Re: ATM Foucault - Interferometer Test Comparison

[Michael Peck <mpeck1@ix.netcom.com>]

"Thomas Bennett" <tbennett@impulse.net> wrote:


>While rummaging through the ATM archives, I ran across the following post
>fragment from about a month ago:
>
>I am not aglass pusher nor do Iunderstand it very well, therefore I need
>some help in determining the quality of a mirror.
>I bought a mirror this pastyear that was a 16" Meade . I sent it off
>to have it tested and if neccessary have it refigured and coated. The
>mirror came back with the following results.
>residual wavefront variations evaluated at fringe centers (in waves)
># of points peak valley P-V RMS Strehl
>Ratio
>525 0.149 -0.132 0.281 0.066
>0.844
>residual wavefront variations over uniform grid (in waves)
># of points peak valley P-V RMS
>Strehl ratio
>2783 0.162 -0.137 0.299 0.064
>0.849
>
>
>If memory serves me, I believe that the poster bought this mirror from me

>as a refigure project. Prior to selling it, I e-mailed him the results of a
>Foucault test on this mirror to give him some indication of its quality and
>being an e-mail packrat, the results were archived. So looks like what we
>have here is a comparison of a Foucault test and interferometer test on the
>same mirror. In case anyone is interested, the Foucault test results were:

If I remember the original thread correctly the interferometer results were 
for the refigured mirror, so even if it was the same chunk o' glass these 
tests may not be on the same mirror.

Anyway, Dave Rowe's Figure45 calls it P-V = 0.39, RMS = 0.074, Strehl = 
0.81 from your foucault data. That's consistent with the refigure job 
having improved the figure, at least a little bit.

I don't see an indication of the test wavelength in this snippet of 
interferometry data. If the test was done at ~630nm the foucault data and 
interferometry are consistent, because I think Figure45 references 
wavefronts to 550nm. That would make the P-V and RMS values a factor of 
630/550 = 1.15 worse.

Mike Peck

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Michael Peck
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