Two-dimensional illumination surfaces of echelle order three of the VIS arm for a 0.4″ slit (left) and a 5.0″ slit (right). The x-axis corresponds to the spectral dispersion direction, the y-axis to the cross-dispersed direction. The relative illumination is plotted on the z-axis. The maximum impact of the inhomogeneities is on the order of ±2.0%. The blue highlights and shadows were introduced to facilitate the visualization of the three-dimensional structure.