de Cuyper, J.-P. ; Hensberge, H.

published in Astronomical Data Analysis Software and Systems IX, 216, pp. 535 (2000)

Abstract: Bias frames taken with CCDs should be homogeneous in intensity. In practice, they fulfill this requirement often sufficiently well, although deviations from homogeneity may be detectable. However, sometimes a clear pattern is superposed on the frame, and this pattern may vary with time. Using a large number of bias frames obtained in one specific run, the time-dependent pattern was found to be built up of a fixed one-dimensional periodic function whose phase-shift varies from row to row. It was successfully removed from calibration and science frames through the use of pre- and upscan ``bias'' pixels available in each of these frames.

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