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 isn't 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 sky frame, processes it if needed, and
creates the output sky flat using appropriate flat field and the keywords
Table 
. The way the command works is
identical to the command SKYCOR/CCD.