The Bias is a constant offset (for each pixel) introduced by the readout electronics of the CCD. Its value is usually between 150 and 250 ADU (for CCD's currently in use at ESO).
As a first step, the short dark and pre-flashed dark exposures have to be averaged to produce the BIAS and pre-flashed BIAS frame. A method which performs the averaging and which at the same time discards bad pixels due to e.g. cosmic events is AVERAGE/WINDOW Before actually executing this command, prepare a catalogue containing all input files to be averaged via:
CREATE/ICAT catname dirspec
Choose appropriate values for the descriptors FLAT_BKG and LHCUTS and run the command
PREPARE/WINDOW inputspec flatbkg lhcuts
where inputspec is either a single file name or the catalogue with the list of input files, to be specified as catname.cat. This program writes the descriptors FLAT_BKG and LHCUTS/R/5/2 into each input frame and will also read the descriptor O_TIME to get the exposure time, prompting for it if the descriptor is not present. After that the various files can be averaged via:
AVERAGE/WINDOW out = catname.cat bgerr,snoise