Editing Mount Wilson Observatory (section)

=== 20-foot Stellar Interferometer ===

The first of these interferometers was the 20-foot Stellar Interferometer. In 1919 the 100-inch Hooker telescope was equipped with a special attachment, a 20-foot optical astronomical interferometer developed by Albert A. Michelson and Francis G. Pease. It was attached to the end of the 100-inch telescope and used the telescope as a guiding platform to maintain alignment with the stars being studied. By December 1920, Michelson and Pease were able to use the equipment to determine the precise diameter of a star, the red giant Betelgeuse, the first time the angular size of a star had ever been measured. In the next year, Michelson and Pease measured the diameters of six more red giants before reaching the resolution limit of the 20-foot beam interferometer.<ref>{{Cite web |url=http://www.mtwilson.edu/vir/100/20fti/ |title=Mount Wilson Observatory |access-date=2015-08-20 |archive-url=https://web.archive.org/web/20150712085353/http://www.mtwilson.edu/vir/100/20fti/ |archive-date=2015-07-12 |url-status=dead }}</ref>