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I’m around 10 hours into polishing, and it looks like I’m very close to polished out! I can see barely any tiny pits under a microscope. I think I’m done.
Next up, figuring. I can test a mirror’s shape easily. I have been doing that already despite not quite being done with polishing.
The problem with a thin mirror is that glass (and anything else) will happily bend a few hundred nanometers under its own weight.
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Polishing continues. I’ve noticed that my short center over center strokes seem to be creating a hill in the center of this mirror, so I’ve switched to longer 1/2-diameter strokes and 1/3 side to side motion in order to wear down the center of the mirror. It seems to be working, according to these ronchi test images before and after. I’m jumping the gun a little by figuring while polishing but there’s a long way to go so I should be fine.
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My pitch lap polishing tool wasn’t making good contact with the center of my mirror. I can tell because I’m just beginning polishing and so I can see that the mirror’s outside is more reflective than the inside. Also, the tool looks unevenly worn.
I tried heating up the tool using a tub of hot water so the pitch would flow and then pressing the tool against the mirror some more (pictured) but it seems to have only helped slightly.
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Another hour and a half of polishing and my piece of glass is looking much more reflective!
I was stroking with tool on top, but I noticed the center of the pitch lap wasn’t making good contact. I suspect the plaster backing tool that the mirror rests on isn’t evenly supporting the mirror; it picked up a slightly wavy surface I was hoping wouldn’t cause issues. When I poured the tool I used wax paper to ensure it wouldn’t stick to the mirror, and the wax paper crumpled slightly from being flat on a non-flat mirror surface.
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I finally started polishing this 12" mirror! It has taken so long and many failures to get here. I took the pitch lap tool that I made and carved some bonus channels into it, squirted on cerium oxide mixed with water, and got polishing. An hour and a half later, the outside of my mirror is polished and now is smooth enough to reflect light! It looks like my pitch lap wasn’t pressed for long rnough to make good contact with the mirror, though, because I can see that only a ring around the outside is being worn down, not the center.
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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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8" telescope: COMPLETE!!
It has literally taken all year, but my second telescope ever is done! It’s made of 3D printed parts, metal tubes, nuts and bolts, and a mirror I hand-polished over the course of 8 months! I’m very proud.
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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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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.