Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21542119g&link_type=abstract
American Astronomical Society, AAS Meeting #215, #421.19; Bulletin of the American Astronomical Society, Vol. 42, p.289
Astronomy and Astrophysics
Astronomy
Scientific paper
High precision (micro-arcsecond) astrometry of nearby bright stars is theoretically (in the photon noise limit) possible with a space coronagraph with the addition of a wide field diffraction limited camera imaging an annulus of background stars around the central coronagraphic field. With micro arcsecond accuracy on a 1.4-m telescope, the mass of all planets that can be imaged by the coronagraph would 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 in the coronagraphic image. While scientifically attractive, this measurement is technically very challenging, and must overcome astrometric error terms, which, in conventional telescopes, are several orders of magnitude above the photon noise limit. This paper investigates how some of these limitations could be overcome with new technical approaches, and identifies outstanding issues.
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 directly on the primary mirror to reduce sensitivity to optical and mechanical distortions.
An example error budget is shown and discussed to identify major sources of error for a 1.4-m telescope imaging a 0.1 squaredeg field of view at the galactic pole. The relative measurements are made over an isoplanatic field much smaller than 0.1 square-degree, possibly as small as 4 arcsec and the photon noise limit within such small fields would be 30 uas, while a 0.3 deg isoplanatic angle would produce a photon limited accuracy of 0.125uas. Ultimately, the actual precision will be driven by wavefront and detector stability over long periods of time, and based on controlling a slew of instrumental errors.
Guyon Olivier
Shao Maxine
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