Lens system configuration for OGLE-2018-BLG-0971. The diagonal line with the arrow represents the source trajectory, and the closed figures composed of concave curves represent caustics. The caustic shape and location evolve over time due to the orbital motion of the lens. The sets of caustics drawn in black, green, and red correspond to the three anomaly epochs t₁, t₂, and t₃ marked in Fig. 1. The right panels display still frames capturing the approach of the source to the caustic at these three epochs. The size of the orange circle on the source trajectory that appears in each right panel indicates the angular dimension of the source relative to the size of the caustic.