High Quality Content by WIKIPEDIA articles! One of the first astronomical interferometers was built on the Mount Wilson Observatory`s reflector telescope in 1920 in order to measure the diameters of stars. The red giant star Betelgeuse was among the first to have its diameter determined in this way. This method was extended to measurements using separated telescopes by Johnson, Betz and Towns (1974) in the infrared and by Labeyrie (1975) in the visible. In the late 1970s improvements in computer processing allowed for the first "fringe-tracking" interferometer, which operates fast enough to follow the blurring effects of astronomical seeing, leading to the Mk I,II and III series of interferometers. Similar techniques have now been applied at other astronomical telescope arrays, including the Keck Interferometer and the Palomar Testbed Interferometer.