Radio ObservatoriesThe second great stimulus to
observatory building was initiated when astronomical observations were extended
beyond the region of visible light seen by normal telescopes. This era began in 1931 with Karl Jansky's discovery in Holmdel, N.J., of radio waves coming from a region in Sagittarius. The development of radio astronomy was slow at first. An electrical engineer in Wheaton, Ill., Grote Reber, built his own radio
observatory, a 31.5-ft (9.6-m) parabolic dish that could be swung in the plane of the meridian. With this backyard telescope Reber proceeded to map, for the first time, the radio Milky Way. During World War II electronic techniques for detecting and analyzing radio waves were developed so rapidly that radio
observatories mushroomed in the late 1940s and through the 1950s, first in Australia, England, and Holland and later in the United States. The north-south imbalance is more severe in radio astronomy than in optical astronomy today, but effective efforts are being made by Australian radio astronomers.Radio telescopes and radio observatories are a more heterogeneous group than their optical counterparts, and radio observations can usually be made day or night and in any weather. Radio observatories are often located in remote valleys, free from local interference. Some are very large. For example, the Very Large Array (VLA) in New Mexico (see National Radio Astronomy Observatory) is 35 km (22 mi) in diameter. It consists of an equiangular Y-configuration, each leg of which is 21 km (13 mi) long and contains a lineup, with variable spacing, of nine solid-surface dishes, each one 25 m (82 ft) in diameter. The 305-m (1,000-ft) Arecibo Observatory dish is fixed in a natural bowl in Puerto Rico. The 5-km (3-mi) telescope at the Mullard Radio Astronomy Observatory (MRAO) near Cambridge, England, consists of a 5-km lineup of four fixed and four movable dishes. Stars emitting extremely regular radio signals, known as pulsars, were discovered at the MRAO in 1967 using a radio telescope covering an area of 1.6 ha (4 acres). England's Nuffield Radio Astronomy Laboratories expanded its own interferometry operations in the 1980s with a multielement, radio-linked network. Any two, or more, radio telescopes can be operated in unison as a Very Large Baseline Interferometer (VLBI). A continent-size Very Long Baseline Array (VLBA), consisting of ten dishes ranging across the United States, was completed in the 1990s. Astronomers have also experimented with a world-spanning 12,900-km (8,000-mi) VLBA and proposed incorporating space-based radio telescopes along with ground-based ones.Atmospheric water-vapor bands seriously limit some infrared and radio observations, and for this reason infrared- and millimeter-wavelength radio telescopes are being built or proposed for the Mauna Kea Observatory.