The 12" Meniscus Mirror on Hill's Space

12" polishing update: Oblate no longer?

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

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

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

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

Creating a mirror cell

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

I have made a contraption called a “mirror cell”

it traps naughty glass so I can examine its sins

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

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

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

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

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

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

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

12" mirror making: take testing seriously

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

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

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

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

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

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.

Polished out?

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

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

12" Mirror Making: Pitch Lap Problems

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

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

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

Starting to Show Reflections

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

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

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

Total polishing time: 3 hours

12" Polishing Start!

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

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

12" Pitch Lap Created

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

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

On to 5 micron, for the third time

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

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

Current cumulative grinding time: 46 hours.

Third time is the charm...

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

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

Scratches, again...

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

Tragedy!

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

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

Rebeveled Edge to Stop Sleeks?

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

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

Sleeks Spotted

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

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

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

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

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

Grinding At 12 Micron, Again

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

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

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

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

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

Back to 30 Micron Grit With Refurbished Tool

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

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

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

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

Tool Refurbishing

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

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

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

Scratches Showing Up

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

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

Crack Coverup

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

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

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

Total time spent grinding out the crack: 7.4 hours

Crack Tragedy!

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

Tragedy!

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

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

Back End

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

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

Total back grinding time: 5 hours 57 minutes

Back Grinding Proceeding Nicely

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

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

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

Meniscus mirror: twyman time!

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

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

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

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

Divot Gone

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

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

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

On to 220

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

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

Focal length flattening

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

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

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

Good grinding progress

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

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

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

Total grinding time: 4.9 hours

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!

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!

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…

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!

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.

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!

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

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) — 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