Re: ATM 6"-8" and 10" - 12.5" mirror/other components

[Buck Childers <buckchil@sound.net>]

On Wed, 23 Jul 1997, Mike Pence wrote:

> I just found a 10" mirror of an f/ratio of between f/5.6 to f/7.  He is
> selling it for $100, but it is not coated.  It is, however, ground and
> polished.  My concern is whether it is ground correctly and if that were
> the case, I'd have to have that redone since I don't have the time nor
> experience to do it.  
> 
> Any thoughts/advice?

Mike, the $100 sounds like a great price.  You will at least want to test
it.  The first thing is that it should be completely polished out.
look for any signs of scattered, reflected light when you pinpoint
magnified sunlight on it's surface.  Even though it is uncoated, if you
have a scope to put it in you can 'star test' it looking at the airy disk
inside and outside of focus.  You can also test it in the scope with a
ronchi screen at the eyepiece.  If you don't have the OTA you can bench
test the mirror using a focault test or ronchi test. Both these test
require a testing setup you can build.  

If any of these terms sound alien to you search for them in the ATM
archives.  After a while browsing through these archives you will probably
feel confident enough that you can refigure the mirror if you need to.  The 
archives are at:
http://www.system.missouri.edu/ics/staff/andy/ATM/search.html

Good luck.
-Buck