Upper panels: spectra extracted from the four analysis strips along x plotted as a coloured solid line and total spectra represented as a black solid line. Lower panels: percentage variation of the strip spectrum with respect to the total spectrum. Dotted lines are ± 2% lines. In the top-left panel we show the CCD0 spectra, in the top-right panel CCD1, in the bottom-left and bottom-right panels CCD2 and CCD3.