Configuration of the lens system for the 2L1S solution. Upper panel: zoomed-out region encompassing the binary lens components, the blue dots marked by M₁ and M₂; lower panel:zoomed-in view of the central magnification region. The line with an arrow indicates the source trajectory, the cuspy closed curve is the caustic, and the dashed circle in the upper panel represents the Einstein ring. In the lower panel, the three empty circles on the trajectory of the source indicate the source locations corresponding to the times of t₀, t₁, and t₂ delineated inFig. 2. The circle size is scaled to θ_E. The gray curves in the lower panel represent equi-magnification contours.