From: "scott rainwater" 
To: lonard{at}hotmail.com, atm{at}shore.net
Reply-To: "scott rainwater" 




My first mirror I had the exact same problems. I tried to correct the
astigmatism by polishing but that took too long. I just went back to a fine
grit size and ground for 15-20minutes while rotating alot. I then polished
while rotating alot. Then the astigmatism was gone. Although your mirror is
much thinner I believe it could work.




>From: "David Lonard" 
>Reply-To: "David Lonard" 
>To: atm{at}shore.net
>Subject: ATM Astigmatism on 20 inch thin mirror
>Date: Tue, 24 Dec 2002 23:59:46 -0600
>
>
>
>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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