First fringe measurements with a phase-tracking stellar interferometer.

A prototype two-telescope stellar interferometer with a 1.5-m base line has been used to track the white-light fringes, 0.4-0.9 microm, from Polaris. Continuous fringe phase and amplitude measurements were made with ~220-photon/4-msec integration time and 1.27-cm(2) collecting area under 2-arc sec seeing conditions. The same control algorithm should be able to track fringes from an 8.7-mg star using the light from two 13-cm (5-in.) telescopes and a 10-msec integration time under 1-arc sec seeing conditions. When tracking, the servo maintained equal path lengths to 0.1-microm rms in the two arms of the interferometer, thus cancelling the path-length variations caused by earth rotation and atmospheric turbulence. In the future, two-color phase measurements will make optical aperture synthesis and optical very long-base-line astrometry possible.