Re: [ATM] Updated web site: mirror testing and Draper machine

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

At 20:56 28-11-04, scott wrote:
>I just wanted to let people now that I have update my web site. The Round 
>Robin testing page has been updated to include a link Roger's 
>interferometer report.  His analysis makes very interesting reading.
>
>http://tlepage.home.mindspring.com/RR%20Reports.html
>

I have to disagree a little bit with one of Roger's conclusions. According 
to numbers Jim Burrows calculated (which I haven't checked) the Foucault 
testers on average were about 25% low in their RMS estimates compared to 
Roger's interferometry on mirrors A and B, where the dominant defects were 
overall correction. That produces Strehl ratio estimates 7.5% and 4.5% too 
high on those two mirrors. The Foucault testers also correctly estimated 
the overall state of correction on mirrors A and B. On mirror C where the 
dominant defect was astigmatism they were significantly too optimistic (by 
about 75%), but again they had the overall correction right. This mirror 
apparently suffered from almost pure primary astigmatism, with any cross 
section looking like a good parabola.

These results tell me that the RMS and Strehl numbers squirted out by 
programs like Sixtests and FigureXP can have a meaningful relationship to 
reality provided the actual errors are reasonably symmetrical. Global peak 
to valley estimates on the other hand are useless, no matter how they're 
measured. Amplitudes of low order aberrations can be useful, but most 
amateurs don't know how to interpret Seidel wavefront aberrations. [How can 
a mirror with 1/4 wave of spherical aberration be good, even excellent? 
Because 1/4 wave according to Seidel is really 1/16 wave.] Zernike 
coefficients would be better, except the traditional definition of Zernike 
polynomials scales them wrong, making it impossible to interpret them 
without additional calculations. Physicists who work on optical problems 
typically use the correct definition of Zernikes. Too bad the optical 
engineers haven't caught on.

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 

_______________________________________________
ATM mailing list http://www.atmlist.net/