Hillescopebuilding on Hill's Space

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!

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!

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?

Check check 1 2 3

hillexed@email.com (hillexed) — Mon, 08 Apr 2024 00:05:57 +0000

There’s a weird bright thing in the sky. I’ve tested my setup and I can definitely see it. Someday I wish it would go away

SUN CANNON: ASSEMBLED

hillexed@email.com (hillexed) — Sat, 06 Apr 2024 02:15:49 +0000

Introducing “Finley”, probably the jankiest and most colorful eclipse imaging setup of all time. The only telescope I have which is enclosed (and therefore safe to use on the sun) is this donated finderscope, meaning I’ll have crosshairs in all my images. I’m taking pictures using my friend’s phone which he gave to me after the screen cracked. I designed a 3D printed adapter so it fits on the sturdy 3D printed mount I designed for the far bigger Hadley telescope. The whole thing looks like it’s made of legos. I hope it works and I get good eclipse pictures!

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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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.

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!

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

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) — 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

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

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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.

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

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

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

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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

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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!