Weighing performance (tellurics) for disk-integrated spectra. First (top) panel: Typical telluric spectrum in HD 209458 epoch 1. Second panel: Relative difference between the master-out spectra accounting or not for telluric correction in weight profiles. Third panel: Same as the second panel, when averaging only two exposures (index 5 and 84). Fourth (bottom) panel: Zoom from the third panel in the region of the sodium doublet, illustrating how shifting telluric lines (reported on an arbitrary scale, in blue and red at indexes 5 and 84) bias the mean over the pixels they absorb when not included in weight profiles.