Authors

Olivier Guyon (1) Michael Shao (2) Marie Levine (2) Robert Woodruff (3) Mark Ammons (1) Eduardo Bendek (1) Joe Pitman (4) Bijan Nemati (2)

Affiliations

(1) University of Arizona (2) NASA JPL (3) Lockheed Martin Corporation (4) Exploration Sciences

Abstract

High precision astrometry of nearby bright stars is theoretically (in the photon noise limit) possible using a space coronagraph combined with a wide field diffraction limited camera simultaneously imaging an annulus of background stars around the central coronagraphic field. If astrometric distortions are calibrated, sub-micro arcsecond astrometric accuracy can be achieved with a 1.4-m telescope in parallel to a 2-day long coronagraphic observation. The mass of all planets that can be imaged by the coronagraph would then be estimated. Simultaneous imaging and astrometric measurements would reduce the number of astrometric measurements necessary for mass determination, and reduce confusion between multiple planets and possible exozodiacal clouds within the coronagraphic image. While scientifically attractive, this measurement is technically very challenging, and must overcome astrometric distortions, which, in conventional telescopes, are several orders of magnitude above the photon noise limit.

In this paper, we propose a new approach to finely calibrating astrometric distortions in the wide field imaging camera of such a system. The astrometric measurement is performed by simultaneously imaging background stars and diffraction spikes from the much brighter coronagraphic target on the same focal plane array. The diffraction spikes are generated by a series of small dark spots originating directly from the primary mirror surface, in order to reduce sensitivity to optical and mechanical distortions. Small scale distortions and detector errors are averaged down to sub-micro arcsecond by rolling the telescope around the line of sight. As an example we show that sub-micro arcsec astrometry is within reach of a 1.4-m telescope having a 0.25 sq deg field of view camera. A preliminary error budget derived from an initial system simulation tool is shown and discussed to identify major sources of error for such a system. Status of an ongoing laboratory test validating the concept is provided.