Residuals when using ThAr lines to predict wavelengths for LFC lines. Left: Residuals for a single LFC exposure plotted with respect to detector position and colored by residual, as in Figure 3. The positions of ThAr lines are overplotted in yellow. In general, residuals are greater between ThAr lines with greater separation. Right: Comparison of polynomial and interpolated wavelength solutions using either just ThAr lines or just LFC lines for a subset of echelle order 94. The shape of the residuals from a polynomial fit is similar whether using ThAr lines or LFC lines. A PCHIP-interpolated wavelength model guided by LFC lines returns the smallest residuals.