Dark frames are exposures without detector illumination. The dark current of the ISAAC SW detector is small, so the dominant feature in these frames is the detector bias, which is also called the zero level offset, since it is not possible to take a zero second exposure with the SW array.
Usually one takes at least three darks and combines them with suitable clipping to create the dark frame that is subtracted from the science data. As the bias is a function of the DIT, the DIT of the science data and that of the dark must match.
Dark frames are acquired through a dedicated template, which obtains (usually at the end of the night) at least three dark frames for each DIT that was used during the night.
The darks can be reduced with the dark recipe (in isaacp). This recipe will produce one dark frame for each DIT. The readout noise is also measured from these frames and the information is written to a readable ASCII file in PAF format.
The dark recipe is actually separated into two sub-recipes, the first one creates the dark and the second one computes the readout noise. Each sub-recipe can be called individually by using the dark-avg and dark-ron recipes in isaacp.
Currently, the dark created by dark-avg is nothing more than an average of the input files (after having sorted them by identical DIT). Cosmic ray rejection will be implemented later.