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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.