Efficacy of the extinction correction, as described in Sect. 4. (a) I_Hα; (b) E(B – V); (c) τ; (d) EM (colour-scale saturated to 1000 cm⁻⁶ pc, the maximum is 2251 cm⁻⁶ pc); (e) n_e (colour-scale saturated to 15 cm⁻³, the maximum is 57.6 cm⁻³); (colour-scale saturated to 0.09 cm⁻³, the maximum is 0.29 cm⁻³). The orange elliptical region gives the area chosen for the LoS estimations. In (e) and (ƒ), ζ Oph is masked to prevent confusion in the colour-scale. Throughout the paper, the uncertainties are shown in grey scale.