Comparison between the S/N obtained from the R50 spectra and that from the OptEx ones in the two arms (integrated, filled symbols) and around the Ca H and K doublet in UVB (computed in the region [3800−4000]Å, empty diamonds) and the MgFe lines in the VIS ([5000−5500]Å, empty circles). The S/N obtained from the optimally extracted spectra is in all but two cases above the one-to-one relation, which is plotted as dashed black line.