Upper panel: maximum detection level of the absorption depth in the Mg i line as a function of the band width. The maximum detection level is reached at the σ-ratio of 2.1 for a band width of 43 km s^-1    ±    3 km s^-1 (AD = 6.2    ±    2.9%). Lower panel: velocity limits, V_min (blue line) and V_max (red line), of the band domains where the maximum of the significance level is reached as a function of the band width. V_min is found to be roughly constant at about  − 62 km s^-1 (stellar reference frame). The position of the highest maximum signal-to-noise ratio at 43 km s^-1 is shown by a vertical dashed line.