PTI measured the rotationally induced oblateness of the rapidly rotating star Altair, finding major and minor axes of 3.46 ± 0.04 and 3.04 ± 0.07 mas. The value of v sin i was determined independently of spectroscopic techniques.

Differential “dual-star” astrometric measurements of the nearby binary 61 Cygni AB achieved a 170 µas precision over a 45-day timescale, yielding proper motions of the components in excellent agreement with the HIPPARCOS determination of system motion.

Combined with radial velocities, Cepheid angular diameters allow determination of the stars’ distances and hence a check on this important link to the cosmic distance scale. PTI measured the change in angular size of the Cepheid ζ Gem, deriving a distance of 362 ± 38 pc.

The Palomar Testbed Interferometer is a near-IR, long-baseline stellar interferometer located at Palomar Observatory in north San Diego County. It was developed primarily to demonstrate the utility of ground-based differential astrometry in the search for planets around nearby stars, and to develop key technologies for the Keck Interferometer and space-based missions.

PTI's dual-star tracking system, the first and (still) only of its kind, simultaneously tracks interference fringes from a target star and a
reference star against which the target is measured. The current operating modes of PTI are visibility amplitude and narrow-angle
astrometry through fringe scanning.

The Michelson Science Center (MSC) currently adminsters scientific operations at PTI on behalf of the PTI collaboration, with funding from the MSC, the SIM-PlanetQuest mission, the PHASES project and the Infrared Processing and Analysis Center. PTI is a testbed for the engineering, techniques, operations, and data reduction for the Keck Interferometer. Data from the visibility mode are made available to the public after 18 months.

Publications: http://msc.caltech.edu/mission s/Palomar/publications.html
Data archive and support: http://msc.caltech.edu/softwar e/PTISupport/