Simulated aperture synthesis ALMA images for a disc at 50 pc, with a transit duration of 2 h. From left to right and top to bottom, we show simulations computed for an observing frequency of 45, 100, 220 (top) and 345, 670 and 870 (bottom) GHz. Axis scales are in arcseconds. Contours start at 0.01 and are spaced by 0.08 mJy beam⁻¹ at 45 GHz, start at 0.08 and are spaced by 0.2 mJy beam⁻¹ at 100 GHz, start at 0.5 and are spaced by 0.5 mJy beam⁻¹ at 220 GHz, start at 0.8 and are spaced by 0.8 mJy beam⁻¹ at 345 GHz, start at 2 and are spaced by 2 mJy beam⁻¹ at 680 GHz, start at 4 and are spaced by 4 mJy beam⁻¹ at 870 GHz.