Upper part: standard deviation of the normalised residuals for a model QSO spectrum with G = 17^m and redshifts z between 0 and 2. Top panel: basis extension with stellar model spectra. Middle panel: basis extension with B-splines. Bottom panel: basis extension with both model spectra and B-splines. The grey shaded region indicates the standard error of the standard deviation. Lower part: same as the bottom panel of the upper part, but for the defocus test.