Re: ATM Astigmatism on 20 inch thin mirror

["Mel Bartels" <mbartels@efn.org>]

'Stig in a 20 inch super thin mirror can easily be generated by lack of a
perfect support when fine grinding.

Since you sound like you have a handle on where to polish to remove the
stig, polishing it out may be faster than returning to fine grinding.
Remember that the odds that some 'stig will remain after the second fine
grinding attempt are non-zero.

Do extend the strokes of your figuring tool out over the edge of the
primary.  Don't try to fix the figure and fix 'stig at the same time - it
cannot be done.  Do keep your method of rotating the mirror small angles
every five minutes or so.  Any imbalance in your support will be averaged
out into zonal issues which can be fixed later.

Mel  Bartels