GP fits to the IP. Top panel: the large black points are the 251 samples with corresponding error bars in the first segment of order 90 (average wavelength λ = 676.5 nm), also indicated as a rectangle in Fig. 3. The thick red line is our reconstruction of ψ using GP regression (Sect. 2.4) after re-centring, in the first iteration. The secondary GP for variance estimation was not used, resulting in a for this fit. The shaded red bands show the 1σ ranges of the model. The dashed blue line is the mean function of the GP, m(x; θ), Eq. (17). The small black points above the model show the residuals normalised with respect to the error on the data, with the grey shaded region indicating ±1σ. Parameters θ (controlling m) and ψ (controlling the correlation matrix K_ij, Eq. (18)) were fitted simultaneously. The magenta dotted line is the difference between the GP model and the mean function, that is the function controlled by K_ij,·. Departures from the Gaussian shape are observed almost everywhere. The thin dotted black line indicates zero flux. Rectangles indicate regions plotted in the bottom panels. Panels with numbers (1)–(4): zoom-ins of the top panel. The number in the top left corner identifies one rectangle in the top panel.