Pa_β line visible in the spectrum of the PN IC1266. Upper panel: although the line is located in a wavelength range with minor atmospheric absorption, it is not well corrected with the classical method. An intrinsic wavelength shift is probably introduced by the radial velocity of the object (colour coding as in Fig. 17). Lower panel: transmission derived with the classical method (green line) shows a broad dip leading to a significant higher flux and different line shape in the corrected spectrum. The transmission derived with molecfit (red) does not show this feature.