Comparison in red of the observed average spectrum of C/2016 R2 extracted on the nucleus (corresponding to about 250 km on either side of the nucleus) and in blue at the extremity of the slit (cometocentric distance ~6000 km). The R branch of the (0,0) band starts at 3909.7 Å and is degraded to the blue side (i.e., lines appear at shorter wavelengths for increasing values of quantum number N), and the P branch starts at 3910.9 Å and returns at the bandhead at 3914.3 Å to shorter wavelengths. Some lines are identified by their type and N″ value (see Fig. 1 for the details), all of them belong to the (0, 0) band. The two spectra are different.