Top: zoom into the red 6640 Å region of the normalised spectral difference corresponding to the last red dot of the 2008 data set shown in the left panel of Fig. 8. A clear interference pattern is visible in the spectral difference (red curve) and is even more pronounced when the difference is smoothed (black curve). Bottom: fourier transform of the spectral difference. A power excess is visible and corresponds to an interference pattern created by an optical element of width 1.51 mm.