This blog is mainly about Telescope making, and some things about my politics. At last we finally have a President that can say "Fool me once; shame on you. Fool me twice; shame on me." instead of mixing up with an old Who song.

Monday, September 01, 2008

Polishing my 12.5"

August was cool. A few days got above 90 but mostly it was in the low 80's and even into the 70's I managed to finish up the fine grinding on my scope in the evenings, in one or two cases I was done with only an hours worth of work, but even still, I kept at it for at least 2 hours on each grit. I did this partly because I wanted to get the ROC as close to 137.5" as possible before going to the next level, even though there were no pits left. At one point, I think it was the 320 grit, I was thinking that I was going to run out of tile. It got awfully close to the concrete. In one place I actually had to dig a grain of sand that was at the same level as the tile and re-seal it with hot wax. But that worked, and the subsequent grits didn't wear any more tile away that I noticed.

Finally it was major clean-up time before going to the polyester polishing pads and Cerium Oxide. I was getting absolutely paranoid.

Then I applied the pads. They look like those flowery bathtub thingies that keep you from sliding around in the shower. I put them in an off-centered hex pattern all over the mirror and cut them where they ran over. I put about a 1/2 teaspoon of CeO2 on it and within two times around the barrel it was already a noticeable shine. But the drag! Man it was like pushing a car around. So I washed off the CeO2 and removed 3 or 4 pads and that made it much easier. In total, I have spent about 3 1/2 hours on it now, and there are no pits left from edge to center. I tried the 10x magnifier - nada, sun test -nada, the lazer test - nada, and I finally found a little hand held microscope down at Radio Shack that goes all the way to 100x. There are simply no pits left... except one set in the very center.

Once while I was polishing and getting over zealous I let the mirror tip over the edge of the tool, and it sheared off chunks of tile. I stopped immediately, cleaned everything and inspected, and sure enough, small scratches. It wasn't too late, I could have gone back to fine grinding, but I decided to continue for a few more whets to see if they would clear up by themselves. Well, for the most part they did, but there are still a couple of the strays, 2mm long, in 2mm patches. But they are within the shadow of the secondary mirror, and one of them would be covered up with an alignment ring eventually anyway, so I'm just going to ignore it and go on.

I really recommend using the pads. It shortened the amount of time I needed to polish the mirror tremendously, and perhaps got me to the point where it is fully polished. I might not have gone that far had I done it with a pitch lap and been tempted to start figureing too soon.

So now I am ready to start getting the shape to a sphere, in preparation for getting it to the shape of a paraboloid. And for that I will need a pitch lap. But while I am waiting for the Lap mold to arrive from Kevin McCarthy at, I can practice using my Foucault tester.

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