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A bias current is routinely applied to CCD detectors to ensure that they are operating in as near as possible linear manner. This current has the effect that a non-zero count is recorded in all pixels. The observer should ensure that a number of 'bias frames' are obtained to adequately estimate the effect. The average of the bias frames should be taken, and the result subtracted from all other frames before using them in a reduction. Failure to account for the bias may have a detrimentel effect on the S/N in the final spectrum.