  A white light vortex coronagraph was used to experimentally achieve
sub-resolution detection. The angular location of the centroid, $\gamma$, and
the angular extent of circular pinhole sources, $\Theta$, were measured to
within errors of $\delta\gamma = \pm0.015\,\lambda/D$ and $\delta\Theta =
\pm0.026\,\lambda/D$. This technique has two advantages over conventional
imaging: simple power measurements are made and shorter exposure times may be
required to achieve a sufficient signal-to-noise ratio.
