The task ech_mdlbck performs this scattered light background fitting and places the resulting fitted values in the `sky fit' arrays used by the extraction routines. The background fitter uses a similar set of parameters to the the normal sky modeller but applies its fits to the sets of inter-order pixels at each X-position in the frame. Note that this involves a lot of I/O and can be quite a slow operation for large frames.
The degree of function to fit and its associated parameters are specified using the same parameters as for `sky' fitting.
Martin Clayton