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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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My 8" mirror that I polished by hand is officially a mirror! The person with a vacuum chamber, who I gave my mirror to two months ago, finally aluminized my mirror and sent me a picture! It’s shiny and reflective like a mirror should be!
Now it just has to survive being shipped back…
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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).
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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.
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My 8" mirror is done; now I need to aluminize it.
The final step of making a telescope mirror, after spending months grinding glass into a parabola, is to cover the perfectly shaped glass in a thin layer of shiny reflective metal.
The most common technique is melting aluminum in a vacuum chamber so the atoms gently float onto the glass and stick. (There’s also a newer spray silvering technique that involves $400 of chemicals but amortizes out to cheap for many mirrors.
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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.
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