Differential phase technique with the Keck Interferometer

We present the motivation and development of the novel "differential phase" technique being developed for the Keck Interferometer with the goal of detecting faint companions near a bright source. The differential phase technique uses simultaneous phase measurements at several infrared wavelengths to detect the astrophysical signature produced by a chromatic, asymmetric brightness distribution. We discuss the origin of the differential phase signature and present results of test observations taken at the Palomar Testbed Interferometer. One important test result is the larger than expected effect of water vapor turbulence on these multi-wavelength observations due to the infrared dispersion of water. In order to reach the design goal of 0.1 milliradians, the phase noise caused by both temperature and water vapor fluctuations in the atmosphere must be corrected, and we discuss several ways to achieve this. By combining the 10 meter Keck telescopes, this technique will enable measurement and characterization of some of the known extra. solar planets, such as the companion to 51 Peg. A large intensity difference exists between the stellar primary and the planetary companion, and the differential phase technique converts the problem of intensity dynamic range to one of phase dynamic range. In addition to enabling the direct detection of extra solar planets, this technique is applicable to other science objectives.