Overview of the Horsehead mane photodissociation region showing the two observed positions. Left: DCO⁺ map (Pety et al. 2007), right: HCO emission map (Gerin et al. 2009). In both maps, The HCO emission peak (PDR position) at RA = 05^h40^m53.9^s, Dec = −02°28′00′′ is shown with a green cross, and the DCO⁺ emission peak (dense core position) at RA = 05^h40^m55.7^s, Dec = −02°28′22′′ with a blue one. The maps have been rotated 14 degree anti clockwise around the position RA = 05^h40^m54.27^s, Dec = −02°28′00′′ and shifted 20′′ to the east to align the PDR front (vertical red line) with the horizontal 0 offset of the map.