Differences between wavelengths of astrocomb lines measured using our empirical IP models and using Gaussian IPs, expressed as a velocity shift between the two (Eq. (27)). Main panel: Δυ, as a function of position on the detector. The zero point of the colour bar is set to Δυ_* = 0 m s^–1, such that the red and blue colours correspond to positive and negative velocity differences. Black contours more clearly visualise the distribution of Δυ_* values across the detector. Contour limits correspond to the median and the central 68% distribution limits. A full line is used for positive contour levels and a dashed line for negative ones. Top panel: the histogram of the values plotted in the main panel. The vertical dashed red lines show the median and the central 68% region limits. The same quantities are shown as horizontal thick black lines in the colour bar to the right of the main panel.