Interferometer, Stellar

an astronomical optical instrument for the measurement of extremely small angular separations (tenths or hundredths of a second of arc) by using the phenomenon of interference of light. It is principally used to measure the angular separation between the components of close binary stars (components that are close in brightness) and the angular diameters of stars.

A distinction is made between simple and periscopic stellar interferometers. The first is an ordinary telescope with an opaque screen placed on the objective; the screen has two openings that are identical in form, for example, parallel slits. In this case interference fringes are observed on a star’s image; their appearance changes as the distance between the openings in the screen is changed and, in the case of binary stars, as a result of the mutual orientations of the line joining the components of the binary star and the openings in the screen. The simple stellar interferometer permits the resolving power of a telescope to be approximately doubled.

In the periscopic stellar interferometer, proposed by A. A. Michelson (USA), an optical system of two pairs of plane mirrors is mounted in front of the telescope’s objective, permitting two widely separated light rays from the source being measured to be directed into the telescope’s objective. This system increases the resolving power of the telescope in proportion to the distance between the outer mirrors. The angular diameters of several stars were measured for the first time in 1920–21 with the help of a periscopic stellar interferometer.