The MOS context is written for the reduction of multi-object spectra obtained with numerous slitlets. Long-slit spectra are considered a special case of multi-object spectra and therefore not treated in a separate chapter. The MOS package is meant especially for the reduction of FORS data but may also be used for other MOS data.
It is assumed that the basic correction for bias, dark, and overscan are performed with standard MIDAS commands (maybe also using COMBINE/LONG to average several frames of the same type). There are no special MOS averaging commands.
During the whole description we assume that the detector is a CCD with the rows along the dispersion direction and the columns along the slit.