Fig. 11. Top cross orders tracing at X=500 of a localization exposure superimposed with the polynomial fit of the background. We notice around Y=500 a little bump due to the contamination by some uncorrected blue wavelength coming from the second order of the cross-disperser. Bottom difference between the fit and the background level f/f=(fimage-ffit)/ffit. The typical error in the background measurements is below 5%.