RE: ATM 8" F/7 Astigmatism

["CSC" <optic@gwi.net>]

Sure.  I used the pinhole to clearly decipher two warped mirrors.  I had
poorly supported them at three points and fast TOT polished in a trifid
pattern.  It was confusing with a slit and KE, but the irregularity was
clear with the star/pinhole.

A 10" f/6 confused me with odd shadow readings which I concluded was due to
astigmatism.  In fact it was a decentered paraboloid.  The pinhole test
revealed the lack of astigmatism.

Try everything, sift it later.

Colin



-----Original Message-----
From: omegatroid [mailto:omegatroid@hotmail.com]
Sent: Sunday, October 19, 2003 3:26 PM
To: optic@gwi.net
Subject: RE: ATM 8" F/7 Astigmatism


Did you ever test a mirror for astig, or did you just see the eyepiece
method in a book?  There are  much better ways that are compatible with a
slit teester.


-----Original Message-----
From:	"owner-atm@shore.net" <owner-atm@shore.net> on behalf of	"CSC"
<optic@gwi.net>
Sent:	Friday, October 17, 2003 11:10 AM
To:	"Atm" <atm@shore.net>
Subject:	RE: ATM 8" F/7 Astigmatism


This is a case where you need more kinds of tests to really confirm what
you're doing, but I believe your results do show astigmatism. replace your
slit with a pinhole (fairly small) and observe the image with an eyepiece.
It should be oblong and will rotate when you rotate the mirror.  .036 is
enough to ruin an image, but not too much to polish out. It could take a few
hours.

Why not try a couple polishing spells to shorten the long diameter's radius?
If done slowly, you will see the .036 drop.  When you can't tell anymore
with certainty, do some long spells of normal polishing, some with MOT, some
with TOT, and blend it all out.


Colin


-----Original Message-----
From: owner-atm@shore.net [mailto:owner-atm@shore.net]On Behalf Of
david.a.stevens@gm.com
Sent: Thursday, October 16, 2003 8:24 PM
To: atm@shore.net
Subject: ATM 8" F/7 Astigmatism




This is the latest for my astigmatic mirror. I have measured across 1
diameter of my mirror ( diameter 1-3 ) and I take an arbitrary measurement
at about the 70% zone. I rotate the mirror 90 deg. and take another
measurement  on the 70% zone  ( diameter 2- 4 ). Dia. 1-3 is closer by .036
and the shadow is oblate sphere ( high edge,high center). Dia. 2-4 is still
oblate but closer to a sphere.

What to do???

A.) Do I keep polishing towards a sphere and hope the astig. will go away
??


B.)  Do I polish along the short diameter ( 1-3) TOT for 15 or 20 min. with
no tool rotation??


Any help is very much appreciated,

Dave