Re: [ATM] 6 point cell support for unknown glass.

[Mark Holm <mdholm@telerama.com>]

Russell Jocoy wrote:
> 
>      Mark,    I have a 3/4" x 13.1" plate glass mirror on a six point cell 
> (using the "bar" method).
>   I am very happy with the results. I do feel that this is the limit for 
> this diameter and thickness.

Plop says this arrangement should have about 3.8nm RMS of deformation
compared to the 2.5nm limit I usually go by.  Now, a little reality
here.  If you really end up with only 3.8nm RMS of cell induced
deformation,  you would never see it.  My 2.5 nm limit is based on the
idea that there will be imperfections, and that those can worsen the
support by a factor of 2 - 4.  At 8nm RMS, you might be able to see the
effect at high power and at 16, you probably could.  So, my rule of
thumb, though conservative, is designed to keep real world deformations
under about 10nm RMS worst case.  If you are not going to be pushing the
magnification up high, then you can let the rule of thumb loosen.  I
have been persuaded to let it loosen for 3 point cells since they are
pretty easy to build with little error.

My advice is based on the idea that one is working with very good
optics, and won't want to be cell limited when you try to push the
magnification up.  If either assumption is false, then loosening my rule
of thumb is fine.

-- 
Mark Holm
mdholm@telerama.com

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