Normally the default `MEAN' algorithm will be adequate. Optionally polynomial and spline functions may be fitted in both X- and Y directions (X is actually along the order). In addition, when it is essential to gain maximum S/N the sky-background errors may be modelled using a monte-carlo simulation (TUNE_SKYSIM=YES). This should be used in conjunction with optimal extraction.
The default behaviour is to estimate the sky errors on the basis of root-N statistics (i.e. error is approx. the square root of the intensity).
Martin Clayton