Re: ATM Interferometry analysis of 25"

[Anthony Stillman <atmer@flash.net>]

Onece more into the breach.


>...25" galaxy mirror
>P-V     0.342
>RMS     0.046
>Strehl Ratio    0.921

>P-V     0.309
>RMS     0.044
>Strehl Ratio    0.927



As I wrote Scott, off list, these numbers are great.  I'm echoing it to the
list as it seems others disagree.  From what I can glean it appears that
some, as a judge of expected image quality are relying irrationally on peak
to valley values in conjuction with misunderstanding the Rayleigh
criterium.

In point of reference I note that in their paper "Effects of wavefront
aberrations on visual instrument performance and a consequential test
technique"  Applied Optics Vol. 26 pp 492-500 (1987), Haig and Burton
conclude that the Stehl ratio is the desired metric.  In this work a
veriety of wavefront aberrations were introduced to produce a
just-noticable-difference (JND) in image quality as compaired to an
un-aberrated image.   The mean Strehl ratio across these tests for the JND
was 0.870

It is generally accepted that a system may be considered diffraction
limited if the Strehl ratio is 0.8 or greater (1).  This criteria
translates into an RMS wavefront error of less than or equal to lambda/14

Based on the numbers above, this mirror, made by John Hudek, isn't just
good, its fantastic.

Now, no doubt someone, I know not who, is going to be ticked at me.


Anthony



1)  Marechal, Schroeder, Malacara (both of them)