Brightness distribution maps, in Jy/beam, for two cases considered in section A. The top plot shows the case of an error-free observation at 144 GHz with ALMA in configuration C = 20, combined with single-dish. The bottom plot shows the case of an observation with the same combination of instruments, but with 0.6′′ pointing errors on all antennas, thermal noise as described in the text, and uncorrected atmospheric phase noise with a 45° rms phase difference on 300-m baselines. Contours on both plots correspond to the 3σ_S sensitivity limit given by the ALMA Sensitivity Calculator, where σ_S = 17.41   μJy for the typical atmospheric conditions in band B = 4.