Re: ATM Am I done yet?

["James W. Burrows" <burrjaw@earthlink.net>]

At 11:56 2002-09-17 -1000, Mike Warne wrote:

>Suddenly I am at ~ 1/6 wave PV.
>Sixtests shows strehl of .94
>
>I often here people talk about 1/8 wave being their target or goal.  I was 
>surprised to see .94 strehl with a 1/6 wave pv.
>
>So.. I am at the cross-roads now,  should I leave it as-is or work it a 
>little more... I know it will be my decision in the end..

Once again I'll quote, as many times in the past, Lord Rayleigh saying 1/4 
wavefront of primary spherical aberration caused a 20% reduction in the 
"irradiance at the Gaussian focus" (which is correct to 4 decimal places) 
and more than that caused image degradation.  The relationship between P-V 
and Strehl ratio is completely different for other aberrations, 
particularly the amateur-generated ones <g>.  Thus, the important part of 
Rayleigh's statement is the 20% reduction; all the optics gurus agree that 
a Strehl ratio of .8 "implies a good quality image".

I've experimented with smooth theoretical aberrations, trying to find the 
1/4 wave mirror with the worst Strehl ratio - it wasn't too bad, something 
like 22.5 nm surface RMS, .77 Strehl ratio.  On the other hand, some 1-wave 
mirrors, with most of the error in the central zones, passed with .8 Strehl.

On to Mike's mirror.  Sixtests says 10.2 ± 3.6 nm (2 sigma) surface RMS on 
the first diameter, and 10.0 ± 3.6 nm on the second.  I'd say you're done; 
it's almost statistically impossible that it exceed the 20 nm RMS which 
gives .8 Strehl.  Try a star test for a second opinion, taking into account 
seeing, collimation, diagonals, eyepieces, etc., etc., then package it up 
and off to the coaters.

         -- Jim Burrows
         --              mailto:burrjaw@earthlink.net
         --              http://home.earthlink.net/~burrjaw
         -- Seattle      N47.47233, W122.36620 (WGS84)