The ratio of the variance added due to an astrometric error Δx to that of the background, V _ast/V(B), as a function of the source flux F and Δx. The upper x-axis shows the corresponding S/N for detection of the source (Ofek & Ben-Ami 2020), while the vertical dashed line shows the position where the variance of the source flux in the central pixel is equal to the variance of the background (i.e., to the left of this line we are in the background-noise-dominated regime).