APS as a function of spatial frequency, k. The solid black line shows the APS of the interloper-contaminated map (i.e. CO+[C_I] + [C_II]) with no mask. The solid green line shows the intrinsic [C_II] APS (from the unmasked contamination-free [C_II] map). Coloured crosses show the [C_II] APS estimates, obtained from masking the interloper-contaminated map with the different masks from r_σ = 1.5 to 3.0. Coloured points linked by dotted lines show the residual interlopers’ APS measured on the masked interloper map (i.e. CO+[C_I] after masking) at (from left to right) z = 5.2, 6.5, and 7. The beam APS at the corresponding observed frequency is shown in grey. Vertical lines highlight the range k = 0.12–0.24 arcmin⁻¹.