= Gravitational-lensing caustic
{title2=$\boldsymbol\beta(\boldsymbol\theta_{\rm crit})$}
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.
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