Astrometry and Photometry with Coronagraphs

We propose a solution to the problem of astrometric and photometriccalibration of coronagraphic images with a simple optical device which, intheory, is easy to use. Our design uses the Fraunhofer approximation of Fourieroptics. Placing a periodic grid of wires (we use a square grid) with knownwidth and spacing in a pupil plane in front of the occulting coronagraphicfocal plane mask produces fiducial images of the obscured star at knownlocations relative to the star. We also derive the intensity of these fiducialimages in the coronagraphic image. These calibrator images can be used forprecise relative astrometry, to establish companionship of other objects in thefield of view through measurement of common proper motion or common parallax,to determine orbits, and to observe disk structure around the starquantitatively. The calibrator spots also have known brightness, selectable bythe coronagraph designer, permitting accurate relative photometry in thecoronagraphic image. This technique, which enables precision exoplanetaryscience, is relevant to future coronagraphic instruments, and is particularlyuseful for `extreme' adaptive optics and space-based coronagraphy.Comment: To appear in ApJ August 2006, 27 preprint style pages 4 figure