Steps of the data preparation applied to a representative spectral region of the dataset obtained on the night of February 04, 2017. The vertical axis in panel A represents flux in arbitrary units, while in all other panels it represents time, expressed as phase. Panel A: original spectrum observed at mid-transit, where telluric absorption lines from H₂O can be identified. Panel B: original spectral matrix extracted using CARACAL, where major S/N differences are observed between the spectra (horizontal stripes), and prominent telluric H₂O emission can be identified. Panel C: normalized spectra, where a 3σ clipping has been applied to filter out outliers and where telluric masks have been applied (white, excluded pixels). All spectra are now normalized to a common continuum. Panel D: resulting spectra after one SysRem pass, where major residuals can be observed, especially from emission lines. Panel E: spectra after nine SysRem passes, where most of the telluric contribution has been removed and any exoplanet signal, if present, is buried in the noise.