Atmospheric transmission at the APEX 12 m sub-millimetre telescope site in the frequency range from 600 to 630 GHz for typical values of pwv columns in our observations: 0.2 mm (blue) and 0.4 mm (green). The dashed and dash-dotted black lines correspond to the J = 1−0 transition of H³⁵Cl and H³⁷Cl, respectively, with the grey shaded regions marking the sideband coverage of our observations.