Setting, Saving, and Retrieving CCD Keywords

Since the number of parameters involved in a complete CCD reduction is quite large, most command parameters have default values. These defaults are taken from the CCD keyword structure. So, after the data have been read in and organized, you can set the CCD keywords to control the CCD reduction process. Basically, one can divide the CCD keywords in three categories:

• general keywords e.g. for telescope, instrument/detector specifications (which CCD has been used; what is the useful area on the chip; what is the read-out noise, its gain; what is the overscan area) and the entries, needed for the various steps in the calibration sequence. Most of the keywords are filled by the LOAD/CCD command (see below).
• calibration keywords related to the handling of calibration data, like fitting of the overscan area, the creation of master calibration frames, the method of bad pixel correction, the creation of illumination and fringe frames. For combining the calibration frames in master calibration frames the system is provided with a set of keywords for each exposure type (see below);
• reduction keywords for controlling the reduction of the science frame(s). Examples are: should the overscan area be used to determine the bias; should we correct for bad pixels; is dark current to be subtracted.

Most of the CCD commands get their input parameters mainly from the CCD keywords. However, most commands also accept a limited number of input parameters on the command line. These parameters will supersede the corresponding keywords. However, apart from a few cases the keyword setting itself is not modified: principally, keywords can only be changed by the SET/CCD command. For handling all these keywords the CCDRED package in equipped with five commands: HELP/CCD, SHOW/CCD, SET/CCD, SAVE/CCD and INIT/CCD.

HELP/CCD without parameters gives an overview of all CCD commands available. With an existing CCD keyword as parameter the command will show the present values of that keyword and a short explanation of it use.

SHOW/CCD returns you the current CCD keyword contents. Given the number of keywords, to display the information the keywords are grouped and displayed according to their functionality:

• GE keywords concerning general information;
• BS keywords for combining bias frames;
• DK keywords for combining the dark frames;
• FF keywords for combining the flat fields;
• SK keywords for combining the sky frames;
• OT keywords for combining other exposure types;
• OV keywords for fitting the overscan region;
• MO keywords for the mosaicing commands;
• FX keywords for the bad pixel correction;
• IL keywords for the creation of an illumination frame;
• FR keywords for the creation of a fringe frame;
• SC keywords for control of the actual reduction sequence.

With SET/CCD a maximum of 8 keywords in the CCD context can be changed in one go.

After (partially) having finished a CCD reduction the user may want to store the current keyword setting. This can be done using the command SAVE/CCD sav_table. The command will store the keywords in descriptors of a CCD save table with descriptor information only. The keyword setting can be restored by the command INIT/CCD sav_table, where sav_table is the table containing the CCD keyword setting, previously saved.