[ATM] Re: ATM Digest, Vol 2, Issue 19

[mdholm at telerama.com (Mark Holm)]

artbianconi@blast.net wrote:



>Has it ever occurred to anyone that concentrating the weight of the entire 
>mirror on two points at the bottom, in a narrow 25 to 30% portion of its 
>perimeter, just might cause the glass to bulge some nearby?  Has anyone here 
>ever seen someone rotate the mirror 45 or 60 degrees and test it again? 
>Probably not. It probably wouldn�t make a difference anyway, the �flat� 
>doesn�t move. 
>
>  
>

With only an 8 inch diameter by 1 inch thick mirror, I can easily see 
the deformation from the support pegs under the Foucault test, 
especially if the mirror is setting so the load is carried near the 
front surface.

In the final stages of testing, I test on at least three diameters. 
There are better tests than Foucault and Ronchi for checking 
astigmatism. They are described in the standard texts, such as Texerau, 
and have been discussed on this list several times.

Many atm's have experienced the problem of grinding and polishing in 
astigmatism due to a poorly supported mirror.. Some have even done it on 
purpose to make a special purpose astigmatic mirror. The key to avoiding 
astigmatism is both to have adequate support and to rotate frequently to 
average out the effects of poor support.. (It helps a lot to start out 
with uniform glass.)

The question of adequate support during testing has also come up more 
than once. For most atms, vertical testing isn't an option. The mirror 
has to be rotated to horizontal. A sling seems to work out well for most 
larger mirrors, but as mirrors get larger, and or thinner in relation to 
their diameter, even sling support begins to show deficiencies.

Mark Holm
mdholm@telerma.com