[APML] BIG QUESTION

[Glenn Shaw <glennshaw@alaska.net>]

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


--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>