$\star$ Many cosmic rays

Figure 8.3 shows what an image resembles after corrections for the effectiveness of CCDs. Distinguer another thing that the cosmic ones requires an exerted eye. This pollution forces to make several installations of the same field to be able to eliminate them.

Silicon is an excellent detector of cosmic rays, it is not thus astonishing to note that the very great majority of the visible objects is the trace of those. The number of cosmic by CCD was determined by holtzman199ä, it is of 3600 cosmic rays for an exposure time 2000 seconds, which represents a rate of approximately 2 cosmic a second and by CCD For an exposure time 2000 seconds, approximately 4% of the pixels are touched by cosmic. If one considers two 2000 seconds installations, approximately 1000 pixels will be affected on the two images, these pixels will be thus unusable.

The reading time of CCDs is relatively long: 3 minutes in the normal mode for the long durations, so as to minimize the noise of reading. Not to assign more than 10% of time to the reading forces to make only 2 installations for one hour a total integration. It is thus not possible to multiply the installations to eliminate the cosmic ones.

Figure 8.3: Rough image of the WFPC2. This figure shows the number of cosmic rays for 1000 seconds a typical exposure time.