Flux measurements of the two flux (bandpass) calibrators over a period of ∼100 days that spans our observation period. These measurements, color-coded by frequency, are taken from the ALMA Calibrator Source Catalogue at 91.5, 103.5, and 337.5 or 343.5 GHz. The solid lines are polynomial fits, from which spectral indices and the 233 GHz flux as a function of time are inferred. In the case of J2258-2758, a few direct 233 GHz measurements are also available and plotted. The dashed line, labelled “|spix|,” shows the absolute value of the spectral index. The vertical bar corresponds to the date of the 2014 UN₂₇₁ observations.