Palomar Testbed Interferometer in the context of Interferometer

The Palomar Observatory is an astronomical research observatory in the Palomar Mountains of San Diego County, California, United States. It is owned and operated by the California Institute of Technology (Caltech). Research time at the observatory is granted to Caltech and its research partners, which include the Jet Propulsion Laboratory (JPL), Yale University, and the National Astronomical Observatories of China.

The observatory operates several telescopes, including the 200-inch (5.1 m) Hale Telescope, the 48-inch (1.2 m) Samuel Oschin telescope (dedicated to the Zwicky Transient Facility, ZTF), the Palomar 60-inch (1.5 m) Telescope, and the 30-centimetre (12-inch) Gattini-IR telescope. Decommissioned instruments include the Palomar Testbed Interferometer and the first telescopes at the observatory, an 18-inch (46 cm) Schmidt camera from 1936.

Altair is the brightest star in the equatorial constellation of Aquila and the twelfth-brightest star in the night sky. It has the Bayer designation Alpha Aquilae, which is Latinised from α Aquilae and abbreviated Alpha Aql or α Aql. Altair is an A-type main-sequence star with an apparent visual magnitude of 0.77 and is one of the vertices of the Summer Triangle asterism; the other two vertices are marked by Deneb and Vega. It is located at a distance of 16.7 light-years (5.1 parsecs) from the Sun. Altair is currently in the G-cloud—a nearby interstellar cloud formed from an accumulation of gas and dust.

Altair rotates rapidly, with a velocity at the equator of approximately 286 km/s. This is a significant fraction of the star's estimated breakup speed of 400 km/s. A study with the Palomar Testbed Interferometer revealed that Altair is not spherical, but is flattened at the poles due to its high rate of rotation. Other interferometric studies with multiple telescopes, operating in the infrared, have imaged and confirmed this phenomenon.