Calibration curves of the α₁ and α₂ parameters depending on the S/N value of a spectrum for the erf Fourier filter. Top: calibration of α₁. The dots represent the best value according to the simulations (see Fig. B.2) that produce the least standard deviation (weighted, as explained in the text) between the noise-free and the smoothed spectrum. The error bars correspond to the 5% best values of the simulations. Two stars were used for the calibration (highlighted by the colour of the points). The calibration curves were fitted by a third-degree polynomial function in log-log space. Bottom: calibration of α₂ with the same procedure. The calibration curves were fitted by a second-degree polynomial function in log-log space.