Laboratory demonstration of a focal plane coronagraphic interferometer designed for anti-halo apodization of starlight

The coronagraphic focal plane interferometer reflects away the core starlight with a mirror in the focal plane and uses it to form a coherent interferometric reference beam. This is used in a Mach-Zehnder configuration with phase shifting to measure the complex amplitude of the star halo speckles in the focal plane where the interference takes place. We present results from a laboratory prototype in which the speckles are suppressed over half the field by modifying the waverront in a pupil plane with a MEMS deformable mirror, based on a Fourier transform of the complex halo derived from the focal plane interferometric data. Even deeper suppression of the residual stellar halo over the full 360 degree field will be possible by explicitly constructing an "anti-halo" from the reference beam; a new technique for exoplanet imaging (Codona and Angel, 2004). We present the design and current status of a laboratory prototype to study antihalo apodization. The spatially-filtered core starlight will be modulated by deformable mirrors in a Michelson configuration to form a temporally-coherent copy of the measured residual complex halo, with the same amplitude but opposite phase (i.e. an "anti-halo"). Using components with only modest control accuracy, the method has the potential to reduce an already low residual halo by an additional two decades.