Very Large Telescope is one of the world’s largest and most powerful telescopes. It was built at Paranal Observatory on Paranal Mountain, about 80 miles (130 kilometers) south of Antofagasta, Chile. Although its name suggests that it is a single telescope, it actually consists of eight telescopes. The four major telescopes were completed in 2000. They are mainly used independently. Four smaller auxiliary telescopes were finished in 2007. The smaller telescopes are moveable, and all the telescopes can be used together. This enables astronomers to operate the telescopes in various ways. For example, the astronomers can make any number of the eight telescopes work together as if they were a single device.
The builder and operator of the VLT is the European Southern Observatory— ESO << EE soh >> —an organization of 15 countries: Austria, Belgium, Brazil, Czech Republic, Denmark, Finland, France, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland, and the United Kingdom. ESO’s headquarters are in Munich, Germany. ESO designed the telescopes, and European companies built them. Most of the instruments for the telescopes were built in a cooperative effort involving ESO and research institutes in member companies.
Each Unit Telescope ranks among the world’s largest optical telescopes—the main mirror of each measures 27 feet (8.2 meters) in diameter. The total light-collecting area of the four main mirrors is about 2,300 square feet (210 square meters). When the four telescopes are operated in combination, they have the light-gathering power of a single telescope whose mirror is 52 feet (16 meters) in diameter.
The other four telescopes, called Auxiliary Telescopes, are much smaller. Their main mirrors measure about 71 inches (1.8 meters) in diameter.
ESO named the four Unit Telescopes in the language of the Mapuche people, most of whom live in central Chile. The four are: (1) Antu, which means sun in the Mapuche language; (2) Kueyen, which means moon; (3) Melipal, the Mapuche name for the Southern Cross, a constellation; and (4) Yepun, the Mapuche name for the planet Venus when it is seen as the evening star. Antu made its first observations in May 1998; Kueyen in March 1999; Melipal in January 2000; and Yepun in September 2000.
Instruments.
Each Unit Telescope of the VLT is equipped with about a dozen instruments that enable it to observe objects in both visible light and infrared radiation. Visible light and infrared radiation are two forms of electromagnetic radiation, which moves through space in waves. The different forms of this radiation differ in wavelength, the distance between the crest of one wave and the crest of the next. Infrared radiation has longer wavelengths than does visible light.
The instruments on the telescopes include cameras and spectrographs. A spectrograph spreads out radiation collected by a telescope into a spectrum (band) according to the wavelength of the radiation. The spectrum of visible light, for example, is a rainbowlike band of colored light. Red light is at one end of the spectrum, and violet light is at the other. Astronomers study an object’s spectrum because it contains information about the object’s temperature, density, chemical composition, and even its distance.
In 2009, the VLT received the first in a series of instrument upgrades. The X-shooter spectrograph will allow the telescope to collect data across the entire spectrum, from infrared light to ultraviolet light, at the same time. Until the X-shooter, large telescopes could only collect data in a small spectrum of light at any one time. Because the X-shooter can cover a broad spectrum of light, it can do the work of several telescopes at once.
Imaging systems.
A major feature of each Unit Telescope is an active optics system. ESO first developed active optics in the 1980’s for its New Technology Telescope at the La Silla Observatory, near La Serena, Chile. An active optics system uses a computer to control the shape of the main mirror. This system keeps the images created by the telescope as sharp as possible as the telescope turns to follow the stars. The computer corrects the shape of the mirror by moving 150 pistonlike hydraulic supports on which the mirror rests. The supports move only a few thousandths of a millimeter.
Each Unit Telescope also has an adaptive optics system. This kind of system corrects a blurring effect caused by movements of the gases that make up Earth’s atmosphere. These movements cause blurring by distorting the path of the light coming from the object being observed.
The main part of an adaptive optics system is a small, flexible mirror. This mirror intercepts the light after it has been gathered by the main mirror but before it reaches the focus of the telescope. To correct for blurring, computer-controlled devices called actuators can change the shape of the flexible mirror up to 100 times per second.
VLT scientists can use a technique called interferometry to produce extremely sharp images. Light beams from the four Unit Telescopes and the four Auxiliary Telescopes travel through tunnels to a common focus in an underground laboratory. The resulting images are so sharp that it will be possible—at least in theory—to produce a picture of a 2-meter-long object at a distance of 400,000 kilometers. Thus, the VLT could produce a picture of an astronaut on the surface of the moon.
Observations with the VLT.
In 1999, astronomers at the VLT began to study some of the faintest and most remote galaxies in the universe. Other researchers are investigating the presence of black holes at the centers of certain galaxies. Although it is not possible to see a black hole directly, the VLT can detect what goes on in a black hole’s immediate vicinity.
Another major area of investigation is the formation of stars inside dense clouds of gas and dust. To study star formation, the VLT uses instruments that are sensitive to infrared radiation. These instruments can “look” into the centers of the clouds.
Astronomers could use two or more Unit Telescopes to observe a single object in different ways at the same time. For example, astronomers could mount a camera on one telescope and a spectrograph on another. The ability to make many kinds of observations would be especially useful in the study of variable stars and other objects whose brightness changes rapidly. Astronomers could use spectrographic data to determine what changes in temperature, density, and so forth accompany the changes in brightness.
Astronomers also plan to use the VLT to detect and determine the characteristics of planets that are orbiting stars other than the sun. First, the researchers would use infrared-sensitive instruments to obtain images of the planets. They would then use spectrographs to determine the composition of the planets.
