Dark frames are taken rarely. Used CCD modes are: high gain, 4 and 1
ports. The typical exposure time is 1800 sec. The FORS pipeline
generates master DARKs from input raw DARKs with master BIAS
subtracted.

Dark current: is computed in from the median master dark frame level
in ADU/hr/px and transformed into electrons by using the conversion
factor CONAD (in the case of 4 ports a mean CONAD is used).