From: "David Lonard" 
To: atm{at}shore.net
Reply-To: "David Lonard" 



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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