  In quasars which are lensed by galaxies, the point-like images sometimes show
sharp and uncorrelated brightness variations (microlensing). These brightness
changes are associated with the innermost region of the quasar passing through
a complicated pattern of caustics produced by the stars in the lensing galaxy.
In this paper, we study whether the universal properties of optical caustics
could enable extraction of shape information about the central engine of
quasars. We present a toy model with a crescent-shaped source crossing a fold
caustic. The silhouette of a black hole over an accretion disk tends to produce
roughly crescent sources. When a crescent-shaped source crosses a fold caustic,
the resulting light curve is noticeably different from the case of a circular
luminosity profile or Gaussian source. With good enough monitoring data, the
crescent parameters, apart from one degeneracy, can be recovered.
