Example of a single spectrum of TPN as measured in the red arm of EMMI. Panel a) shows the spectrum of diffuse light after background subtraction. The bright rim-dark core structure of the globule is evident. Panel b) shows the total measured signal, i.e. the measured spectrum before the background subtraction when the dominating component is atmospheric emission. Note the positive correlation between the noise of the scattered light and the strength of the atmospheric emission. The extents of the bright rims and the dark core, as defined in Sect. 3.2, are marked with white lines. The location of the deep telluric absorption line at  ~7650 Å is marked with a  ⊕ -sign. The horizontal region located at  ~4.3′ on the spatial axes is the gap between the two CCD arrays. The gap is interpolated along columns (spatial axes). The intensity of the 5577 Å [OI] atmospheric emission line is off-scale. Some stars outside the globule are seen as yellow strikes. The sky north of TPN (upper side of panel a) is dimmer than the sky south of TPN (lower side in panel a) because of the cometary shape of the cloud.