Source trajectory and caustics of OGLE-2015-BLG-1609 for different binary lens topologies: inner-large q (blue line), small q (orange line), and outer-large q (green line). The central caustics of the small q and inner-large q models at the point (θ_x/θ_E, θ_y/θ_E) = (0, 0) are approximately point-like. For clarity, we plot only the small q topology trajectory, as the other trajectories differed only slightly. The orange circle represents the size of the source in the case of the small q topology.