Re: [ATM] Calculate Astigmatism

[Douglas S Angle <Douglas.S.Angle@can.dupont.com>]

At 2004-03-18 21:11 -0500, Robert Turner wrote:
>I was wondering if it would be possible to calculate astigmatism from
>focault readings.

to which Jim Burrows replied:
>The main trouble is the astigmatism that messes up the image the most is
>primary astigmatism...  To evaluate that with Foucault is nearly
impossible because it
>needs ROC measurements almost to the nanometer on different diameters.


which kind of surprised me, because
At 08:17 10/18/2000 -0400, Douglas S Angle wrote:
>I've been fighting a problem with astigmatism on my 16" mirror.  Below
>are the foucault readings, and the Sixtests output, based on two
>different diameters.  Both are pretty close on their own, but I can't
>decide if the difference between the two is significant.
>...
>Sixtests input files:
>October 16 arrow at 12 oclock ...
>October 16 arrow at 9 oclock...

to which Jim Burrows replied (Thu, 19 Oct 2000 19:31:13):
>This is sort of guess-work, but I'd be most worried about the different
>focal lengths of the two diameters.  Running sixtests, I get 2498.6 mm for
>12 o'clock, 2499.0 mm for 9 o'clock = 0.4 mm difference.  Born & Wolf,
>"Principals of Optics", 7th edition, p. 491, eq. (27) says the focal
>tolerance (.8 Strehl at the focus offset) is 2(f/)^2*L, L the
>wavelength.  For Doug's mirror, 2(2500/406)^2*550E-6 = .04 mm.  Since we're
>only talking about half the mirror, maybe you could get away with .1 mm;
>.4's too big.

I thought my question in 2000 was essentially the same as Robert's last
week.  But "0.1mm" is a far cry from "almost to the nanometer".  I still
haven't quite figured out if the sixtest output of focal length is the
actual focal length, but I was satisfied with the answer at the time, and
proceeded on that basis.  Now I'm back to being confused.  What am I
missing?

Thanks,
Doug



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