Wavelength measurement error, ∈_λ, as a function of detector position for our empirical IP models (left) and for the Gaussian IP approximation (right). Colour schemes in both panels are identical and centred at 0 m s⁻¹ (white) to highlight the vast improvements associated with the former. Contours for the empirical IP case (left) were not informative so are not shown. Plotted contour levels for the Gaussian IP case (right) correspond to the 16th and 50th value percentiles (−14.06 m s⁻¹ and −3.59 m s⁻¹, respectively) while values above the 68th percentile (i.e. above 0.93 m s⁻¹) do not cluster at any specific location so that contour is not shown. Smaller panels show the histogram of values shown under them. The median and the central 68% distribution limits are shown as vertical dashed red lines. The same quantities are shown as thick black lines in the colour bar to the right of each main panel.