Sky background definition

  The previous methods allow to correct for the interorder scattered light. The sky background component can be measured in spectra taken with an extended slit using the two commands DEFINE/SKY and EXTRACT/SKY. The command DEFINE/SKY allows to define the offset limits of up to two sky windows, usually on both sides of the spectrum. The command EXTRACT/ECHELLE performs an average extraction of the sky background using these offsets, optionally fiters for cosmics, and provides an extracted spectrum in a format similar to the one resulting from EXTRACT/ECHELLE. The two extracted images can be subtracted for sky background correction. It can be noted that the background measured by the command EXTRACT/SKY includes also the interorder scattered light components and makes in principle unnecessary the correction described in the previous Section.