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It looks like there were pockets of air under the pitch. Leaving it alone for a week meant the bubbles seem to have popped and some areas sank downwards.
This turned down edge is slowly going down but it still feels like an endless quest with no end in sight.
Total grinding time so far: 8 hours
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This pretty art piece is actually a precise measurement of my progress in telescope mirror grinding. It looked fun enough out of context I decided to post it!
You’re looking at multiple overlaid square images. Each one is a Ronchi test, which tells me the shape of my mirror on a nanometer scale in the shape of the red stripes. As I polish the mirror, it changes shape, so after 20 minutes of polishing I want to measure the new shape.
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you can mirror grind in two ways: mirror on top or tool on top. Apparently if you have a turned down edge to fix it you do center over center strokes with amplitude 1/3 of the length, according to a video by Gordon Waite. I did it for two hours yesterday and made little progress. Today I took another look at the video and noticed he was doing it tool on top.
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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.
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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.
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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.
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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!
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.
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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.
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Finally finished making a pitch lap for @BeasMeeply’s mirror! Yahoo! Now I can start polishing… as soon as I get some cerium oxide to polish with.
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I got a mirror that’s 2.3 inches wide. However, several weeks ago, I printed a mirror holder that was designed to fit a mirror 2.46 inches wide. Sure, I could just use the bigger holder, but the bigger the secondary holder the more light it blocks from reaching your mirror. Is it worth a smaller secondary mirror holder that will block half a square inch less light? Yes, I decided. So I opened up the model and slightly scaled it down.
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