Illustration of the main processing steps applied to one frame of our dataset. Left: raw detector frame. The brick-wall pattern of the slitlets is visible as the telluric absorption lines are shifted in adjacent slitlets. Middle: reduced frame. The missing data in the middle is due to cropping the frames, essentially removing slitlets at the border of the image. Right: residuals after PSF subtraction via spectral PCA. The two horizontal lines at 2.24 and 2.30 μm mark the position of the mask applied to the data to hide the strong stray light features present in slitlets 2 to 6 and 12 to 15 (cf. Fig. C.1). The slitlets are numbered from 1 to 32, and span the image from bottom to top (i.e. from south to north).