[ATM] Polishing to a sphere

[Gilbert Gagne <ggagne@clearwire.net>]

Hi all!
I've been manually polishing a 12" plate glass mirror trying to get to a 
sphere. The back has been ground to the same radius of curvature as the 
front which is 134.4" (f/5.6) and it is 0.75" thick, with max. wedge of 
0.0005". It ultimately will be a "flex" mirror, but first its figure 
must be made spherical. It is completely polished (CeO) out but its 
figure seems to be resisting all efforts to be spherical. The polishing 
lap has been properly prepared full size with gugolz 64 pitch and 1" 
squares (not touching). Polishing was done with 1/3 stroke MOT 
center-over-center. The last hour of polishing has been done in 10 
minute spells alternating with half hour cold pressing at about 20 lbs. 
To get uniform pressure on the convex top when cold pressing, I use a 
12" beach ball nearly filled with water. The Ronchi-grams below (80 
lines/inch) were taken with a webcam and red LED after allowing several 
hours cooling time. They are separated, in time, be three polishing 
spells. Its hard to see much change. I must be doing something wrong, 
but what?

http://www.atmlist.net/contrib/ggagne-at-clearwire-dot-net/113-1.jpg
http://www.atmlist.net/contrib/ggagne-at-clearwire-dot-net/113-5.jpg

Constructive comments will be greatly appreciated.
Gil Gagne
Port Orange, FL
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