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

["Jerry" <wa4guu@verizon.net>]

Hello Mike 

The moment I wrote it I stopped and considered that I should not say "that
is astigmatism" and even considered other words. But then I realized that a
question would be raised by someone. And in the case of astigmatism I
confess I'm not sure I totally grasp the unvarnished truth of primary
astigmatism, or any other astigmatism precisely. I'm not so sure that
general asymmetries aren't astigmatism.

And then like people saying that the millennium began on Jan. 1, 2000 it
comes to be easier to switch than fight.

So from this level of confessed ignorance and trying to get a better
understanding and without really disagreeing with you let me ask more
specifically. 

To my eye the most prominent defect I see on Mirror C is the square shaped
defect of 4 high "corners" in this case not quite equally spaced around the
circumference.

Now if they had been equally spaced and of equal magnitude and the mirror
not otherwise flawed you would have no primary astigmatism. While there
would be many different radii on different diameters any two diameters at
exactly 90 degrees to each other would have the same radii. The 90 degree
requirement for it to be astigmatism is not met. What then do we call that
aberration.

The 90 degree definition implies that if only two of the high corners 180
degrees apart were there it would be astigmatism. What is the name of the
aberration when you have that twice on the same surface?

As for seeing it in the Foucault test, do you think that I would not have
been able to see them if there were only two raised areas separated 180
degrees?

Jerry


-----Original Message-----
From: Michael Peck
Sent: Tuesday, August 30, 2005 7:42 PM
To: 'ATM'
Subject: 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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