Title of Presentation: Closed-Loop Adaptive Suppression of Residual Coronagraph Halos using a Focal Plane Interferometer and Anti-Halo Apodization
Primary (Corresponding) Author: Johanan L. Codona
Organization of Primary Author:
Abstract: High-contrast imaging systems for studying extra-solar planets require contrasts of 8 -- 10 decades at angles of a few \lambda/D, requiring unprecedented precision and control. Of the methods proposed to accomplish this, many involve some form of coronagraph, where a focal plane mask is used to block the bright star, simulating an artificial eclipse. However, wavefront imperfections of as little as 8 pm, or transmission variations of as little as 0.01% cause planet-like speckles to appear at 10^{-8} of the star, causing confusion.
We have developed an interferometric focal-plane wavefront sensor using the starlight normally discarded by the coronagraphic focal plane mask to form an interferometric reference beam. The interferometer permits us to estimate the phase and amplitude of the residual halo relative to the bright center of the star's diffraction pattern. The measurement is efficient and has the significant benefit of being able to overcome the effects of incoherent background noise such as zodiacal light, even when the halo is much fainter. We have demonstrated its use in halo suppression by driving a pupil DM in closed loop.
We have also developed a new halo suppression method, Anti-Halo Apodization (AHA), which uses the coronagraph's discarded starlight as the raw material to construct a coherent negative copy of the halo directly in the focal plane. A major advantage of the AHA approach is that it uses an attenuating filter to set the overall level of the antihalo, rather than the ultra-precise control needed with a pupil DM. This allows us to achieve a great deal of additional halo suppression with relatively low-precision phase modulators: two additional decades of suppression can be acheived with 0.1 radian of phase control. The AHA benefits can be used to ease tolerances throughout the system.