Variability of the FP cavity width with regard to the input value for methods FP1 (FP lines) and FP2 (HC lines), offset by 0.5 μm for visibility (top), and residuals to the fits, offset by 0.05 μm for visibility (bottom). The results from the two methods are very similar. For method FP1, some outliers (poorly fitted FP lines) can be seen, while for method FP2 the number of lines drops off towards low line numbers (i.e. high wavelengths) and no lines are selected for the reddest order. For scale, a cavity width offset of 0.25 μm corresponds to a velocity offset of 6.12 km s⁻¹, using the FP equation and the initial cavity width value of 2d = 24.4999 mm.