Position and detector normalisation derived from sky spectra. The lines in the left panels show the ratio of the background spectra measured from individual slitlets relative to the average spectrum of all slitlets, with the line colours indicating the “y” position of a slitlet within the field of view. Clearly, there are wavelength- and y-dependent systematics. The right panels show the ratios after applying the position- and wavelength-dependent flux scale correction.