AAS 207th Meeting, 8-12 January 2006
Session 154 Planetesimals, Protostellar Disks and Cosmic Rays
Oral, Wednesday, 10:00-11:30am, January 11, 2006, Balcony C/D

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[154.08] Automatic Recognition of Cosmic Rays at Deep Impact CCDs

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

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