High on the desolate Plaines of San Agustin 50 miles West of Socorro, New Mexico is a scene right out of science fiction. The Very Large Array or VLA is the world's largest radio telescope. 27 individual dishes, each 82 feet in diameter. The dishes form a "Y" and are movable along a special pair of railroad tracks to create an effective diameter of from 2000 feet to 19 miles.
The VLA is part of the National Radio Astronomy Observatory (NRAO) which was funded by the National Science Foundation.
Our radio eyes to the universe were opened by Karl Jansky in the early 1930's. Working at Bell Laboratories, Jansky was attempting to discover the source of interference to experimental microwave transmission. Because the interference repeated exactly every 23 hours, and 56 minutes, he correctly theorized that the origin must be extraterrestrial, and probably came from the center of the Milky Way.
As Jansky was finishing his work on microwave interference, Grote Reber, a young radio engineer working in his spare time constructed a 30 foot parabolic dish in his backyard in Wheaton, Illinois. Reber tried to observe Jansky's cosmic radiation, but was unable due to limitations of his equipment. By improving his equipment and observing in a lower 160MHz band, Reber was able to map the radio sky by the early 1940's.
Raber published a paper in the Astrophysical Journal in 1944. The paper contains a detailed radio map of the sky. The concentration of radiation towards the galactic center and the plane of the Milky Way are clearly shown. The now well known intense radio sources of Cygnus A and supernovae Cassiopeia A are also clearly shown.
Professor Jan H. Oort, director of the Leiden Observatory in Holland was very interested in Reber's article. Oort realized that the radiation observed must extend over a broad spectrum. Oort knew that if some monochromatic line radiation existed in the radio spectrum, significant advances could result because it would identify the elemental source. 21.1 cm (1,420MHz), corresponding to the neutral hydrogen of interstellar space was suggested. That emission was detected in 1951 at Harvard University.
The pace of radio astronomy research quickened after World War II. In 1946, construction was started on a 66 meter dish at the University of Manchester in England. This was followed by a 76 m dish at the same institution. For many years this instrument at Jodrell Bank was the world's largest.
In 1961 the National Science Foundation began planning a very large radio telescope array. An experimental model with a three dish array was built in Green Bank, West Virginia at the National Radio Astronomy Observatory. In 1972, Congress authorized construction and appropriated funding for the 27 dish Very Large Array (VLA). An isolated site was selected at 7000 feet above sea level on the Plains of San Agustin in New Mexico. Construction began in 1975 and the last telescope was completed in 1979, and the fully completed VLA was dedicated on October 10, 1980. The total cost of the VLA was $78.6 million dollars.
The VLA may be reconfigured. It acts like a zoom lens. With the dishes at their farthest distance (the D configuration) the entire array is approximately 20 miles in diameter. In this configuration it acts like a wide angle lens, providing low resolution imaging of a large area. In the closer together configurations, like the A configuration, where the diameter is less than 0.4 miles a high resolution "telephoto" image is achieved.
A crucial part in the formation of a radio image is the combination of all 27 telescope signals in the correlator. The travel time of the 27 signals must not differ from each other by more than 1 billionth of a second (the time it takes a radio signal to travel about a foot). Since the telescopes can be separated by many miles, the actual travel time can be 1/10,000 of a second or more. These differences are removed by delaying the signals of some telescopes before they are combined in the correlator. The "correlated" data is sent to a high speed computer for processing and storage. At its peak observing rate the VLA generates about 10 million numbers every minute.
In the computer a huge data base is checked for a variety of defects which can occur during observation. Every 30 minutes the VLA observes standard radio sources in order to calibrate the image processing. This edited data is then sent to the image processing computer which calculates the radio image using a Fourier transformation. This transform turns the data into an image. Each pixel represents a location in the sky and the number associated with the pixel is assigned a color.
Color photograph are often used to display radio images. It should be noted that the colors are not real colors, but assigned to represent information like signal strength or frequency.
The 1997 Warner Brothers science fiction movie Contact was filmed at the VLA. It stars Jodie Foster and was based on Carl Sagan's book about Earth's first encounter with an alien civilization.
© 1999 Michael Conti