Difference between the LFC wavelength solution and the ThAr-based 1D (left) and 2D (right) solutions for the red detector arm of the La Silla/HARPS spectrometer. Each vertical panel shows one spectral order with the longest wavelength order positioned at the bottom. The differences between ThAr and LFC solutions in m s⁻¹ are plotted as red lines against detector pixels in the X-axis. LFC solutions were constructed separately for each spectral order. Blue crosses indicate the positions of the ThAr lines used for the construction of the wavelength solutions. In all cases, a 5th order polynomial was used for the dispersion direction. The 2D solution included an additional 3rd order cross-dispersion component as well as the corresponding cross-terms.