Re: ATM John Sherman's concern with 22" Mirror

["Jan van Gastel" <jhm.vangastel@wanadoo.nl>]

My 20" mirror shows very difficult visible rings (sometimes invisible)
outside focus and I was told that this is caused by the fact that isn't
cooled off enough. Can both be true? I'd like to have an explanation,
because I don't understand how this works.

Jan
http://www.geocities.com/janvangastel/
----- Original Message -----
From: "Alan Garcia" <ngarcia@columbus.rr.com>
To: <atm@shore.net>
Sent: Friday, April 20, 2001 11:14 PM
Subject: ATM John Sherman's concern with 22" Mirror


> I concur with
> one respondent that the mirror may be over corrected, but from Mr.
Sherman's
> description I believe there is more going on here.  His description of the
> diffraction rings inside focus(or the lack thereof)  is EXACTLY what my 2"
> thick primary looks like inside focus until it cools off.  Until I get the
> mirror to within a reasonable degree of the ambient air temperature, it
> looks horrible!  However, once it cools, I get what are for all practical
> purposes text book diffraction rings inside focus.
> Alan Garcia
>