Re: [ATM] Working with Large Thin Mirrors

["Bob May" <bobmay@nethere.com>]

Astigmatism is a bit more difficult to evaluate with such a thin mirror.
I'd start looking at whether the problem is from the mounting of the mirror
in the test stand or actually real astigmatism in the mirror itself.  You
also need to evaluate how much astigmatism there is in the mirror if that is
where it is coming from.
The VERY important thing when grinding and polishing, especially with a thin
mirrror, is to insure that you don't tend to grind or polish in a repetitive
manner where the work is preferentially done in one direction.  This makes a
saddle shaped surface and since the glass is so thin, it can often stress
itself so that the saddle shape stays when polishing in other directions.
The barrel walk becomes extremely important as well as learning how to turn
the glass with each and every stroke across the lap.  I'll also note that
the mirror needs to be constantly turned upon its support to insure that the
shape of the support doesn't get ground into the mirror's surface.  This is
one that is often ignored by the beginner thin mirror grinder.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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