Visualization of the D_eff(λ) determination. The top panel shows the individual measurements of the FP effective gap size obtained from the ThAr lines (blue) and the interpolated smooth function (red). For better visualization, a value of 15.210 mm, approximately the mean D_eff, is subtracted from measurements and D_eff(λ) function. The bottom panel shows the corresponding residuals. Uncertainties of the measurements are based on the combined laboratory wavelength and line fitting error. Points rejected by the sigma-clipping procedure are shown in gray. A representation of the smoothing kernel used by the interpolation scheme is displayed in orange.