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RE: [ATM] Another Astigmatism question, but at the sphere stage
Well it seems tetrafoil it the aberration of interest. To me anyway.
That seems to be astigmatism twice. At least astigmatism a second time 90
degrees to the first.
But since on mirror C the 4 high areas are not at exactly 90 degrees it is
primary astigmatism not tetrafoil that is the major defect. So it is primary
astigmatism that I am seeing and the question of whether astigmatism can be
seen in Foucault is answered.
Right?
Jerry
-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Dale Eason
Sent: Tuesday, August 30, 2005 9:45 PM
To: atm@atmlist.net
Subject: RE: [ATM] Another Astigmatism question, but at the sphere stage
One way to represent the surface of a mirror is with
Zernike polynomials.
I like the simulator at the following link:
http://wyant.opt-sci.arizona.edu/zernikes/zernikes.htm
Dale Eason
--- Jerry <wa4guu@verizon.net> wrote:
> 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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