Flowchart of CCF mask generation, illustrated with HD 209458. Thresholds applied in each step (magenta lines) can be adjusted based on the resulting line selection displayed as 2D distributions (green and red disks show lines kept and excluded, respectively), occurence histograms, and cumulative functions of the line weights. We illustrate how telluric contamination is flagged by showing the master telluric spectrum at its minimum (blue) and maximum (red) doppler shifts during the two epochs, relative to the master stellar spectrum (black profile). The stellar line at ~7223.7 Å is excluded due to contamination by the telluric line highlighted at its mean position by the dashed grey line.