Calculated atmospheric zenith transmission of the 440 GHz (~68O µm) submillimetre window for the site of the APEX telescope for three values of the precipitable water vapour column: 0.23 mm (dark blue), 0.40 mm (red), and 1.00 mm (orange). The grey shaded region marks the range of frequencies covered by all the HFS splitting transitions between ²Π_3/2, N = 1, J = 3/2 →²Π_1/2, N = 1, J = 1/2 of CD.