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S.I. Ipatov, M.F. A'Hearn (University of Maryland, College Park), Deep Impact Team
The number of cosmic rays on images made by different
cameras (HRI VIS, MRI VIS, ITS VIS, HRI IR) during the
flight of Deep Impact to Comet Tempel 1 was studied for
out-of peak and in the peak (during a flare) of solar
activity. Both dark images, which contain only cosmic rays,
and normal sky images were considered. We analyzed the work
of several programs (imgclean, crfind, and
di_crrej) written by several authors and deleting cosmic
rays from one image. These programs run well in many cases,
but usually they do not work well with raw images, some of
the programs have problems with infra-red images and with
long (oblique entry) rays, and they delete pixels near the
edge of a comet. For infra-red images, imgclean has
less problems than other two programs. We have developed an
algorithm which allows one to recognize most (but not all)
cosmic rays using only one CCD image and which works both
with raw and calibrated images. In some cases (e.g. for
deleting cosmic rays near a bright star), it works better
than the above programs, but for many calibrated images it
has no advantages. After the work of our program, pixels of
deleted cosmic rays look like neighboring pixels.
Crfind and di_crrej only find pixels corresponding
to cosmic rays. Analysis of different dark and usual visual
images showed that for exposure time t>4 seconds most
objects on an image consist of not more than 4 pixels and
these objects are caused mainly by hits of cosmic rays.
Glitches of large rays have a linear form in contrast to the
more circular form for stars. We considered that an object
is a ray if the ratio tpix/(dx2+dy2)<0.17 (where dx
and dy are maximum differences of coordinates x and y
of an object, respectively, increased by 1; tpix is the
number of pixels constituting the object). For most HRI and
MRI visual images made during low solar activity at t>4 s,
the number Nsc of objects on image per second per
square centimeter of CCD was about 2-4, both for dark and
usual images, and mainly there were no rays consisting of
more than 2t pixels, where t is the exposure time in
seconds. For t=30 s there were about 150, 100, 50, and 40
objects consisted of 1, 2, 3, and 4 pixels, respectively, on
one dark image, and about 170-200, 110-150, 40-60, 40-50
objects on a typical sky image. At high solar activity,
Nsc sometimes exceeded 10. The ratio of the number of
cosmic rays consisting of n pixels obtained at high solar
activity to that at low solar activity was greater for
greater n, e.g., it was about 1-2 for n=1 and about 4-10
at n=5. At 0.004 If you would like more information about this abstract, please
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.