My third telescope project pushes the boundaries of telescope technology, using kiln draping techniques to create a thin yet constant thickness meniscus-shaped mirror that will give bright views for cheap. This mirror will be 12" f/3.3, but only 0.5" thick. Only a few people in the world have tried making mirrors this thin and this fast.
Making mirrors this thin is a bit heretical for amateur telescope making. Telescope mirrors must hold the correct shape to within a fraction of a wavelength of light (500 nanometers). Glass may look rigid, but on small scales, glass is floppy and will happily bend under its own weight like a piece of paper. To resist bending, traditional mirrors are almost an inch thick (or often more).
However, large volumes of glass may take a long time - hours, even - to cool down when brought from warm indoors to cool outdoors. Because materials expand and contract with temperature, the difference in thickness between a larger edge and smaller center will change the mirror’s shape during those hours of cooling down. Traditional mirror owners must wait many hours for their mirror to cool down to ambient temperatures before the stars will look pinpoint. A meniscus mirror is thin - only 0.5" thick - and its curved shape gives it a constant thickness the entire way through, reducing the problems of cooling.
My 0.5" thick meniscus mirror with 0.25" sagitta, bending-wise, should like an 0.75" thick traditional mirror with a thickness of 0.5" at its center. Less glass means it is easier to carry and cools down faster in the cold night air.
Also, buying thick glass cylinders costs hundreds of dollars. I bought a glass countertop from an used furniture store for $20 and cut a hole out of it.
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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
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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.
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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.
12" meniscus mirror: Grinding start!
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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