Vortex-phase filtering technique for extracting spatial information from   unresolved sources
  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. 
Garreth J. Ruane, Prachyathit Kanburapa, Jiaxuan Han, and Grover A.   Swartzlander Jr
physics.optics