To: "John Sherman" ,
From: Mark Suchting 
Reply-To: Mark Suchting 


At 09:24 PM 8/10/2003 -0700, John Sherman wrote:

>A few people asked me if I still have the original interferometer data from
>my Galaxy mirror. Yes, I do. See www.johnspics.com/hi/hudek.jpg

The shame about this mirror ( in its original form ) is that most of the
error is just pure overcorrection ....1.6 waves  in fact, which refocuses
at best fit to 0.4 waves P-V ( with focus subtracted,  as a shown on the
interferogram). This equates to 15% overcorrection for a 22" F4.5.   
Another 30 minutes polishing down the 70% zone would have produced
a much finer result

I note also that the section showing what `terms'  have been removed from
the data has been chopped off the printout  ie focus , astigmatism. . I'm
well familiar with what  Zygo `Zap' software printouts should look like in
completeness.

I would suspect from the three equispaced bumps on the phase map, that
astigmatism has been subtracted too. This seems to be standard practice
amongst companies offering interferograms, because of the astigmatism
induced by self weight deformation, but I would question how much effort
goes in to analyzing how much real astigmatism these mirrors might have,
before it is `subtracted out'.

Probably the only way to produce an honest interferogram of a large thin
mirror is to build a testing tower so that the optical axis can be vertical
and the mirror lying flat on a flotation cell. Standard practice for large
professional mirror labs.

~Mark Suchting

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