Differences between astrocomb line position measured by fitting two different IP models to the data, Δx_* = x_*,_G − X_*,E. The subscripts indicate which IP model was used to determine the centre during line fitting (G for the Gaussian and E for our empirical IP models). Main panel: Δx_* as a function of position on the detector. The zero point of the colour bar is set to Δx_* = 0 pix, such that the red and blue colours correspond to positive and negative differences. Contours show the spatial distribution of values more clearly, with contour levels corresponding to the median and the central 68% distribution limits. Top panel: the histogram of the values plotted in the main panel. The bottom x-axis is in units pixel and the top axis in units m s⁻¹ (1 pix = 820m s⁻¹). The vertical dashed red lines show the median and the central 68% distribution limits. The same quantities are shown as horizontal thick black lines in the colour bar to the right of the main panel.