Diy on Hill's Space

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.

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" 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.

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.

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.

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…

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…

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

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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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 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

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.

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!

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!

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

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!

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.

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.