Dark creation

After the preliminary steps including addition of variance, the dark is merely filed in the calibration system, using the frame number and the exposure time in the name.

When multiple darks of the same exposure time form part of the same grouped observation, these darks are averaged before filing with the calibration system. Such averaged darks have the group number instead of the observation number in their names.
[_AVERAGE_DARKS_ and IRCAM, UFTI, and UIST variants; _GET_DARK_NAME_]