Channel maps of the DR21 Main outflow in HCO⁺. Contours of the blueshifted HCO⁺ emission (in blue) and redshifted emission (in red) are integrated over velocity steps of 5 km s⁻¹ and are plotted over the corresponding grayscale. The full velocity range is from 5 to 45 km s⁻¹ relative to the source velocity (−3 km s⁻¹) and the contour levels correspond to 5, 10, and 20 σ_rms. The black dashed circle shows the area of the DR21–1 core in Cao et al. (2019). The green dashed lines denote the half-circles used to derive the outflow opening angles (see Fig. 7).