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In some instances it is desireable to have both long and short frame times. One example would be the case of a grating spectrum of a bright source. The images of the zeroth order, and possibly bright spectral lines could be saturated or highly piled up with multiple photons, while the continuum and weak lines are not. If the exposure time is made very short the pile-up may be avoided, but the efficiency of the observation is greatly reduced by the need to wait for the full 3.3 seconds for the framestore array processing.
With Alternating Exposure Times all CCDs are clocked in unison, but have two exposure times. One short exposure is followed by n long exposures (where n is configurable up to 15). The short exposures are used to reduce photon pileup, and the long exposures are normal ones for fainter observations. A typical choice of long and short exposure times would be 3.3 seconds and 0.3 seconds. In this case, if the duty cycle of long exposures is 1:n (short:long), then the efficiency, , is: