Astronomical interferometry, from the visible to sub-mm waves

Abstract

The history of astronomical interferometry dates back to the 19th century, when Fizeau, Stephane and later Michelson set out to measure the angular diameters of stars. Whereas Stephane could only obtain an upper limit of0.158 arcseconds for the stars he observed in 1874, Michelson published interferometric diameters of Jupiter's Galilean moons in 1891. These early experiments used masks with two small openings placed on the aperture of astronomical telescopes to perform double-slit experiments ala Young. Around 1920 Michelson increased the resolution by placing a periscope on the front-end of the 100inch telescope on Mt. Wilson; this arrangement enabled him and his collaborators to determine the orbit of the binary system Capella, and to measure the diameter of Betelgeuse. After these initial successes, technological difficulties slowed further progress for several decades. In the 1950's and 60's Hanbury Brown, Twiss, and collaborators invented intensity interferometry! and used it to measure the diameters of 32 stars this is still the best data set on hot stars available today. The era of modern stellar interferometry began in 1974 when Labeyrie successfully combined the light from two telescopes spaced by 12 m. This breakthrough was soon followed by the construction of similar instruments, which took advantage of modern electronic detectors and computer control to track and stabilize the interference fringes with real-time servo loops, which drive optical delay lines that compensate for the optical pathlength difference and thus maintain coherence between the two arms of the interferometer (see Figure 1). This active pathlength controlwith millisecond time resolution is a crucial aspect of ground-based astronomical interferometry, because the Earth's rotation and random atmospheric fluctuations have to be tracked continuously.