For each order, we plot the average shift (diamonds) over time (dots). The drift relative to the reference spectrum is expressed in pixels (1 pixel equals to 2.7 km s⁻¹). In the left panels, for the three nights, we have taken as reference spectrum the average over the spectra taken in position A (in black) while in the right panels the average over the spectra taken in position B (in red). The average shifts of the spectra over time acquired with one nodding position are shifted with respect to those taken in the other position. These shifts present themselves in a linear fashion, as a function of spectral order. The amplitude of the shifts does not depend on the night considered.