As was stated in the LWS observers manual, the LWS instrument does not observe the sky background automatically. It is up to the observer to take a source and an off-source measurement if he wants to be able to subtract the background. The background radiation can be especially bright in the 157 micron line with long exposures (e.g. faint sources like galaxies). For subtraction of the astronomical background the most important step is to rebin the background spectrum (which is in a separate data product if the user asked for a background spectrum) to the same wavelength scale as the source spectrum. After that the background spectrum can be simply subtracted from the source spectrum. The ISAP software package does not have a function to perform this operation.