Comparison for HR10 (top) and HR21 (bottom) of our cross-correlating mask (NAC RE mask; red line) to a standard one (black line; e.g. the one that would be used to compute the Gaia-ESO CCFs) computed from a spectral synthesis with Arcturus stellar parameters. Our NACRE mask includes only weak and isolated lines. Despite our strong constraints, more than 20 lines are kept in both HR10 and HR21 masks for Arcturus.