Re: ATM Astigmatism on 20 inch thin mirror

["Mutalib Abdallah" <omegatroid@hotmail.com>]

If I remember correctly, stress-induced astigmatism was used in Art 
Leonard's unobstructed designs.  He used a warping harness.   It should be 
possible to calculate for your mirror the amount of astigmatism that a small 
force, say 1/10 ounce, induces.  Then you will better understand your 
problem.   In fact, there may be published articles by Art in the Maksutov 
Circulars that show you how to calculate this.

Can you guarantee that the terry cloth provides equal support to within 1/10 
ounce in all sectors of your mirror?  I think not.

I would not give up yet.  You have created a problem for yourself, and now 
it is up to YOU to solve it.  That means that books from a long time ago and 
advice from people who have never actually done such a thing (including 
me)... are all worthless.

Your biggest mistake was probably failing to realize how flexible your glass 
is.   I suspect that if you apply a load by hand to it, you will be able to 
SEE the warping in the reflection of distant objects... no tester needed.  I 
also suspect that the figure will be distorted in worse ways than 
astigmatism.   Remember the fairy story of the princess who was identified 
as such because she complained about the discomfort of a pea under her 
mattress?  That mattress is your glass.  It tends to pass the smallest lumps 
underneath to the surface.





>From: "David Lonard" <lonard@hotmail.com>
>Reply-To: "David Lonard" <lonard@hotmail.com>
>To: atm@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
>
>
>
>
>
>_________________________________________________________________
>MSN 8: advanced junk mail protection and 3 months FREE*. 
>http://join.msn.com/?page=features/junkmail&xAPID=42&PS=47575&PI=7324&DI=7474&SU= 
>http://www.hotmail.msn.com/cgi-bin/getmsg&HL=1216hotmailtaglines_advancedjmf_3mf

_________________________________________________________________
MSN 8 limited-time offer: Join now and get 3 months FREE*. 
http://join.msn.com/?page=dept/dialup&xAPID=42&PS=47575&PI=7324&DI=7474&SU= 
http://www.hotmail.msn.com/cgi-bin/getmsg&HL=1216hotmailtaglines_newmsn8ishere_3mf