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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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.
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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.
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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.
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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
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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.
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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.
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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).
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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.
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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.
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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.
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