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Mirror Figuring

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I cast some concrete tools reinforced with glass fiber using the blank as a mould. I then covered the surface with 1" square tiles. As the curve had already been generated I started with sandblasting grit. This turns to sludge pretty quick but is cheap and I think leaves smaller pits than 80 carbo. The marks left by the diamond wheel were quickly removed then it was on to medium then fine powers. The only hiccup occurred when I knocked the side of the mirror against a piece of wood when moving it. The crack while small and not near the surface was still unwelcome.
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I had started the mirror in June and by August I had a mirror I could start to figure. The parabolisation was difficult (f now 3.8) and lots of correction is needed. I ended up using 3 different sized laps and a small tool. When I finally got the reading to 1/4 wavelength I had picked up some astigmatism. The mirror was 1/4 over in one axis and 1/4 under in the other. For such a large mirror this isn't bad (I think). However I tried to fix it but made things worse. Ended up going back to medium grit and starting again. October I was again figuring. Again got the figure close when astigmatism set in. Gave up at this stage and sent the mirror to be figured professionally by Norman Oldham (Oldham optical). The problem was apparently that the back of the mirror wasn't flat. Norman ground the back flat then completed the figuring. Norman also made a 100mm flat.

 

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Copyright 2006 Steve Chambers. All Rights  Steve@pmdo.com

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