Phase-Sorting Interferometry for High-Contrast Imaging with the GMT
Johanan L. Codona
Univ of Arizona, Steward Observatory
High-contrast detection and characterization are plagued by subtle telescope imperfections and non-common-path (NCP) aberrations, leading to many long-lived speckles in the search area near a star. These can be corrected using the AO DM, but the complex halo of the star must be known in the science camera focal plane in order to compute the DM corrections. Phase-sorting Interferometry (PSI) is a technique to measure the science path halo on-sky and has been demonstrated at the MMT in the L and M band. PSI makes an interferometric measurement of the halo including NCP aberrations without the use of any extra hardware or sensing light introduced into the focal plane. The only requirement is that the science camera be read out fast enough to image the varying speckle intensities over the region of interest near the star. The PSI method uses the AO system’s wavefront sensor telemetry to estimate the residual wavefront error over a series of synchronized science camera exposures, and a mathematical model of the telescope to compute the phase and amplitude of the dynamic halo speckles. The computed speckle phases are used to analyze the science frames into four statistical interferograms which are combined in the usual way to derive the complex halo in the science focal plane. This allows calculation of a DM correction update, enabling a halo-suppression closed-loop to be implemented. We explore the potential for using this method with the GMT AO system and simulate a GMT anti-halo servo for various near-to-mid infrared bands combined with a 6-decade high-contrast phase-apodization coronagraph implemented using machined phase plates. The result is a robust, low-overhead, easy-to-use, high-contrast imaging technique suitable for both search and spectral characterization very near stars.