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The read out noise for the CCD chips used on La Palma is given in Table 2.1. This is typically 5-10 electrons per pixel for the GEC P8603 and about 60 electrons per pixel for the RCA SID501
Cosmic ray events become important for longer exposures. During an exposure cosmic rays hit the CCD. When a pixel is hit, its charge content is vastly increased and the resulting `spike' in the image needs to be removed during the data reduction stage. This can be done either by numerical filtering techniques using the fact that the appearance of a cosmic ray event is quite distinct from that of an astronomical object, or by combining multiple exposures of the same object. The former process is likely to be interactive and (therefore slow) and a ``patch'' with information in the strike zone is irretrievably lost. The latter is recommended.