Jet Propulsion Laboratory, California Institute of Technology
The limitations to astrometric accuracy from ground interferometers for application to planet detection are surprisingly small. One exploits the narrow-angle nature of the problem to make the position measurement relative to an angularly nearby reference. For references within 20 to 30 arcseconds of the target and observations at K, cophasing can be used to increase sensitivity in order to use very faint references. With 1.5 to 2.0 m telescopes at a good site, near full-sky coverage is available. Accuracies at the tens of microarcsecond level in an hour of integration time should be possible, enabled by long interferometer baselines as well as the small star separation. Development of interferometry at Keck Observatory has commenced, with the initial phase to be the combination of the two 10-m telescopes. Among its initial objectives is characterization of the exozodiacal environment of nearby stars in preparation for future space missions for direct planet detection. The proposed second phase would add outrigger telescopes to enable synthesis imaging as well as narrow-angle astrometry for an extensive planet search. Technology development for Keck has been ongoing with the Palomar Testbed Interferometer. The interferometer uses a 110 m baseline with 40 cm apertures at K, and achieved first fringes last year. The interferometer architecture is similar to that proposed for Keck, with active delay lines and fringe tracking, and a dual-star configuration. Current engineering work is directed toward validating the narrow-angle astrometry technique and investigating the achievable astrometric accuracy.