Re: ATM Autocollmination Flat Tolerance-2

[Mark Suchting <masuch@dap.CSIRO.AU>]

On Tue, 30 Jul 1996, ric rokosz wrote:

> With what you've said about flatness,it makes me ask the obvious question 
> with regards to substrate for the flat.Would plate glass be in the 
> acceptable range,or would coarse annealed pyrex be the neccesary 
> material,for the atm?

Ric

If the `flat' could be located in a temperature stable enviroment it 
would probably be adequate, with some insulation around the edge of thr disc.
However, if you are investing the time to make a reference element 
it makes sense to use the best material you can afford. 

With plate glass running at typical thickness of 1-1.25" it would be
unlikely to produce a flat more than 12" anyway, and the differnence in
cost would not be great. I made a 16.5" plate glass flat from 1.25" 10
years ago but it was more of an exploration in the Ritchey-Common test. I
suspected that it suffered astigmatism in the test stand....so something
thicker may have been better. 

Mark