Input for the command can be either a single sky frame, or the Association Table containing a column with the names of the master sky frames. In the first case the output is, obviously, a single illumination frame. In case the Association Table is given as input the name of the output frame names will be the names of the input frames extended with `_ill'. In addition, the illumination frames will be stored in an illumination column in the Association Table. This column can be used in the pipe line processing of the science frame. Default input is taken for the keyword CCD_IN. Smoothing parameters are taken from the the IL_... keywords, which are listed in Table 3.9.
After the illumination frame has been created, one can multiply the original flat field by the illumination frame, resulting in an adjusted flat field. This approach clearly has the advantage of speeding up the calibration process since it requires one calibration frame and two computations (scaling and dividing the illumination correction) less. The output frame is called sky flat. It is the flat field that has been corrected to yield a flat sky when applied to the observations.
Having done this, this sky flat can be used as the final one. How good this new flat field is can be checked by correcting the blank sky once more, using this sky flat. If the result is not satisfactory one can try to play with the smoothing parameters, or else ask for help of an experienced observer.
To create the sky flat fields the command SKYFLAT/CCD is available. As input it takes the blank master sky frame, processes it if needed, and creates the output sky flat from the smoothed processed sky and the appropriate flat field. The smoothing parameters are stored in the keywords displayed in Table 3.9. The way the command works is identical to the command SKYCOR/CCD. The default names of output sky flats are the same of the original master flat field frames. Hence, the input master flat fields, use to flat the master sky frame, will be overwritten.