Generation of a CCF mask for HD 209458. Top panel: Master disk-integrated spectrum over all out-of-transit 1D spectra, before (blue) and after (black) normalization with the stellar continuum (red). Shaded blue areas are excluded due to strong telluric contamination. Middle panel: final mask lines, identified in the master spectrum by their minimum (green disks) and surrounding maxima (red disks). Bottom panel: zoom highlighting the range of each mask line (shaded bands) and, when cross-matched with VALD, their origin species.