Better Flat-Field Models

Using a median filter for the flat-field model, by setting fltfit=median in echmenu is generally fine, however, sometimes this may not give the best result. For example, if the flat-field lamp has some a feature in its spectrum in an area which you want to use (not the best situation in the first place... but) then taking the median over a range of a few pixels (the default is 10 pixels) will not work well if the size of the feature is similar.

There are alternative fitting schemes, the most useful probably being spline. It may be useful to hone the flat-field model interactively, this can be done by setting tune_ffinter=yes. The order of fit can then be adjusted interactively.