Change in airmass (top), precipitable water vapour (middle), and signal-to-noise ratio (bottom) during observations (mid-transit ~00:39 UT). Vertical dashed lines indicate the first, second, third, and fourth contact, respectively. The lack of data around −2.3 h was due to a loss in the tracking system. The post-transit observations not included in the analysis are shown in grey. The out-of-transit red diamonds were used to compute the master spectrum of Fig. 2.