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Section 1 described how in a well-designed CCD camera system, observations are limited by 4 chip parameters only: pixel size, quantum efficiency at the required wavelength, saturation level (electrons/pixel), and readout noise. One further factor may limit exposure time - cosmic- ray detection rate. The saturation level and the cosmic-ray detection rate limit the maximum exposure time, as detailed in Section 3b. First we consider calculation of the minimum exposure time, set by the signal- to-noise ratio (s/n) required.