Steps of the data preparation applied to a representative spectral region of the dataset obtained on the night of 31 March 2022. The vertical axis in panel A represents flux in arbitrary units, while in all other panels it represents time, expressed as the planet’s orbital 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 CR2RES, where major S/N differences between spectra (horizontal stripes) and prominent telluric H₂O absorption (vertical stripes) can be identified. Panel C: normalised and masked spectra, where opaque telluric windows are excluded (white stripes). Panel D: residual spectra after one SysRem pass, where telluric residuals can be observed. Panel E: residual spectra after nine SysRem passes, where most of the telluric contribution has been removed. Nodding position effects were corrected in all panels by interpolating the pixelwavelength solution from nodding position B to position A.