To: atm{at}shore.net
From: Jim Burrows 
Reply-To: Jim Burrows 


At 15:19 2003-05-06 -0500, Stephen C. Koehler wrote:

>Can anyone give me any pointers to papers, etc., on simulating atmospheric
>turbulence?

I think any college optics text would do the job.  I use M. V. Klein &
T. E. Furtak, "Optics", ISBN 0-471-87297-0, but I'm sure there's
plenty others.

>Suiter uses a fractal perturbation of the phase portion of the
>pupil function.  I'm not sure that is the best that can be done.  In
>particular, star test patterns created this way
>
>     1. tend to lack the radial spikes you often see in an actual star test,
>     2. don't display the cellular nature of actual turbulence, and
>     3. don't show much of a difference inside and outside focus.
>
>It's possible that I'm limited by calculating diffraction patterns using
>the Fraunhofer approximation.  I suspect that real turbulence affects more
>than just the phase of the pupil function.
>
>I haven't yet tried mucking with the transmission function.  Perhaps that's
>where the spikes come from.

The above optics book says Fraunhofer (far-field) is OK for images near the
focus.  Since you're fiddling with the phase across the pupil, you're
already "mucking with the transmission function", the only thing
left (besides details of the phase variation in time and space) is the
transmission function magnitude, which probably wouldn't have much effect.

It could be that you're not using enough points in the FFT - the distance
between the points determines the highest frequency visible, i.e., the
spikes.

         -- Jim Burrows
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         -- http://home.earthlink.net/~burrjaw
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