Gaussian centroid wavelengths as a function of slit position for the prominent O i line at 5577.339 Å. The median of all fitted wavelengths is shown with a vertical red dashed line, the corresponding air reference wavelength in dashed black. The difference between the measurement and reference is − 0.037 Å and the standard deviation of the fitted values is 0.011 Å. The cross-dispersed direction corresponds to the y-axis, the spectral dispersion direction to the x-axis.