Palomar Testbed Interferometer, Astronomical research facility at Palomar Observatory, San Diego County, United States

The Palomar Testbed Interferometer was a near-infrared, long-baseline stellar interferometer at Palomar Observatory in north San Diego County, California. Built by Caltech and the Jet Propulsion Laboratory, it combined light from separated apertures to measure stellar diameters, shapes and masses, and carried a "dual-star" system that observed two stars at once to cancel part of the atmospheric seeing.

The instrument was developed by Caltech and the Jet Propulsion Laboratory, operating at Palomar Observatory from 1995 to 2008. Its creation marked an important advance in interferometry techniques for astronomical observation.

Scientists at the facility produced over 50 academic papers examining stellar masses, diameters, and shapes through advanced interferometric techniques.

The facility was located on the grounds of Palomar Observatory in the hills east of San Diego. Visitors can tour the observatory to learn about its installations and exhibitions, though the interferometer itself is no longer operating.

The Altair finding came from PTI's "dual-star" system, which observed two stars at once and cancelled part of the blurring caused by the atmosphere. That same setup supported measurements of stellar diameters, shapes and masses across more than 50 refereed papers.