High precision astrometry with a diffractive pupil telescope
Submitted to ApJ
pdf file
Abstract
Astrometric detection and mass determination of Earth-mass exoplanets requires sub-uas accuracy, which is theoretically possible with an imaging space telescope using field stars as an astrometric reference. The measurement must however overcome astrometric distortions which are much larger than the photon noise limit. To address this issue, we propose to generate faint stellar diffraction spikes using a 2-D grid of regularly spaced small dark spots added to the surface of the primary mirror. Accurate astrometric motion of the host star is obtained by comparing the position of the spikes to the background field stars. The spikes do not contribute to scattered light in the central part of the field and therefore allow unperturbed coronagraphic observation of the star's immediate surrounding.
Because the diffraction spikes are created on the primary mirror and imaged on the same focal plane detector as the background stars, astrometric distortions affect equally the diffraction spikes and the background stars, and are therefore calibrated. We describe the technique, detail how the data collected by the wide field camera is used to derive astrometric motion, and identify the main sources of astrometric error using numerical simulations and analytical derivations. We find that the 1.4-m diameter telescope, 0.3 deg2 field we adopt as a baseline design achieves 0.2 uas single measurement astrometric accuracy. The diffractive pupil concept thus enables sub-uas astrometry without relying on the accurate pointing, external metrology or high stability hardware required with previously proposed high precision astrometry concepts.
SIMULTANEOUS EXOPLANET CHARACTERIZATION AND DEEP WIDE FIELD IMAGING WITH A DIFFRACTIVE PUPIL TELESCOPE
In preparation
pdf file
Abstract
High precision astrometry can identify exoplanets and measure their orbits and masses, while coronagraphic imaging enables detailed characterization of their physical properties and atmospheric compositions through spectroscopy. In a previous paper, we showed that a diffractive pupil telescope (DPT) in space can enable sub-μas accuracy astrometric measurements from wide field images by creating faint but sharp diffraction spikes around the bright target star. The DPT allows simultaneous astrometric measurement and coronagraphic imaging, and we discuss and quantify in this paper the scientific benefits of this combination for exoplanet science investigations. We show that using both
measurements to identify planets and measure their masses offers greater sensitivity and provides more reliable measurements than possible with separate missions, and therefore results in a large gain in mission efficiency. Our preliminary analysis, performed for a 1.4-m space telescope, shows that the masses of Earth-like planets can be measured to 10% precision around nearby stars. The DPT is especially powerful at identifying potentially habitable planets in multiple systems, where astrometry alone would require many measurements over a long time baseline. In addition, the combined measurement allows direct measurement of stellar masses to percent-level accuracy, using planets at test particules. We also show that the DPT maintains the full sensitivity of the telescope for deep wide field imaging, and is therefore compatible with simultaneous scientific observations unrelated to exoplanets. We conclude that astrometry, coronagraphy, and deep wide field imaging can be performed simultaneously on a single telescope without significant impact on the performance of any of the 3 techniques.