An illustration of the quality of the spectra for the 5780 and 5797 Å (left) and 6614 Å (right) DIBs. For each target all observed spectra were co-added in the heliocentric reference frame. The five targets shown have comparable E(B−V) values (Tables A.1 and A.2), and thus have similar 5797 Å DIB strengths. The well established, strongly variable 5780/5797 ratio can be seen in the spectra, with the intensity of 5780 absorption inversely related to the molecular gas fraction, f_H2 (Tables A.1 and A.2). The five spectra are shown superimposed on each other at the top of the panels. Note that because of generally poor observing conditions there are numerous weak and narrow atmospheric water features present (particularly noticeable around 5788–5792 Å) that could influence the 5797 Å profile. In the future these features will be removed using the method described in Sect. 6.