Re: [ATM] pitch&strehl.

[Jim Burrows <burrjaw@earthlink.net>]

At 2005-07-14 07:11 -0400, Mark Holm wrote:

>Also, I am not sure you can get the Strehl ratio of a multiple surface 
>system by multiplying Strehl ratios of the individual surfaces.  The 
>Strehl ratio calculation from RMS is decidedly non linear and estimating 
>the RMS of a system from the RMS of each component is also non linear.

It turns out that you can multiply the Strehl ratios of multiple surfaces 
to get the system Strehl, subject to two assumptions:
         1)  Strehl ratio is given by Mahajan approximation = 
1/exp((4*pi*R/L)˛), R = surface RMS, L = wavelength
         2)  Surface errors are uncorrelated => system RMS˛ = sum((surface 
RMS)˛).
That square in Mahajan's formula makes the Strehl ratio linear in the RMS 
squared.

         -- Jim Burrows
         -- mailto://burrjaw@earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)  

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