Femtolens Imaging of a Quasar Central Engine Using a Dwarf Star   Telescope
  We show that it is possible to image the structure of a distant quasar on scales of $\sim 1\,$AU by constructing a telescope which uses a nearby dwarf star as its ``primary lens'' together with a satellite-borne ``secondary''. The image produced by the primary is magnified by $\sim 10^5$ in one direction but is contracted by 0.5 in the other, and therefore contains highly degenerate one-dimensional information about the two-dimensional source. We discuss various methods for extracting information about the second dimension including ``femtolens interferometry'' where one measures the interference between different parts of the one-dimensional image with each other. Assuming that the satellite could be dispatched to a position along a star-quasar line of sight at a distance $r$ from the Sun, the nearest available dwarf-star primary is likely to be at $\sim 15\,\pc\,(r/40\,\rm AU)^{-2}$. The secondary should consist of a one-dimensional array of mirrors extending $\sim 700\,$m to achieve 1 AU resolution, or $\sim 100\,$m to achieve 4 AU resolution. 
Andrew Gould and B. Scott Gaudi