ATM - Astig and MOT

[Richard Schwartz <richas@idt.net>]

The last mirror I made was an 8", only 3/8" thick.   I had a terrible
time with astigmatism because I polished with the tool on top and the
mirror resting on about 20 pitch bumps.

I corrected the astigmatism by taking the mirror off of the machine and
working by hand with the mirror on top.   I was careful to touch the
mirror only at or near its center.  The idea was to let the mirror
surface come into equilibrium with the pitch lap surface in a
stress-free condition.   Anyhow, it worked.

So if you want to avoid astigmatism, you should let the mirror relax on
top of the pitch lap for long periods of time, then polish with no
stress applied to the mirror (touch the mirror only at its center).   In
fact, you might save a little work by beginning this technique in the
last few wets of final fine grinding.

. . . Richard