Figure 8

Figure 8: Tangential and radial caustic points. Tangential caustic points, D - = 0, occur on the axis of symmetry through the observer. A (point) source at a tangential caustic point is seen as a (1-dimensional) Einstein ring on the observer’s sky. A point source at a radial caustic point, D+ = 0, appears “infinitesimally extended” in the radial direction.