[ATM] Second Thoughts on Mirror Support

[djd521 at comcast.net (Don D'Egidio)]

Dale,

I didn't know the Foucault test was blind to astigmatism. There is no signs of any when using my  scope. I was present at one of our clubs TM class and there was a mirror under test that was cemented to the adjustment plate and it showed definite signs of astigmatism, which disappeared when it was cut free from the plate. I think it was a 6" F8 mirror and after it was cut free showed a very decent figure. What a difference the method of mounting made to the testing.

Don

----- Original Message ----- 
From: "Dale Eason" <atmpob@yahoo.com>
To: "The Amateur Telescope Makers List" <atm@atmlist.net>
Sent: Tuesday, February 03, 2004 14:16
Subject: Re: [ATM] Second Thoughts on Mirror Support


> Thanks Don,
> The reason I asked was that much of the distortion for
> a mirror in the horizontal position as you described
> would be due to astigmatism.  The Foucault test is
> blind to astigmatism.  So saying that the mirror
> tested fine under these conditions does not confirm
> that the cell did not induce astigmatism distortion.
> 
> 
> Dale
>