Observed spectra at the dust peak of N₂D⁺ and DCO⁺, with the original angular resolution (in black histograms). The transition is labelled in the upper-left corner of each panel. In red, the best-fit solution found with MOLLIE is shown overlaying the observations (see Sect. 3.2). Underneath each line we present the residuals of the fit (difference between the model and the observation). The vertical blue bars show which hyperfine component has been used to derive the molecular column density, when the hyperfine structure has not been neglected.