Post-coronagraph wavefront sensing for the TMT Planet Formation Imager

Direct detection of exo-planets from the ground may be feasible with the advent of extreme-adaptive optics (ExAO) on large telescopes. A major hurdle to achieving high contrasts behind a star suppression system (10(-8)/hr(-1/2)) at small angular separations, is the "speckle noise" due to residual atmospheric and telescope-based quasistatic amplitude and phase errors at mid-spatial frequencies. We examine the potential of a post-coronagraphic, interferometric wavefront sensor to sense and adaptively correct just such errors. Pupil and focal plane sensors are considered and the merits and drawbacks of each scheme are outlined. It is not inconceivable to implement both schemes or even a hybrid scheme within a single instrument to significantly improve its scientific capabilities. This work was carried out in context of the proposed Planet Formation Imager instrument for Thirty Meter Telescope (TMT) project.