Change in the ¹²CO v = 0 J = 2 − 1 velocity measure as a function of the aperture size. The velocity measures are extracted for a range of aperture sizes and noise levels following the same procedure as outlined in Step 2 in Sect. 5.2. The left panel is constructed for a velocity profile with β = 1 and the right panel for β = 5. Velocity measures extracted from the red (blue) wing are indicated with ‘red’ (‘blue’) in the legend in the panels.