Exoplanet detection and characterization using combined coronagraphy and sub-uas astrometry from space

Combining high-contrast imaging and astrometry in a single space mission would enable efficient detection and characterization of single- and multiple-planetary systems around nearby stars, allowing determination of planetary mass, composition, atmosphere, and system architecture. These science goals can be achieved using a 2m wide-field (>0.1deg²) class telescope equipped with two instruments: a high-performance coronagraph to perform direct imaging, and a wide field camera to achieve sub-microarcsecond astrometric accuracy. However, these measurements are only possible if there are no relative distortion changes between astrometric observations. At sub-microarcsecond accuracy regime, even space optics suffers from dynamic distortions in the optical system and dominates the error budget. We propose to utilize a diffractive pupil, in which an array of dots on the primary mirror generates polychromatic diffraction spikes in the focal plane to calibrate the dynamic distortions of the optical system. According to simulations, this technique would allow to obtain 0.2 microarcsecond single-visit precision astrometric accuracy. In this paper we present the laboratory results that demonstrate the diffractive pupil concept on wide-field images. We also discuss simulations and experiments performed at the NASA Ames ACE test bed, demonstrating that the diffractive pupil does not affect the coronagraph performance down to 2×10-7. Finally, we assess the compatibility of a diffractive pupil telescope with a general astrophysics mission, showing that the spikes do not impact wide-field observations.