VLA Field Day

Abstract

think it's all papier mache. I think all those papers that are coming out are hoaxes. So spoke an astronomer walking through the corridor at the recent meeting of the American Astronomical Society. He was joking, of course -or was he quite? It is hard even for experienced astronomers to truly assimilate the reality of an instrument like the Very Large Array of radio telescopes that stands on the plains of San Augustin in west central New Mexico. Managers of many large astronomical observatories like to give occasional tours to visiting astronomers. It helps give the visitors, who are potential users, a feel for what they can do with the facility before they draw up plans for an observational program. It this case it may have meant even more. So, after the AAS meeting ended in Albuquerque, two busloads of astronomers with one SCIENCE NEWS reporter along set out for the VLA site. The route ran down the Rio Grande valley to the little town of Socorro, where the VLA'S administrative offices are located. Socorro has the windswept appearance of so many far western towns. The houses and the few trees seem to huddle together for company against the wideness and openess of the landscape. The climb from Socorro to the VLA site leads past the village of Magdalena, which looks even lonelier amidst the landscape. The Plains of San Augustin are an ancient lake bed at about 7,000 feet above sea level surrounded on all sides by mountains going as high as 10,000 feet. So flat are the plains and so complete the surrounding that drainage is a problem. When it rains shallow transient lakes appear here and there on the plain and persist for days. The VLA is not a Potemkin interferometer. The 27 antennas that may be deployed at any one time may be white, but papier mache they are not. From the center of the Y-shaped arrangement, they march off, 11.18 miles to the north, 13 miles to the southeast and southwest, like files of futuristic Dutch windmills, gleaming against the dunnish colors of the landscape. Some local residents have deplored the esthetic effect. more serious complaint from neighboring ranchers is that the VLA'S access roads make things easier for rustlers. The wild west never dies; it just adopts tractor trailers. The location was chosen to be high, remote, lonely and bordered by mountains so as to minimize interference from terrestrial broadcasts and artifacts with the celestial emanations the VLA is made to observe. It does that. It also prompted comparisons by visiting astronomers. Over the bologna sandwiches at lunch came tales of the loneliness and altitude at places from Mauna Kea to Cerro Tololo to Sunspot, N.M., to interior Oregon. Perhaps one reason south central Arizona is a favorite spot for telescopes is that the astronomers can go home to Tucson when their observing is done. Most of the VLA staff choose to live in Socorro. A modern piece of astronomical equipment consists of a computer with a telescope attached. So runs another corridor comment. In radio astronomy the data never went directly from human retina to human brain for obvious reasons, but even in optical astronomy now the computer does everything but trigger supernovas. The VLA can be described as a complex computer system with 27 radio receivers hung at the foci of 27 reflecting dishes as its nerve endings. Yet one should not neglect these nerve endings. It is at them that the physics and astronomy is done. Waves received from a given astronomical source at different points can be added together to give sum and difference waves. Generally the waves received at different points will vary in phase and in amplitude. Traditional interferometry, as this adding and subtracting of signals received is called, worked mainly with the phase difference. In visible light these differences produce a pattern of alternating light and dark lines called fringes when the addition of signals is visualized. The term is still used even though the work is mostly done with computers. Now more and more information is drawn 0