Input and Output

  The COMBINE/CCD command offers the possibility to combine a number of input images using different combining methods. COMBINE/CCD takes three input parameters at maximum: the exposure type of the images to be combined, the input frames themselves and and output frame. The various command options can be chosen by setting a number of specific keywords.

The first input parameter should contain the exposure type of the images to be combined. Possible choices are: BS (for bias); FF (for flat fields), DK (for dark), SK (for sky images), and OT (for others). The combining options the command offers are controlled by a set of exposure type dependent keywords, all starting with this two letter identification that has been given as the first input parameter. These keywords control various combining methods, scaling and offset corrections, as well as weighting (see below).

The second input parameter is the input frames to combine. The input can be provided in different ways:

1. by a list of images; e.g. frame01,frame02,frame03;
2. by a MIDAS catalogue; e.g. framecat.cat;
3. by a MIDAS Data Organizer (DO) output table (with the extension .tbl), the Association Table (see Section ).

The parameter for the output frame is required in case the input for the second parameter is a catalogue or a string of input frames. In the case of DO input (association) table, from the name of the output calibration frame in the table the command extracts the names of all requested single calibration frames and combines these frames in the output frame. The name of the output master frame can be indicated with an asterisk, meaning that all associated single calibration frames have to be combined. In that case the names of these single frames are taken for the calibration column in the Association Table, e.g. BIAS, DK, etc. See also Section . By default, the input is taken from the keyword CCD_IN.

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.