Gravitational-lensing caustic ($\mathbf{\beta }({\mathbf{\theta }}_{\mathrm{crit}})$)

A caustic is the source-plane image of a gravitational-lensing critical curve. Crossing a generic fold caustic creates or destroys a pair of images of opposite parity. For a circular lens, a tangential critical circle maps to an aligned-source point, while a radial critical circle can map to a finite source-plane circle. Critical curves and caustics consequently occupy different planes and must not be identified as the same angular locus.