[ATM] Another automated Foucault success

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

Group:

Now that I've joined the burgeoning ranks of amateur interferometerists 
I've finally had a chance to validate the work I wrote up years ago on 
"semi-automated" Foucault testing 
(<http://home.netcom.com/~mpeck1/astro/autof/autof.htm>). So far I've only 
done a couple of interferometric test sessions, both on the little 6" 
mirror I wrote about there.

Here's the main result. The Foucault test did what I intended it to do, 
which was to accurately measure the average radial surface profile.

For more details, I've uploaded some graphs:

a) The interferometrically tested wavefront map: 
<http://home.netcom.com/~mpeck1/astro/temppix/wf_inter.png>. This map is 
based on an average of measurements of 10 interferograms from my second 
successful testing session. Units here are waves at the test wavelength of 
633nm.

b) Surface error profiles along 3 diameters compared to the Foucault test 
estimate: <http://home.netcom.com/~mpeck1/astro/temppix/xsections.png>.

c) Estimated Zernike coefficients for the first 5 spherical aberration 
terms (3rd through 11th order), comparing Foucault to interferometry. I was 
going to offer a long winded explanation of what the coefficient values and 
error bars mean, but I'll pass for now. If you care feel free to ask.

This mirror also has considerable primary and 5th order astigmatism, which 
really surprised since it's a small, regulation thickness mirror that was 
polished and figured with a full size lap - and with a pro standing over my 
shoulder to boot. The astigmatism makes the difference between a marginally 
passable and marginally failing mirror.

My estimated RMS error from foucault testing was 15.2 nm, for a Strehl of 
0.89. Interferometry sez it's 25.9 nm, Strehl = 0.70. P-V estimated from 
Foucault was 59.7 nm on the surface or 0.22 waves on the wavefront. The 
interferometrically estimated P-V was 157.6 nm (0.57 waves on the wavefront).

We still don't know why automated Foucault testing sometimes works and 
sometimes fails. I'm not prepared to buy James Lerch's theory that it has 
something to do with astigmatism, at least not yet.

Mike Peck

_________________

Michael Peck
email mpeck1@ix.netcom.com
Wildlife photography page http://home.netcom.com/~mpeck1/index.html
Amateur telescope making http://home.netcom.com/~mpeck1/astro/astro.html 

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