Wavefront control architecture and expected performance for the TMT planetary systems imager

Abstract

The Planetary Systems Imager (PSI) instrument for the Thirty Meter Telescope (TMT) will employ high-contrast imaging techniques to study exoplanets and disks around nearby stars. Thanks to the large TMT aperture, it will have the sensitivity and angular resolution to characterize the atmospheres of a large and diverse set of exoplanets, and will study circumstellar disks to constrain planetary systems formation and evolution. One of PSI's most challenging goals will be to image Earth-size exoplanets in the habitable zones of nearby M-type stars and characterize their atmospheres, looking for the spectral signatures of biomarkers. Signal-to-noise considerations based on photon noise shows that this will require PSI to deliver near-IR raw contrast of at least 1e5 at the location of the first Airy ring, which is approximately 2 to 3 orders of magnitudes beyond what the current best AO systems achieve. To do so, the PSI wavefront control architecture must overcome multiple challenges that are either not addressed in existing AO systems, or for which TMT PSI's requirements are oder(s) of magnitude tighter than currently encountered. (A) Wavefront chromaticity between the wavefront sensor and the science camera(s), as well as within the science wavelength range, must be solved for. (B) The stellar flux available for sensing is too small to meet the 1e-5 raw contrast requirement using current sensing approaches. (C) Temporal lag needs to be at least 10 times smaller than in current AO systems. (D) Non-common path errors (NCPE) must be measured and corrected at the nanometer level We show that these challenges are addressed by combining several wavefront sensors at multiple wavelengths, and employing sensor fusion and predictive control algorithms to maximize sensitivity and minimize temporal lag. A wavefront control loop operating from the post-coronagraphic focal plane is particularly essential, as it addresses NCPE errors and chromaticity. Together, predictive control, sensor fusion and the use of high efficiency wavefront sensing options will address the stellar flux challenge. We review architecture trades and outstanding main questions. We describe ongoing AO prototyping efforts an current ground-based facilities, as well as laboratory efforts.