Illustration of the Molecfit telluric correction on a single line from the sodium D-line doublet in the observed spectrum of WASP-76b on the night of 2018 October 31 in all exposures. The vertical dashed purple line represents the expected line position. Top left: Sodium line prior to the telluric correction. Top right: Sodium line after the Molecfit telluric correction. Bottom: Ratio of the nontelluric-corrected over telluric-corrected spectral lines. Inset: Flux and air-mass variation over the transit observation. As expected for observations later in the night, with higher airmass, the removed telluric features are stronger.