Ratios |β^⋆|∕|β| plotted against β in units of θ_E. The source positions β^⋆ are obtained by means of a numerical optimization of the cost function h(β^⋆) defined in Eq. (48). The resulting source mapping β^⋆(β) is slightly anisotropic: |β^⋆| is larger than for ∠β > 45° and smaller for ∠β < 45°. The color-coding refers to the azimuth angle ∠β in the source plane. When a source crosses the tangential caustic curve, the mapping shows β ^⋆ (β) discontinuities. Two particular jumps are highlighted: (1) sources depicted with triangles pass by the cusp located on the β_x-axis (the corresponding β_x∕θ_E is identified by the dashed vertical line); and (2) sources depicted with squares pass by the cusp located on the β_y -axis (the corresponding β_y∕θ_E is identified by the dash-dotted line.)