Calculated atmospheric zenith transmission of the entire 450 μm submillimeter window for the site of the APEX telescope for three values of the precipitable water vapour column: 0.25 mm (dark blue), 0.40 mm (orange), and 1.08 mm (red). The grey shaded regions mark the 8 GHz frequency windows covered by each of the two sidebands (separated by 8 GHz) of the SEPIA660 receiver. The dashed brown and pink lines mark frequencies of the p-H₂O⁺ and ArH⁺ lines discussed in this work, respectively.