2354 20160701171704.0 ASTROimport-511 de Cuyper, J.-P. 2000 The Removal of Periodic Read-Out Patterns from Science Frames 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. Hensberge, H. Astronomical Data Analysis Software and Systems IX 216 2000 535 http://esoads.eso.org/abs/2000ASPC..216..535D published in NONREF