Telescope

I INTRODUCTION

Telescope, device that permits distant and faint objects to be viewed as if they were much brighter and closer to the observer. Telescopes are typically used to observe the skies.

For hundreds of years, telescopes were the only instruments available for studying the planets and stars. Even today, space probes can reach only our closest neighbors in the heavens, and scientists continue to rely on telescopes to learn about distant stars, nebulas, and galaxies. Telescopes are the fundamental research instruments that enable astronomers to tackle scientific questions about the birth of the universe (see Big Bang Theory; Cosmology); the emergence of structure in the early universe; the formation and evolution of stars, galaxies, and planetary systems; and the conditions for the emergence of life itself.

Most telescopes work by collecting and magnifying visible light that is given off by stars or reflected from the surface of planets. Such instruments are called optical telescopes. Conventional optical telescopes use a curved lens or mirror to collect light and bring it to a focus, a point in space where all the light rays converge (see Optics). A small magnifying lens, called an eyepiece, placed at the focus allows the image to be viewed. In astronomical research, cameras or other instruments placed near the focus make a precise recording of the light gathered by a telescope. The visible light collected by a telescope is divided into component wavelengths, or colors, through a process called spectroscopy. This powerful technique, which uses a prism or diffraction grating, essentially “decodes” starlight to yield information about an object's temperature, motion and other dynamics, chemical composition, and the presence of magnetic fields.

Light rays, however, are just one part of what scientists call the electromagnetic spectrum (see Electromagnetic Radiation). Just as stars emit visible light, they also give off other types of electromagnetic radiation, including radio waves, microwaves, infrared light, ultraviolet light, X rays, and gamma rays. All these forms of electromagnetic radiation are emitted as waves.

Rapid advances in astrophysics and optical technology, coupled with the advent of the space age, broadened telescope technology in the last quarter of the 20th century. Astronomical telescopes today come in a wide variety of shapes and sizes, dictated largely by the portion of the electromagnetic spectrum the telescope is designed to view. Telescopes today view the entire spectrum of electromagnetic radiation sweeping the universe. Each new advance in wavelength coverage has dramatically altered our view of the universe.

Many telescopes are Earth-based, located in astronomical observatories around the world. But only radio waves, visible light, and some infrared radiation can penetrate Earth's atmosphere and reach the surface of our planet. To overcome this problem, scientists have launched telescopes into space, where the instruments can collect waves from the other regions of the electromagnetic spectrum (see Space Telescope).

II OPTICAL TELESCOPES

There are two main kinds of optical telescopes—refracting and reflecting. Refracting telescopes use a lens to magnify objects; reflecting telescopes use a curved mirror.

A Refracting Telescopes

Refracting telescopes, or refractors, use a glass lens to bend, or refract, starlight and bring it to a focus. The lens is convex, meaning that the center of the lens is its thickest part, and the lens becomes thinner toward its edges. A convex lens bends light at the edge of the lens to a greater angle than light coming through the center, so all of the rays converge to a focus. The distance between the lens and the place where the rays converge is called the focal length of the lens. A refracting telescope's light-gathering power is proportional to the size of the objective, or main, lens and to the ratio of the focal lengths of the objective lens and the eyepiece.

Refracting telescopes are typically hampered by chromatic aberration, which causes different colors of light to come to a different focus because every color has its own degree of refraction. Chromatic aberration causes the image of a star or planet to be surrounded by circles of different colors.

Another fundamental limitation of refractors is that lenses with diameters beyond 40 in (100 cm) are impractical because they weigh more than half a ton and sag under their own weight, distorting the starlight. They cannot be supported from behind, as optical mirrors are.

B Reflecting Telescopes

A reflecting telescope uses a precisely curved mirror instead of a lens to collect starlight. The mirror is concave—that is, shaped like the inside of a dish—a shape that brings reflected light waves to a focus at a point above the mirror. Reflecting telescopes are especially useful for gathering light from dim objects. A reflecting telescope's light sensitivity increases with the square of the diameter of the telescope's mirror, so doubling the mirror's diameter increases light-gathering power by a factor of four. Not only can a larger telescope see fainter objects, but it can also obtain the data in a fraction of the time required for a smaller telescope. Larger reflecting telescopes can typically detect objects that are a millionth or a billionth the brightness of the faintest star seen by the human eye.

The ideal mirror for a reflecting telescope has a parabolic or hyperbolic shape that brings distant light rays to a precise focus. Such mirrors are difficult to make because the curvature of the mirror's surface changes with its distance from the center, unlike a simpler, though not as precise, spherical reflector. A telescope mirror is cast from special molten glass that will not significantly expand or contract with temperature once it cools and hardens. Pyrex glass has been commonly used, and newer materials include borosilicate glass and a glass-ceramic composite.

The molten glass is cast as a mirror blank, a flat, thick disk that approximates the size of the finished mirror. It then must cool slowly to avoid cracking. Once cooled, the flat mirror blank's surface is ground and polished to the right shape using a computer-controlled polishing tool that rubs a liquid slurry of fine abrasive across the glass. This process must be extraordinarily accurate—differences in the surface must be smaller than a fraction of the width of a human hair. A fine layer of aluminum is deposited on the glass to create a reflective surface.

A technique that reduces some of the time needed to grind a mirror to shape was developed in the 1990s. The glass is spun into the desired shape while it is still molten. Rotational forces move some of the glass toward the edge of the spinning container and into a shape called a paraboloid. After it cools, a spin-cast mirror does not require laborious grinding to remove excess glass.

Astronomers seek ever-larger mirrors to increase the power and efficiency of telescopes. However, huge mirrors are expensive and difficult to make, and they are challenging to move while tracking celestial targets. One particularly daunting problem is that a solid glass mirror is heavy. The 200-in (508-cm) Hale telescope on California's Mount Palomar weighs 14 tons.

In the 1990s a daring and innovative design broke the mirror size barrier. Each of the twin Keck telescopes, located in Mauna Kea, Hawaii, combined 36 hexagonal 72-in (183-cm) mirrors together, like bathroom tiles, to behave like one immense 400-in (1,016-cm) mirror having four times the collecting power of Palomar (see Mauna Kea Observatory).

In some telescopes designed in the 1990s, the mirror's weight has been dramatically reduced by sandwiching a honeycomb pattern of glass ribs between a thin, but rigid, concave mirror and a flat back plate. Engineers have even developed meniscus mirrors—mirrors that are too thin to support their own weight. An adjustable framework supports the meniscus mirror, and servomechanical actuators, controlled by computer, continually adjust the shape of the mirror as it tracks celestial targets. Actuators are also critical to the operation of segmented mirror telescopes, like Keck, that require that a number of smaller mirrors operate as if they were one large mirror.

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.

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.

V ULTRAVIOLET TELESCOPES

Some of the hottest and most energetic stars in the universe are visible in the ultraviolet region of the spectrum (see Ultraviolet Astronomy). However, this light is largely blocked by Earth's atmosphere and so can only be studied from space. In the 1980s and 1990s a series of highly successful Earth-orbiting observatories explored the ultraviolet universe, most notably the International Ultraviolet Explorer (IUE), the Extreme Ultraviolet Explorer (EUVE), the ASTRO space shuttle observatory, and the Hubble Space Telescope (HST).

Ultraviolet telescopes are similar to optical reflecting telescopes, but their mirrors have special coatings that reflect ultraviolet light very well. Ultraviolet telescopes provide much information about interstellar gas, young stars, and the gaseous areas of active galaxies.

VI X-RAY TELESCOPES

X-ray astronomy was developed in the early 1960s when simple X-ray detectors were mounted on high-altitude rockets. Astronomers were surprised to discover X rays streaming from many energetic astronomical objects. Space-based X-ray astronomy was pioneered in 1970 by the U.S. Explorer 42 (Uhuru) satellite, which mapped the sky for X rays. Two important new X-ray telescopes were launched in 1999: the National Aeronautics and Space Administration's (NASA) Chandra X-ray Observatory and the European Space Agency's X-ray Multimirror mission (XMM).

Some X-ray telescopes are built like optical reflecting telescopes. The main mirror of these telescopes must be nearly cylindrical, as opposed to the dish-shaped optical mirrors. X rays from a targeted object hit the mirror at such a shallow angle that they just graze it in order to be reflected into the detector. Some X-ray telescopes are just X-ray detectors that can be pointed at sources. To block out X rays not coming from the target, most detectors are surrounded by a cylinder of X-ray absorbing lead.

VII GAMMA-RAY TELESCOPES

Gamma rays are electromagnetic radiation with wavelengths even shorter than X rays. Some of the most catastrophic events in the universe, such as neutron star collisions and black holes, blast high-energy gamma rays across space. Since gamma rays cannot penetrate Earth's atmosphere, they must be observed from space. In the early 1990s the Compton Gamma Ray Observatory (GRO) found that mysterious gamma-ray bursts are evenly distributed across the sky. Because of their even distribution, astronomers believe that these bursts are extraordinarily powerful events that occur in normal galaxies. Many astronomers believe collisions between two neutron stars or between a neutron star and a black hole produce these bursts.

Gamma-ray telescopes consist of two or more gamma-ray detectors in a line. A detector is triggered by any gamma ray that passes through it, no matter what direction the gamma ray is traveling. To observe gamma rays from a particular source, then, at least two detectors are placed in a line pointing to the source. Only a gamma ray from the targeted source will pass through both detectors.

VIII HISTORY

The fundamental optical principles of the telescope were first described in the 13th century by English scientist Roger Bacon. Dutch spectacle-maker Hans Lippershey is credited with inventing the first telescope in the year 1608, when he discovered that a distant object appeared to be much closer when viewed though a concave lens and a convex lens held in front of each other. He mounted the lenses in a tube to make the first crude refracting telescope.

Early telescopes were not used to explore the heavens; rather, they were employed for military purposes, to detect advancing armies or ships. News of the telescope's invention spread rapidly through Europe. Glass grinding and polishing techniques, which had been developed since the 13th century, made it easy for the telescope design to be constructed and improved.

Science historians credit Italian scientist Galileo with the first use of the telescope for scientific observations of astronomical objects. In 1609, using a homemade telescope that could magnify objects to 20 times the size seen by the naked eye, Galileo discovered four moons orbiting the planet Jupiter. By the end of the following year, he had used his telescope to resolve the Milky Way Galaxy into countless stars, see dark spots on the Sun, and map the face of the Moon. The rapid rate of these discoveries and the extraordinary new insights they offered are unique in the history of astronomy.

Telescope technology took a giant leap forward in the 17th century. In 1663 Scottish astronomer James Gregory first conceived the reflecting telescope. A major departure from the refractor, Gregory's design brought light rays to a focus by bouncing them off a curved mirror rather than bending them. English mathematician and scientist Isaac Newton was the first to build this new type of telescope, in 1688. Scientists quickly found that reflecting telescopes produced optically better images than refractors because the mirrors built for reflecting telescopes could be made much larger than the lenses needed for refracting telescopes.

Early reflecting telescope mirrors were made of speculum metal (a copper-tin mixture). Soon, larger and larger reflecting telescopes were being made. By the mid-1800s Irish astronomer William Parsons built a 72-in (180-cm) reflecting telescope in Ireland that enabled him to study details in nebulas, fuzzy patches of light scattered across the sky that contain clues to a far vaster and more complex universe than imagined in his time.

Parson's telescope remained the largest telescope in the world until the construction of the 100-in (254-cm) Hooker telescope on Mount Wilson (see Hale Observatories) in 1917. It was powerful enough to resolve stars in neighboring galaxies, providing conclusive proof that the Milky Way was just one such group of stars in a universe filled with galaxies.

In 1950 the Hale Telescope went into operation and remained the best in the world for nearly half a century. It was used to refine measurements of the rate of the expansion of the universe and discovered new phenomena, such as quasars.

IX NEW DEVELOPMENTS

Telescope technology continues to advance in all fields of astronomy. Several new optical telescopes designed for interferometry are being built. Georgia State University's Center for High Angular Resolution Astronomy (CHARA) began construction of five 3-ft (1-m) telescopes at the Mount Wilson Observatory in California in 1995. The telescopes should become operational in 2000. The United States, the United Kingdom, Canada, Chile, Argentina, and Brazil joined forces to build two 26-ft (8-m) telescopes, one in Mauna Kea, Hawaii, and one in Cerro Pach?n, Chile, in 1996. The telescope in Mauna Kea, called Gemini North, became operational in 1999, while the telescope in Chile, called Gemini South, became operational in 2000.

A team of scientists from the University of Arizona, Ohio State University, and German and Italian astronomical research institutions cast the largest single-piece mirror ever in 1997 for the Large Binocular Telescope (LBT). The LBT was dedicated in October 2004 at the Mount Graham Observatory in Arizona. Only one of its two 27.6-ft (8.4-m) mirrors had been installed, however. When fully complete, the LBT will provide an image comparable to that of a single 75-ft (23-m) telescope.

The launch of Japan's Space Observatory Program satellite in 1997 enhanced the radio astronomy program called the Very Long Baseline Interferometer (VLBI), creating a radio telescope larger than Earth. The satellite and about 40 Earth-based radio telescopes combine signals to produce radio images about three times clearer than was previously possible.

The Chandra X-ray Telescope, launched by NASA in July 1999, began sending images in January 2000. In its first few months of operation, it revealed numerous black holes in the centers of galaxies, including a particularly low-temperature black hole at the center of the Andromeda galaxy, a neighbor of the Milky Way.

Contributed By:

Ray Villard