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Cosmic ray events

These are an important source of additional noise on a CCD, limiting in practice the exposure time. Cosmic rays which strike the detector produce bright and narrow noise spikes in the signal. The effect on the data depends exactly on where on the detector the event occurs. The best way to deal with cosmic rays is to break long exposures into several shorter ones. This has the major disadvantage of increasing the total readout noise. If the total number of events is small they can usually be succesfully removed by software.