Background spectrum determined from off-centre raster positions subtracted

In the wavelength range of PHT-S the dominant background component is the zodiacal light (apart from localised regions like HII regions, reflection nebulae, or the galactic plane). In the 5.8-11.7 um regime the zodiacal flux was clearly measurable for PHT-S (see Leinert et al., 2002), and its contribution is not negligible in the case of fainter stars. When the observer performed a raster measurement, a background spectrum was determined from the off-centre raster positions, and this off-spectrum was subtracted from the on-target spectrum as part of our processing scheme.