Re: [ATM] Calculate Astigmatism

[Jim Burrows <burrjaw@earthlink.net>]

At 2004-03-18 21:11 -0500, Robert Turner wrote:

>I was wondering if it would be possible to calculate astigmatism from
>focault readings. My first guess was to take focault readings at different
>orientations and then determine the amount of astigmatism. From that
>calculate a strehl based on the spherical abberation and estimated
>astigmatism. Is this possible and where would I start looking for
>information on coming up with something?

The main trouble is the astigmatism that messes up the image the most is 
primary astigmatism, and it depends on other things - the mirror cell, 
tilt, etc.  To evaluate that with Foucault is nearly impossible because it 
needs ROC measurements almost to the nanometer on different diameters.

To estimate the RMS due to higher-order astigmatism (wiggles on the 
Foucault profile plots), my first guess would be to take the root mean 
square of the RMSs along each diameter tested.  I.e., let Rn, n=1, 2, ... 
up to the number of diameters tested, be the RMS output for each profile, 
then the total RMS would be
         sqrt[(R1²+R2²+...)/n].

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

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