Weighing performance (stellar lines) for intrinsic spectra. Top panel: Mean of intrinsic spectra along the transit chord of HD 209458b in the region of the sodium doublet, when accounting (black) or not (magenta) for the disk-integrated spectrum in weight profiles.Bottom panel: Relative difference between the magenta and black profiles, overlapped with the contribution of the disk-integrated spectrum to the weight profiles (arbitrary scale, in blue and red for two exposures at ingress and egress).