Example of the cross-correlation of two profiles from different exposures. Upper panel: normalized intensities of the Mg ii λ2796 Å line from the z_abs = 1.5864 system (Sect. 4.2) extracted from exposure #2, setting 760 (solid line) and from exposure #1, setting 760 (dotted line). See Table 1. Lower panel: the dependence of  – the sum of squares of the intensity differences over all pixels covering the line profiles – as function of the velocity shift, δv. The profiles become fully aligned when the dotted line profile is shifted by  ≈  − 350 m s^-1 ( ≈ 1/4 pixel size).