COMBINE/CCD takes three input parameters at maximum. The first one is the exposure of the calibration frame to be combined. Possible chose are BS for bias frames, DK for darks, FF for flat fields, and SK for sky frames combinations. The second one is name of the data set. Here, the input can either be a single data set or a MIDAS CCD reduction table (with extension .tbl) containing one or more data sets (science frames) including the name of the master calibration frame(s) that are to be produced. The third parameter is only used in case the input for the second parameter is a single data set. By default, the input is taken from the keyword CCD_IN. If CCD_IN is empty, the CCD reduction table stored in keyword CCDREDUC will be used.
In case of single data set input the command will directly go to the CCD association table. From that table all associated calibration frames of the requested exposure type will be collected in a temporary MIDAS catalogue. Hereafter, the combine command will use that catalogue as input for the actual combining routine. the exposure type ( e.g. bs for bias, dk for dark, etc. The default name of the output master calibration frame will be taken from the keyword CCD_OUT, or it that is empty, the naming convention i.e. the name of the input science frame will be used, extended with _`exp' (where `exp' indicates the exposure type, e.g.) _bs).
In case the input is a CCD reduction table the command will execute
a similar procedure as above. In this case, for every data set the command
reads the names of the output calibration frames from the CCD reduction
table. As can be understood from Section 
,
in principle, each output calibration frame can be the result of a
combination of calibration frames belonging to any data set. Hence,
after having checked that the output calibration frame does not exist
yet, from the name of the output calibration frame the the name of
the data set is extracted, all associated calibration frames are
collected, and finally the requested output calibration frame is produced.
In addition to the output calibration frame the combined sigma frame can be
generated. This frame is the standard deviation of the input frames about
the output frame.
Before the actual combining is done the exposure type descriptors of the input frames are compared with the descriptor type stored in the keyword `exp'_TYP. In case this keyword is filled with `*' or `?' all exposure types are allowed. Else, a fatal error will follow if the keyword content is not equal to the exposure type(s) of one or more input frames.