Visibility amplitudes for the source model in Figure 1. Green, blue, and gold points show the (u, v) locations of for the shortest baselines. The red and orange contours show the regions where, respectively, the zeroth- and second-order approximations to the amplitudes differ from the true amplitudes by less than 0.2 Jy. The bottom panel shows horizontal and vertical slices of the amplitudes (solid lines) and the dotted lines show the second-order expansion.