Top: difference between the reference S value S_{E = 100} calculated without ISM absorption, and S_MW obtained considering two non-saturated (log N_CaII = 11.5) ISM absorption components as a function of velocity separation (Δvr in km s^-1) of each ISM feature from the line centre. With increasing Δvr, the two features move in opposite directions. The black dashed line shows the case of a single ISM absorption component moving redwards of the line centre. The short dashed vertical lines are colour-coded by the absorption features shown in the insert and indicate their position relative to the line centre. The insert shows the photospheric plus chromospheric flux (the photospheric spectrum is shown by a dashed line) in black and, as an example, absorption components lying at a velocity separation of 0.0 (red), ±10.0 (blue), ±30.0 (green), and ±50.0 (orange) km s^-1 from the line centre. Bottom: as the top panel, but considering saturated (log N_CaII = 14.0) ISM absorption lines.