Telescope

C

Resolution

An optical telescope’s resolution—the ability to see fine detail—increases with mirror or lens size. However, Earth’s turbulent atmosphere provides a practical limit on resolution because it blurs incoming starlight. This effect makes stars appear to twinkle at night.

With the use of computers, astronomers are developing adaptive optics that essentially take the blur out of starlight. Astronomers use computers to analyze the blurring created by the atmosphere and compensate for it by rapidly distorting the mirrors in a reflecting telescope. The Keck II telescope at Hawaii’s Mauna Kea Observatory was outfitted with such technology in 1999, enabling it to take pictures that are 20 times more detailed than before. Telescopes using adaptive optics can resolve something the size of a quarter at a distance of more than 50 kilometers (30 miles).

D

Optical Interferometry

A new technique in optical astronomy is to combine signals from telescopes in separate locations so that the resulting image is equal to that received from one giant telescope, a method called optical interferometry. In 2001 the European Southern Observatory opened the largest optical interferometer, the Very Large Telescope (VLT), in the Atacama Desert in northern Chile. The VLT combines the light from four 323-in (820-cm) telescopes and several smaller telescopes to produce an image equivalent to that of a 630-in (1,600-cm) telescope.

Optical interferometers are useful for resolving the separation between relatively bright, closely paired objects, such as double stars. Astronomers hope this technique will eventually make it possible to directly image small, Earth-sized planets orbiting distant stars.

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E

Recording Images

Throughout most of the history of astronomy, scientists have viewed celestial objects through a telescope’s eyepiece. When photography was invented in the 1800s, one of its first applications was to attach a camera to a telescope to make a photograph of the Moon. Photography permitted astronomers to record and archive what they saw. Photographic time exposures exceeded the eye’s sensitivity and recorded very faint objects, often in rich colors.

Today, photographic film in telescopes has been largely replaced by solid-state detectors called charge-coupled devices (CCDs). These thumbnail-sized silicon chips are divided into millions of picture elements, called pixels, that convert incoming starlight into an electric charge that is read by computer. The resulting mosaic of bright and dark pixels creates a picture. CCDs provide much greater sensitivity and contrast than photographs do, and the image is automatically recorded in digital form for subsequent storage and enhancement by computer image processing. CCDs can also record more wavelengths of light than cameras can, from the visual edge of the ultraviolet region to the near-infrared.

III

Radio Telescopes

Radio astronomy was discovered in 1931 when Bell Telephone Laboratories engineer Karl Jansky, using a makeshift antenna, realized that annoying radio static was actually coming from the core of our galaxy. This was the first time that scientists realized that radio waves could come from nonterrestrial sources. In the years since, many major discoveries in radio astronomy have similarly occurred by accident or coincidence, including the detection of active galaxies, pulsars, and the glow of the big bang itself.

The fundamental design of a radio telescope is similar to that of an optical telescope, but radio telescopes must be larger because they are looking at longer wavelengths of electromagnetic radiation. Radio waves are typically between 1 m (3 ft) and 1 km (0.6 mi) in length, while visible light waves are only about 1 micrometer, or 0.001 mm (0.00004 in) long. Radio waves can be focused and gathered more easily than light waves because of their length. As a result, the bowl-shaped surfaces of radio telescopes do not need to be as smooth as their optical counterparts and are crafted of steel and wire mesh.

Radio astronomers have a unique advantage because faint radio signals can be detected around-the-clock, while the electromagnetic radiation from the Sun makes observing other wavelengths difficult during the day. The energy radio telescopes receive from distant sources is extraordinarily weak, less than the energy released when a snowflake hits the ground. To detect these faint sources, radio telescopes must be located in valleys and other areas naturally shielded from artificial radio waves. The largest radio telescope dish, built into a bowl-shaped valley in Arecibo, Puerto Rico, is 305 m (1,000 ft) across (see Arecibo Observatory).

To see objects in as much detail as a large optical telescope, a radio telescope would need to be about 50 times the size of the Arecibo telescope. By simultaneously linking signals from two or more radio telescopes in separate locations, a technique called radio interferometry, astronomers create a huge telescope whose power is equal to a telescope as large in diameter as the separation between the two smaller telescopes. If more telescopes are added, the resolving power is even greater.

One of the largest radio interferometers is the Very Large Array (VLA) near Socorro, New Mexico. It is a Y-shaped array of 27 dish-shaped antennas 25 m (82 ft) wide, extending over three arms 21 km (13 mi) long. The VLA can see objects emitting radio waves 1,000 times more sharply than optical telescopes can see light-producing objects. The power of the VLA is dwarfed by the VLBI (Very-Long Baseline Interferometer), which consists of ten dish-shaped antennas, each 25 m (82 ft) in diameter, strung between Hawaii and the United States Virgin Islands. The VLBI is equivalent to a single telescope almost 8,000 km (5,000 mi) across. One problem that plagues radio telescopes, the VLA in particular, is interference from ground-based sources of radio waves. As cellular phone companies, television broadcasters, and air-traffic controllers use up frequencies in the radio wave range, radio astronomers struggle to keep frequencies important to their research free of interference.

IV

Infrared Telescopes

Infrared astronomy permits scientists to explore the dark dusty region of space both within and beyond our galaxy to uncover clues about the birth of stars, formation of planetary systems, behavior of comets and planetary atmospheres, the core of the Milky Way Galaxy, and the birth of some of the most distant galaxies in the universe. Despite the fact that Earth’s atmospheric water vapor absorbs some infrared light, research can be performed from dry high-altitude observing sites and aircraft. Even better is infrared astronomy from space-based telescopes, which offer a crystal clear view, free of the background glow produced by Earth’s atmosphere (see Infrared Space Observatory).

Infrared telescopes use the basic design of an optical reflecting telescope, but have a detector at the focus that sees only infrared light. Because heat produces infrared radiation, the signal that an infrared telescope receives can be contaminated by the heat of the atmosphere if the telescope is Earth-based, as well as by the heat produced by the telescope itself. To adjust for this contamination, telescopes often take frequent readings of the background radiation away from the object being observed. The background radiation is then subtracted from the final image of the observed object. Infrared telescopes are also cooled to very low temperatures to reduce heat contamination of the image.