| 1 min
| 126 words
Unable to find image 6966242-8-telescope-mirror-image.png 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.
Read More
| 1 min
| 138 words
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.
Read More
| 1 min
| 212 words
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.
Read More
| 1 min
| 145 words
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.
Read More
| 2 min
| 232 words
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.
Read More
| 2 min
| 242 words
Unable to find image verystraight_20240610_150033.jpg Unable to find image 6533527-8-mirror-grinding-image.png 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!
Read More
| 1 min
| 171 words
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.
Read More
| 1 min
| 148 words
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.
Read More
| 1 min
| 127 words
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.
Read More
| 1 min
| 117 words
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.
Read More