Per-phase differences (black lines) between observed WASP-107 b spectra and an inverse model of the mean nightly telluric absorption with a mean PSF profile scaled to the appropriate airmass. This spectral segment is totally dominated by telluric features and contains no significant stellar lines. Vertical offsets between each subsequent phase were added for visibility. A blue overplotted spectrum shows the mean telluric spectrum for each night at the lowest airmass. Note that substantial structure in the residuals is only visible for two phases on Night 2 (third from bottom, and topmost), which is indicative of not only the PSF changes in those phases but also the general stability of the PSF otherwise.