Re: ATM Astigmatism on 20 inch thin mirror

["Bill Kelley" <flex@commspeed.net>]

Hey David,

Looks like the mother of all TDE!  If you keep your strokes short  it will
get worse because you are polishing the center deeper. Long W strokes will
slowly resolve the edge. If the mirror is supported without flexure, and you
are rotating both the mirror and the tool, as you get rid of the TDE the
astigmatism should clear up.

It may seem counter-intuitive, but, TDE is treated by polishing the edge.
IOW you have a hole in the center, you can't raise the hole, so you must
polish the edge down. As you do this a raised zone may develop in the
center - easy to remove later.
Good Luck and Seasons Best to you and our wonderful List,

Bill Kelley

Subject: 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