ATM Astigmatism on 20 inch thin mirror

["David Lonard" <lonard@hotmail.com>]

Hello Group,

I would like to thank all those who have given advice.
So far the biggest consensus that I have received thus
far has been to extend my stroke to bring the sphere out
further to the edge, which I have been doing and seems
to be working well.  I have still been concentrating a
lot of strokes along the more spherical axis since I
still think that the edge is higher along this axis.
Along the more hyperbolic axis, I am using a longer W
(or V?) stroke.  I have an updated ronchigram photo at

http://www.geocities.com/lonard3/mirror.jpg

I have also made what I hope is a more reasonable star lap
as was suggested.  Several of you have said that I should
wait on the star lap, but I have tried using the full
lap and things go wildly oblate spheroid when I do so.
At any rate, the ronchi bands are getting rather straight
with this current lap so I think it is currently useful.

To answer your question about machine work Dennis, when
the surface came to polish, the ronchi pattern was packed
with multiple nasty zones, too many and too deep to honestly
know if I had astigmatism at that point.  The 12/24/02 image
is the product of probably 20 hours of subsequent hand
polishing.  I actually did put it back on the machine,
but strong zones started to return, so I stopped.  To
be fair, my machine isn't very mechanical sound, made
out of wood and was practically destroyed during the
violent hogging out process.

I'm sure that there is still plenty of interest in a
20 x .875 inch mirror so I will try to give more
updates.

David






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