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RE: [ATM] Another Astigmatism question, but at the sphere stage

At 12:16 8/30/05, Jerry wrote:

>What would be the proper term for the shapes and the aberration caused by
>these different defects?

I like "localized defects," but there's nothing 
wrong with old mirror makers' jargon like 
"primary ripple, dog biscuit, zones," etc. I have 
no doubt at all you can see stuff like that with 
Foucault, in fact I've written a post or three on 
that subject myself. The original question in 
this thread concerned primary astigmatism though, 
which in classical aberration theory is focus 
shift with azimuth angle where the angle between 
the long and short focus is 90°.


>What would be the proper terms regarding the shape of Mirror C of the "Round
>Robin Test Mirrors"? What parts of the shape are astigmatism, general
>asymmetry, and localized defects?

The major defect in mirror C was pure primary 
astigmatism as classically defined. I've lost the 
links to most of the various reports that were 
posted, but here's James Lerch's writeup: 
<http://lerch.no-ip.com/atm/MRR/Mirror%20C%20Results.htm>. 
I think his interferometry was basically 
consistent with others who did it, and his report 
shows enough astigmatism to make the difference 
between a good to excellent mirror and a mediocre 
one. Notice he didn't mention astigmatism in the 
Foucault part of his writeup. I think, from 
memory, that Guy Brandenburg was right that 
nobody in the mirror round robin who did Foucault 
testing picked up on the astigmatism.

A fringe traced interferogram doesn't really have 
the spatial resolution to detect defects like you 
were describing. Foucault is better for that -- 
in fact digitally captured images might be a little *too* unforgiving.

Mike Peck


_________________

Michael Peck
email mpeck1@ix.netcom.com
Wildlife photoblog! http://wildlife-pix.com
Amateur telescope making http://home.earthlink.net/~mlpeck54/astro/astro.html 

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