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ATM Astigmatism on 20 inch thin mirror



Hello group,

I would like to get some input on a mirror that I'm working
on.  It was a 20 inch .875 inch thick (at edge) float glass
mirror blank purchased from Dan Cassaro to work on as a first
time mirror project, in spite of the many warnings that I have read
about trying such a bold attempt.  I'm figuring it as a f/4.7
parabola.  Images from a recent ronchi test (.35 inches
outside ROC, 200 line/inch grating, moving source) are included
at

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

The blank was ground and initially polished by machine,
then polished by hand using a 16 inch tool.  Lately, I have
been using a star lap (pictured at the above http link),
to keep the figure from wanting to assume a more spherical
or oblate spheroid shape.

So far, I realize that I have a severe problem with
astigmatism.  In the two images, the mirror has been
rotated 90 degrees along the axes of the minimum and
maximum amount of parabolization.  I'm presently attacking
the problem by concentrating strokes along the more
spherical axis using mostly COC with a slight amount
of W stroke.  It appears to be working to some extent,
but is resolving very slowly.  More recently, I have also
been using a shorter stroke to attempt to correct what
I perceive as a broad rolled edge along the more hyperbolic
axis.

I would like to know if I'm doing the right thing.
Can this be fixed without going back to grinding?
I don't know how I managed to wind up with the astigmatism.
On the machine, the mirror was not rotated to avoid
generating a new center.  Upon hand polishing, I have
been rotating the mirror and have kept a wet thick teri
cloth towel underneth it.

Thanks for any tips.

David Lonard





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