Retrieval of the LSF. A fraction of an ESPRESSO spectrum of the hot star HD 164402 is superposed (in red) in the barycentric frame, and the downloaded atmospheric O₂ transmittance spectrum adapted to the observing conditions is overplotted as well (in black). A zoom into one of the O₂ line is inserted. The data were kept in the barycentric frame, as issued from the pipeline, while the TAPAS transmittance is in the geocentric frame. This explains the Doppler shift between the two. This Doppler shift is accounted for in the computation of the LSF.