Left panel: line position difference in velocity units of the isolated confirmed U lines (i.e. with a corresponding match in the literature). The colour scale located at the right edge of the plot denotes the normalized line intensity in a logarithmic scale. From 690 to 744 nm, we can see the residuals of the eight standard lines used to calculate the k̄_VIS. The x-axis is shortened for readability reasons at 744 nm. Right panel: distribution of the residuals. The distribution is centred on −0.12 ± 8.19 m s⁻¹, where the uncertainty is calculated as the weighted standard deviation plus the statistical error introduced by the standard lines used for calibration.