The read out noise for the CCD chips used on La Palma is given in Table . This is typically 4-5 electrons per pixel.
Cosmic ray events become important for longer exposures, and are a worse problem with Tektronix chips than EEV. 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.