Overview of the 3.3 GHz CH line observations towards W51. Panel (a) displays the ATLASGAL 870 μm emission at contour levels starting from 5×, 10×, 20×, 40×, and 80 × 1σ where 1σ = 0.5 Jy beam⁻¹ overlaid atop the 3.3 GHz continuum emission along with the synthesised beam of the VLA observations (filled white circle) as well as the central beam of upGREAT/SOFIA (red) and the LABOCA/APEX beam (teal) for comparison. The position from which the spectra are extracted is marked by the beam in black and labelled E; we also mark the positions of W49 M and N. Panels (b), (c), and (d) display the integrated intensity contours of the 3.264 GHz, 3.335 GHz, and 3.349 GHz CH lines, respectively, overlaid on top of the 3.3 GHz continuum emission. The intensities are integrated over a velocity range between range between 61 and 70 km s⁻¹, except in the case of the 3.335 GHz line whose absorption component is integrated between 60 and 66.6 km s⁻¹, while the emission between 64 and 72 km s⁻¹. The contour levels mark 5×, 10×, 20×, 40×, 80×, and 160× 1σ where 1σ = 2.80, 0.97, and 2.80 K km s⁻¹ for the 3.264 GHz, 3.335 GHz, and 3.349 GHz CH emission, respectively (solid dark blue curves) and the 3×, 4×, and 5 ×1σ levels where 1σ = 2.31 K km s⁻¹ for the 3.335 GHz absorption component (dashed white curves).