remember, the less bits you have to store a wave, the more distorted it’ll sound. If sin(x) can take any value between -1 and 1, it’s converted to 8-bit number that’s rounded to any value between 0 and 255. But if 8 notes can be played at once, and we divided by 8, we’re effectively adding waves of the form sin(x)/8, which is rounded to an integer between 0 and 32.

This rounding has two effects: first, using the same speaker, our original 0-255 wave moves the speaker eight times more distance compared to an 0-32 wave, so the new wave sounds much quieter*. (This problem once tricked me into thinking my code stopped working, when really it was just too quiet to hear.) Second, rounding a wave to one of 32 values adds a LOT more distortion than rounding to one of 256 values, and you can hear that distortion very strongly.

*the wave is eight times weaker, but we hear volume logarithmically so it only sounds log_2(8) = 3 times weaker.

You could think of this as a “bit budget”: you can either spend your 0-255 range on one high quality wave, or give each wave 1/8th of the total range to represent them so that you can add up to 8 waves at once at the cost of bitcrushing each individual wave.

Use 16-bit outputs. That way you can play multiple waves at once without running into the bitcrushing problem as much, because an 0-65536 range divided by 8 is still a very high quality 0-8192 range. You’ll still have to make the speaker louder to counteract the volume loss from dividing your waves to play multiple notes at once, however.

Also, there’s intense time pressure

Humans can hear waves up to 20kHz, which means if you want to be able to output all possible waves a human can hear, your microcontroller needs to choose which value to output 40,000 times a second. That includes doing any math to compute sin(x), adding the waves from multiple notes at once, etc. It’s not too bad if you only play one note at once - in fact, for a square wave you can simply use a timer - but if you want to play multiple notes at once, time starts to become a problem.

An Arduino runs at a 16mHz clock speed by default. That means whatever code you write to choose which 0-255 or 0-65536 value to output, that code needs to involve less than 16000000 / 40000 = 400 machine instructions. Your function needs to be FAST. If it takes too long, you’ll get distortion!

Writing fast code is hard. I’ve successfully gotten up to 4 waves added together on an Arduino, but but as soon as I tried adding a few more additions to bring it up to 6 waves at a time, my code took too long to compute, lagged, and made weird digital noises instead of my crisp waves. Want to compute (wave1 + wave2 + wave3)/3? Too slow. The ATMega chip doesn’t have a division instruction built in, so dividing by 3 takes as much time as a good fraction of the rest of my code combined.

A library called Mozzi solves the gotta-go-fast problem by having its fast function be “read one value from the precomputed buffer and output it”, then updating that precomputed buffer only once every 256 function calls. Another technique for outputting waves quickly is called “wavetable synthesis”, where you save many values of sin(x) for various x in a table 1024 entries long, and that way accessing table[x] is very quick. Other projects go fast by abandoning the Arduino’s ATMega328 chip for an even faster computer, such as a Teensy. In fact, the device you’re reading this on is much faster than either of those, and that’s why people usually make music with computers nowadays.

In summary

Computers are made of workarounds upon workarounds upon workarounds, modern processor speeds are a miracle we take for granted, and I’d say music was a mistake but actually the cool noises you get at the end are worth it.

Time

hillexed@email.com (hillexed) — Fri, 11 Nov 2022 03:04:39 +0000

And the days go on and on and on

Aiming

hillexed@email.com (hillexed) — Mon, 01 Jan 0001 00:00:00 +0000

My Hadley has two upgrades that help it aim.

Finderscope

The first upgrade is an 6x50 finderscope that says “orion” on it. I found it in my local astronomy club’s shelf of spare parts, bought it for $10, and designed a 3D printed adapter for it to fit onto my Hadley. It features integrated crosshairs and helps me aim at planets at a glance!

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Sliced Pifinder

The second upgrade is the Sliced PiFinder. It’s a targeting computer that permanently lives on my telescope - which is an incredible sentence. We truly live in the future.

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The PiFinder is a device that uses a raspberry pi and a camera to take pictures of the sky and compute where in the sky your telescope is aiming, even with high light pollution. Since it knows where the telescope points, you can select a particular galaxy or nebula and it will tell you how to push the telescope to get there.

PiFinders are an open-source project that can also be bought for $500 from the designer. Mine was built for $110 - a mere slice of the cost. The stock PiFinder is designed for a Raspberry Pi 4 using the Raspberry Pi High Quality Camera ($50) with a $50 lens, but I 3D printed and assembled the parts myself, used a scavenged battery pack and previous-generation Raspberry Pi 3 from a defunct project, and a $30 IMX462 camera and $12 lens.

Fixing the Astigmatism

hillexed@email.com (hillexed) — Mon, 01 Jan 0001 00:00:00 +0000

When I tried to focus my Hadley on mars, I couldn’t see very much. It looked like a blob.

When I looked at Saturn, I noticed a sort of double-saturn effect.

The Hadley’s default telescope mount was very wobbly. I assumed that the pattern I was seeing of two saturn images was actually one Saturn image vibrating back and forth quickly.

Based on this assumption, I designed a much stiffer truss mount. Inspired by Ivan’s open-source Hadley mount which used a truss design but sized for metric screws and rods, I designed a printable Hadley mount focused on a truss design to reduce wobbling and cheap EMT steel tubing. While the Hadley used aluminum rods that cost $30 for 9 feet, one 10-foot tube of EMT cost $7! Eventually I open-sourced my design; it’s available to download for anyone who has a Hadley!

After spending a long time building my design… Every star still looked like two points close together when viewed through the Hadley.

Undeterred, I redesigned the mount to be even stiffer; going through three iterations of increasingly stiffer mounts and even adding cross-bracing.

I took a look… still double saturn.

It took looking at the double-double in Lyra to figure out the issue. I should have seen two clusters of two stars. I did see two clusters of two stars - but each cluster seemed to be aligned the same direction. That direction didn’t match the star charts - and it didn’t even match the direction that my telescope should have been wobbling in. My “extremely fast vibrating” wasn’t vibration at all.

The problem? Astigmatism in my primary mirror. The Hadley telescope relies on a $30 cheap chinese mirror, and not all mirrors are made equal. I had bought a mirror off a random vendor from Amazon, and instead of a perfectly spherical shape, my mirror focused light to two slightly different places. As confirmation, removing the lower optical assembly containing the mirror and putting it back on rotated 120 degrees rotated the abberations.

Eventually I reported my findings to the Hadley discord, and Zane Landers offered to ship me a replacement 4.5" mirror. Saturn has looked sharp ever since.

The Mirror Making Process

hillexed@email.com (hillexed) — Mon, 01 Jan 0001 00:00:00 +0000

(This page is a work in progress!)

I dove into the rabbit hole of optical fabrication and made my own mirrors for my 8" Leavitt telescope.

When we think of mirrors, we think of mirrors like those on a bathroom wall, which are flat. Telescope mirrors are curved, to focus light to a point. This curvature focuses parallel light rays everywhere on the mirror’s surface to the same place, increasing the amount of light the eye can see when looking through a telescope. This curve must be smooth to within a fraction of a wavelength of visible light - 500 nanometers. It is incredible that we can make surfaces that smooth using just muscle power.

There are four main steps to mirror making: rough grinding, fine grinding, polishing, and parabolizing.

Rough grinding carves a depression of the right curvature in a piece of glass.
Fine grinding smooths the surface using a succession of smaller and smaller grits.
Polishing uses a new tool and cerium oxide to make the glass reflective for the first time, creating a reflective sphere.
Parabolizing, also known as “figuring”, slightly adjusts the extremely precise sphere to create an extremely precise parabola.

Then there is one final step: aluminizing. After the first four steps a mirror is a fully usable precisely shaped reflective piece of glass, but covering the glass with a thin layer of shiny metal increases the amount of light reflected to around 90%. This step requires a vacuum chamber. I don’t have a a vacuum chamber. Instead, I did what most amateurs do and shipped my mirror to someone else. Aluminizing was its own shenanigan - more on that below.

The Materials

Telescope mirror glass has gotten expensive nowadays. I was very lucky: I got mine through social media. I had blogged about the process of creating my first 3D printed telescope, Hadley. When I mused online about falling down the rabbit hole of mirror making, I recieved a wonderful comment: @BeasMeeply on cohost had an abandoned mirror making project that had sat in a closet gathering dust for 10 years, and graciously offered to send it to me for the cost of shipping. After that, I had no choice but to dive in.

Making tools

Making a mirror involves rubbing two surfaces against each other to make each other smooth, one concave (the mirror) and one convex (the tool). In the olden times, glass was cheap and a second piece of glass would make a good tool. Now that 8" glass circles are almost $100, a cheaper way is to cast a plaster disk. There are two requirements for this: 1) waterproof plaster, and 2) porcelain tiles embedded in the plaster, to match the hardness of the glass and ensure the soft plaster doesn’t simply get ground away.

Dentists use waterproof plaster called “dental stone” to take tooth impressions, and it works great. Unfortunately, dental stone is usually sold in 45 lb boxes for $50, and I only need 2lb or so. I got my dental stone from… having a dentist appointment and asking my dentist nicely!

I got hexagonal porcelain tiles from Home Depot for $5. I learned the hard way to keep the webbing on.

Dental stone hardens incredibly quickly - within 10 minutes of adding water and mixing. I learned the hard way to set a timer for four minutes before pouring.

Polishing

When I got the mirror, first I used spherometer to measure the curve - same curvature all around, accurate to within 0.0002 inches. I put it in a foucalt tester, and after lots of help and fiddling I got some pictures of the result:

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The fact that it’s transparent enough that I can see through the glass and get a result in the foucalt test tells me @BeasMeeply got through grinding and polishing, and now I can skip to making a pitch lap.

Homemade 3D printed ronchi tester

I built a 3D printed Ronchi tester to test my mirror’s surface shape. Handheld and easily portable, it uses an LED, a 9 volt battery, a switch, and a Ronchi grating of evenly spaced lines sent to me by a telescope-making friend in France. It lets me bounce light off the mirror and

For more on the Ronchi test, see here.

Depending on where each part of the mirror focuses light, the pattern of lines that is reflected back to the LED will look longer or shorter. I made this infographic of how to interpret test results:

The Ronchi test is great at detecting astigmatism, turned down edges, and overall parabolization progress. It is so sensitive that you can see air turbulence or uneven heating from the wobbling of the lines! However, that sensitivity requires taking pictures of the Ronchi pattern and comparing them on a computer, using this wonderful calculator by Mel bartels. A spherical mirror shows up as perfectly straight lines up and down. However, detecting straightness by eye (or even by computer) is very tough, and if you want a 1/10th wave finish or better, slight surface issues can be visible if you are good at dialing in a Foucalt test to the center of curvature but escape a Ronchi test. I have found that sometimes seeing an issue in a Foucalt tester, I have gone back and seen the issue in the Ronchi.

For my mirror, I used my Ronchi tester to guide overall spherical progress as well as parabolization, but used the Foucalt test with a Couder mask to take numerical readings for the final stretch of parabolization.

Parabolizing

The process of parabolizing involves turning an incredibly precise sphere into an incredibly precise parabola. measuring the mirror’s surface

I tried parabolizing three times, since twice I had to restart. Experts at my local astronomy club were able to spot 1/10th wave defects in a surface I thought was spherical, and insist I smooth out the surface.

In total, polishing and parabolizing took 32 hours of cumulative polishing time. This was not a way to save time or money. It is a lot of time spent moving a piece of glass back and forth. As I write this in 2025, an 8" mirror can be bought from Chinese company GSO for $300 on AgenaAstro. If all you wanted was a mirror, the correct answer is to spend 32 hours working a part time job and use that money to buy a mirror. That said, the commercial mirrors would only be smooth to within 1/4th of a wavelength, while mine is smooth to within 1/20th of a wavelength. Plus, I had fun!

Cardboard Couder Mask

I drew circles on cardboard with a ruler and compass traced them out to create this cardboard mask. This isolates parts of the mirror at different points from the center to the edge. The zone radii aren’t regular, but are instead chosen so that each zone focuses light

The spreadsheet

I made a spreadsheet that would take knife-edge measurements of the Couder mask zones and from there integrate to compute the total shape of the mirror. This helped me with the final few steps of the process.