Observatories, Ground

Astronomers study the universe by measuring electromagnetic radiation—gamma rays , X rays , optical and infrared radiation , and radio waves—emitted by planets, stars, galaxies, and other distant objects. Because Earth's atmosphere is transparent to optical and infrared radiation and to radio waves, these types of radiation can be studied from ground-based observatories. Astronomers must launch telescopes into space in order to study X rays, gamma rays, and other radiation that is blocked by absorption in Earth's atmosphere.

Astronomers make use of ground-based observatories whenever they can. It is about 1,000 times cheaper to build a telescope of a given size on the ground than to launch it into space, so it is much more economical to operate on the surface of Earth.

A telescope can be thought of as a bucket that collects light or radio waves and brings them to a focus. More light can be gathered with a larger bucket. Since most astronomical sources of light are very faint, it is desirable to build telescopes as large as possible. Given current technology, we can build much larger telescopes on the ground than we can in space, which is another reason that ground-based observatories remain very important.

The Locations of Ground-Based Observatories

The best ground-based sites for optical and infrared astronomy are Mauna Kea, a volcano on the Big Island of Hawaii that is 4,205 meters (13,796 feet) high, and mountain peaks in the desert in northern Chile. Other good sites are in the Canary Islands and the southwestern United States. The following are the characteristics that astronomers look for when they select a site for an optical/infrared telescope:

1. Clear skies. The best sites in the world are clear about 75 percent of the time. Most types of astronomical observations cannot be carried out when clouds are present.
2. Dark skies. The atmosphere scatters city lights, making it impossible to see faint objects. The best sites are therefore located far away from large cities. (Even with the naked eye, one can see quite clearly the difference between what can be seen in the night sky in a city and in the country.)
3. High and dry. Water vapor in Earth's atmosphere absorbs infrared radiation. Fortunately, water vapor is concentrated at low altitudes, and so infrared observatories are best located at high altitudes.
4. Stable air. Light rays are distorted when they pass through turbulent air, with the result that the image seen through a telescope is distorted and blurred. The most stable air occurs over large bodies of water such as oceans, which have a very uniform temperature. Therefore, the best sites are located in coastal mountain ranges (e.g., in northern Chile or California) or on isolated volcanic peaks in the middle of oceans (e.g., Mauna Kea).

The Hubble Space Telescope is above Earth's atmosphere, so its images are much clearer and sharper than the distorted images that are observed from the ground. Astronomers are, however, devising techniques called adaptive optics that can correct atmospheric distortions by changing the shapes of small mirrors hundreds of times each second to compensate precisely for the effects of Earth's atmosphere. Even when this technique is perfected, space observatories will still be needed to observe gamma rays, X rays, ultraviolet radiation , and other wavelengths that are absorbed by Earth's atmosphere before they reach the ground.

The requirements for radio observatories are not nearly so stringent as for optical/infrared telescopes, and many types of radio observations can be made through clouds. Therefore, countries that do not have good optical/ infrared sites, such as Great Britain, Japan, the Netherlands, and Germany, have concentrated on radio astronomy.

While they are not bothered much by clouds or city lights, radio telescopes are affected by electrical interference generated by cell phones, radio transmitters, and other artifacts of civilization. Therefore, radio telescopes are often located far away from large population centers in special radio-quiet zones. Also, certain radio wavelengths are reserved for the use of radio astronomy and cannot be used to transmit human signals.

Optical and Infrared Telescopes

There are two main types of telescopes: refracting telescopes, which use lenses to gather the light and form an image; and reflecting telescopes, which use mirrors to accomplish the same purpose. Telescopes are described by the size of the largest lens or mirror that they contain. The largest refracting telescope ever built is the Yerkes 40-inch (1-meter) telescope, which is located in southeastern Wisconsin. Refractors are limited to fairly small sizes for two reasons. First, since the light must pass through a lens to be focused, the lens must be supported around its outside edge, not from behind. Large lenses tend to sag and distort in shape because of the effects of gravity, and the focused image is not as sharp as it should be. Second, because the light passes through the lens, the glass must be entirely free of bubbles or other defects that would distort the image. It is difficult and costly to make large pieces of perfect glass.

Reflecting telescopes make use of mirrors. Since the light is reflected from the front surface, mirrors can be supported from behind and can therefore be made as large as several meters in diameter. The front surface is coated with highly reflective (shiny) aluminum or silver. Since the light in a reflector never passes through the mirror, the glass can contain a few bubbles or other flaws. For these reasons the largest telescopes in the world are reflectors.

Reflecting telescopes are used for both infrared and optical astronomy. Because glass does not transmit infrared radiation very efficiently, refracting telescopes are unsuitable for most kinds of infrared astronomy.

New Technology Telescopes

For about forty years after its completion in 1948, the Palomar 5-meter (16.7 feet) reflector in southern California was the largest telescope in the world. The 5-meter Palomar mirror is very thick and is therefore rigid enough not to change shape when the telescope tracks stars as they rise in the east and set in the west. The Palomar mirror weighs about 20 tons, and a very large steel structure (weighing about 530 tons) is required to hold it. The Palomar telescope is near the limit in size of what can be built for a reasonable cost with a massive, rigid mirror.

In the 1990s, many countries took advantage of developments in technology to build telescopes with diameters of 6.5 to 10 meters (21 to 33 feet). It is now possible to use thin telescope mirrors, which do change shape when they are pointed in different directions. High-speed computers calculate the forces that must be applied to the flexible mirrors to produce the correct shape. These restoring forces can be adjusted many times each second if

necessary. A lightweight thin mirror can be supported by a lightweight steel structure, and telescopes double the size of the Palomar telescope are affordable with this new technology.

At the dawn of the twenty-first century, the largest single mirror that has been manufactured to date is 8.4 meters (27.5 feet) in diameter, and it is scheduled to be installed in a telescope in southern Arizona in 2003. This is probably about the largest single mirror that is feasible. Given the width of highways and tunnels, it would be impossible to transport a much larger mirror from where it was manufactured to a distant mountaintop.

Currently the largest telescopes in the world are the twin 10-meter (33-foot) Keck telescopes on Mauna Kea. These telescopes do not contain a single mirror that is 10 meters in diameter. Rather, each consists of thirty-six separate hexagonal-shaped mirrors that are 1.8 meters (6 feet) in diameter. These mirrors are positioned so precisely relative to one another that they can collect and focus the light as efficiently as a continuous single mirror.

Radio Telescopes

Radio astronomy is a young field relative to optical astronomy. Italian mathematician and astronomer Galileo Galilei used the first optical telescope, a refractor, in 1610. By contrast, American electrical engineer Karl Jansky first detected astronomical radio waves in 1931. Astronomical radio waves cannot be heard. Like light, radio waves are a form of electromagnetic radiation. Unlike light, however, we cannot sense radio waves directly but must use electronic equipment. Radio waves are reflected by surfaces that conduct electricity, just as light is reflected by a shiny aluminum or silver surface. Accordingly, a radio telescope consists of a concave metal reflector that focuses the radio waves on a receiver.

Interferometry

Resolution refers to the fineness of detail that can be seen in an image. The larger the telescope, the finer the detail that can be observed. One way to see finer detail is to build a larger single telescope. Unfortunately, there are practical limits to the size of a single telescope—currently about 10 meters (33 feet) for optical/infrared telescopes and about 100 meters (330 feet) for radio telescopes. If, however, astronomers combine the signals from two or more widely separated telescopes, they can see the fineness of detail that would be observed if they had a single telescope of that same diameter. Telescopes working in combination in this way are called interferometers . For example, infrared radiation falling on the two 10-meter Keck telescopes, which are about 85 meters (279 feet) apart, has been combined, allowing astronomers to obtain the kind of detailed image that they would observe if they had a single telescope 85 meters in diameter.

Radio interferometry is easier than optical and infrared interferometry because radio waves have much longer wavelengths than optical or infrared radiation. The equipment used to measure radio waves need not be built to the same precision as optical telescopes, and radio waves are not distorted very much by turbulence in Earth's atmosphere. For these reasons, radio astronomers have been able to build whole arrays of telescopes separated by thousands of kilometers to conduct interferometry. For example, U.S. astronomers operate the Very Long Baseline Array, which consists of ten telescopes located across the United States and in the Virgin Islands and Hawaii. When combined with a telescope in Japan, this array of radio telescopes has the same resolution as a telescope with the diameter of Earth.

The Future of Ground-Based Observatories

By 2003, fourteen mirrors with diameters larger than 6.5 meters (21.3 feet) will have been installed in optical/infrared telescopes. During the early twenty-first century, these telescopes are likely to produce many impressive discoveries. But astronomers are already planning for the next generation of large telescopes. These will truly be "world" telescopes. The costs, which are estimated to be several hundred million dollars each, are beyond the reach of any single country. Therefore, the new, very large telescopes will be built through international consortia involving many countries.

Astronomers in Europe are exploring the feasibility of building an optical/ infrared telescope that is 100 meters (330 feet) in diameter—about the length of a football field. This telescope is called the OWL telescope, which stands for Overwhelmingly Large Telescope. The mirror would be built in the same way as the Keck mirrors, that is, by combining literally thousands of smaller mirrors to form a single continuous surface. This telescope would be powerful enough to study objects present when the universe was only a few million years old. The current age of the universe is about 14 billion years, and so with a telescope such as OWL astronomers could observe directly the evolution of the universe throughout nearly all of its history.

In radio astronomy, the next major project is likely to be the Atacama Large Millimeter Array (ALMA). The project will be an interferometer that detects radio radiation with wavelengths between 0.350 and 10 millimeters (0.014 and 0.4 inches). The facility will consist of sixty-four radio antennas, each 12 meters (39 feet) in diameter, with the separations between antennas varying from 150 meters (490 feet) to 10 kilometers (6.2 miles). ALMA will be located at one of the driest spots on Earth—a large plateau at an altitude of 5,000 meters (16,400 feet) in the Atacama Desert in northern Chile. Water vapor in Earth's atmosphere absorbs much of the millimeter wavelength radiation that astronomers would like to detect, and so it is important to select an extremely dry site. The facility will be particularly useful for studying how stars and planets form and what galaxies were like when the universe was very young.

see also Astronomer (volume 2); Astronomy, History of (volume 2); Astronomy, Kinds of (volume 2); Careers in Astronomy (volume 2); Hubble Space Telescope (volume 2); Observatories, Space-Based (volume 2).

Sidney C. Wolff

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