This part has already been discussed in
Section 
.
Depending on the keywords SC_BSCOR, SC_DKCOR, and
SC_FFCOR the calibration procedure checks for the availability
of the master bias, dark, and flat field calibration frames. If they
can not be found they will be created using the input frames listed in the
association table, and according to the keyword setting for combining.
The next step after having obtained the calibration frames is to process these frames. For example, after the flat field is created/found it will be checked for its processing status. If it has not been processed any on-going calibration ( i.e. of a science frame) will be interrupted, and the flat field will be processed. After this has been completed processing of the science frame will continue.