Re: [APML] BIG QUESTION

["Paul M. Rybski" <rybskip@idcnet.com>]

Dear Glenn,

      I've done a lot of optically guided spectroscopy in my life.  My 
suspicions are that you are looking at the effect of bad seeing in 
the clouds themselves, not just scattering.  Of course, it could be 
that your optically thin assumption is where the modelling is failing.

Paul

>Hello everybody:
>
>When photographing stars through very thin cirrus cloud, the stars come
>out as "blobs". Why?
>
>The classical manner of treating this problem is to turn it into a
>radiative transfer problem where the ice crystals are replaced by
>equivalent spheres, of 10 to 30 micrometers diamter, with the refractive
>index appropriate for ice substance and use Mie theory, which is the
>general theory for plane wave scattering from spheres. I've done this
>for optically-thin cloud and find that the scattered light lies mostly
>in a cone of 5 to 10 degrees diameter. Thus, in photographing stars
>through optically thin cirrus, one should find the original star as a
>"point" (only limited by the usual aperture, and seeing and optical
>abberations, etc), surrounded by circular smear of several degrees. That
>isn't at all what's found.
>
>What's found is that the star images spread out to several minutes of
>arc. Probably there IS a smear from scattered light, extending out to 5
>to 10 degrees from the star, but not detected in the H-D curve because
>of its low radiance.
>
>Some people like to photograph star fields through thin ci, or,
>equivalently, place a diffraction "screen", literally such as a window
>screen, in front of the objective to bring out the bright stars and
>bring a "constellation" into view. So it isn't necessarily always "bad"
>to photograph through thin cirrus. But theory and results differ and I
>am very curious why.
>
>Ideas?
>
>Glenn Shaw
>
>
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________________________________________
Paul M. Rybski, Ph.D.
Associate Professor and former Chair, Dept. of Physics, and
Director, Whitewater Observatory
Goodhue 320
University of WI-Whitewater
Whitewater, WI  53190-1790

Office FAX:     (414) 472-5633
Email address:  rybskip@uww.edu

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