Telluric lines removal

The stars are observed in different conditions of air mass and humidity and have different radial velocities with respect to the spectrograph. Therefore, positions with respect to the stellar lines and intensities of telluric lines are changing from one spectrum to another. They have to be removed to avoid any earth atmosphere contribution during the comparison between two spectra.

Telluric lines have been identified using the table of Rowland of ``The solar spectrum'' (Moore et al. 1966). Two or three pixels are removed on each side of the central pixel of each telluric line, depending on the equivalent width listed in the Rowland's solar spectrum : smaller or larger than 2 mÅ. Figure 3 shows the order 94 of HD 4306 before and after telluric lines removal.