Telescope

Telescopes vary in shape and size from huge bowl-shaped reflectors that measure up to 1,000 feet (305 meters) across to small binoculars and gunsights. Binoculars are actually two telescopes joined side by side. In most telescopes, a lens or mirror is used to form an image of an object. The image may be viewed through an eyepiece or recorded on photographic film or by electronic devices.

The most familiar telescopes are optical telescopes. These instruments, like our eyes, see visible light. But objects in space give off many other kinds of radiation that people cannot see, such as radio waves and X rays. Astronomers use other kinds of telescopes to observe this radiation.

Optical telescopes form an image of a star or other heavenly body in two ways. In a refracting telescope, light waves enter a glass lens, which concentrates each wave crest at a point called the focus. There, an image is formed that can be viewed with an eyepiece. In a reflecting telescope, a bowl-shaped mirror reflects the light waves to a focus. This design, called a Newtonian telescope, uses a small, flat mirror to reflect the light to an eyepiece at the side. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustrations by Oxford Illustrators Limited.

The Dutch optician Hans Lippershey probably made the first telescope in 1608, when he mounted two glass lenses in a narrow tube. Within a year, the Italian astronomer Galileo built a similar device and became the first person to use a telescope to study the sky. Galileo soon made discoveries that revolutionized astronomy. For example, he discovered that several moons revolve around Jupiter. In 1668, the English astronomer Isaac Newton built a telescope that used a mirror. Today, most large research telescopes use mirrors instead of lenses.

Telescopes can also detect extremely faint objects. In optical telescopes, this ability depends on the amount of light the telescope can collect. The larger the telescope's light-gathering lens or mirror, the more light the telescope can collect. Large research telescopes can gather about 1 million times more light than the human eye can and so detect objects about 1 million times fainter.

Visible light is only one of the many kinds of electromagnetic radiation that reach the earth from space. This radiation moves through space in patterns called waves that differ in wavelength. Wavelength is the distance between the crest of one wave and the crest of the next. The chief types of electromagnetic radiation--in order of increasing wavelength--are gamma rays, X rays, ultraviolet rays, visible light, infrared rays, and radio waves. When electromagnetic radiation interacts with matter, it takes on the character of particles called photons in addition to its wavelike character. Each photon carries a specific amount of energy. The photons of radiation with the shortest wavelengths have the highest energy. Astronomers use special telescopes with electronic detectors to make images of invisible forms of electromagnetic radiation.

Some types of electromagnetic radiation, including visible light and certain radio waves, pass through the atmosphere and can be studied from the earth. But the atmosphere blocks other types of radiation--particularly ultraviolet rays, X rays, and gamma rays. Astronomers use telescopes aboard satellites to observe these forms of radiation.

Telescopes with devices called spectrometers enable astronomers to study individual wavelengths of electromagnetic radiation. These devices spread and separate wavelengths of radiation to form a pattern called a spectrum. Astronomers use spectrometers to determine the temperature and chemical composition of stars, planets, and gas clouds, and to calculate how fast an object is approaching or moving away from the earth.

How optical telescopes work. Optical telescopes use a lens or mirror to collect and focus light waves. Each wave from a faint star is so weak that it can only be detected if its energy is concentrated by a lens or mirror. A lens or mirror makes the crest of a wave come together at a point called the focus. Waves from stars in different locations in the sky meet at different focuses, but all of the focuses lie at an equal distance from the lens or mirror in an area called the focal plane. The distance from a lens or mirror to the focus is called the focal length.

In the simplest telescopes, astronomers place photographic film at the focal plane to record images of objects in space. For direct observation, images can be magnified by an eyepiece. Most eyepieces consist of two small lenses. A viewer focuses the telescope by adjusting the eyepiece to change the distance between the eyepiece and the light-gathering lens or mirror. The eyepiece also has a focal length. The magnifying power of a telescope can be found by dividing the focal length of the lens or mirror by the focal length of the eyepiece.

Types of optical telescopes. There are three main types of optical telescopes: (1) refracting telescopes, (2) reflecting telescopes, and (3) refracting-reflecting telescopes.

Refracting telescopes, also called refractors, have a large lens called an objective lens--or simply an objective--at one end of a long, narrow tube. The objective is convex (curved outward) on both sides so that the middle of the lens is thicker than the edges. The glass slows the light waves as they pass through the lens. A wave is slowed most in the middle of the lens, where the glass is thickest. The lens thus causes the entire crest of the wave to arrive at the focus at the same time.

Refractors with a magnifying eyepiece invert the image so that it appears upside down. Astronomical observations do not require an upright image. But telescopes used to observe objects on the earth, such as binoculars, gunsights, and surveying equipment, use additional lenses or prisms to turn the image right side up again.

Galileo made all of his discoveries using refracting telescopes. Galileo's instruments and other early refractors, however, produced images with rainbow coloring around the edges called chromatic aberration. This coloring appeared because a lens slows blue light more than red, giving blue light a shorter focal length. When white light, which consists of all colors, passes through a lens, only one color focuses correctly.

Astronomers found that gently curved lenses made chromatic aberration less noticeable. But these lenses had long focal lengths and required extremely long tubes. To reduce chromatic aberration, some early telescopes stretched more than 200 feet (60 meters) long. In the mid-1700's, however, astronomers discovered they could make a compound lens of two different types of glass that had a short focal length and almost no chromatic aberration.

Reflecting telescopes, also called reflectors, use bowl-shaped mirrors instead of lenses. The mirror, called the primary mirror, has a surface shaped so that any line across the center of the mirror is a parabola, a curve like the path of a ball batted high in the air. A mirror with that shape, called a parabolic mirror, reflects light rays to a sharp focus in front of itself. There, a second mirror reflects the rays to an eyepiece.

Astronomers generally prefer reflecting telescopes to refracting telescopes. The weight of a large lens can cause it to bend and become distorted. But a large, heavy mirror can be supported from behind. As a result, mirrors can be made much larger than lenses and, thus, can gather more light. In addition, parabolic mirrors are useful because they can collect infrared and some ultraviolet rays as well as visible light.

The English astronomer Isaac Newton designed one of the first reflectors in 1668 to avoid chromatic aberration caused by lenses. He used a small, flat mirror to reflect light from the primary mirror to an eyepiece at the side of the telescope tube. In 1672, a French telescope maker known only as Cassegrain designed a telescope using a small convex mirror in front of the primary mirror. The small mirror reflected the light through a hole in the primary mirror to an eyepiece behind it. This design, called a Cassegrain telescope, is used most frequently by astronomers today for optical and infrared telescopes.

A Cassegrain reflecting telescope uses a primary mirror with a hole in the center to reflect light to a smaller, curved mirror. The smaller mirror reflects the light through the hole, where it may be viewed with an eyepiece or recorded. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustration by Oxford Illustrators Limited.

The mirrors of early reflecting telescopes were shaped like a section of a sphere. A spherical mirror is much easier to polish than a parabolic mirror, but it does not focus light properly. Astronomers developed techniques to make parabolic mirrors in the early 1700's. Early mirrors were made of speculum metal, a heavy mixture of copper and tin that tarnished easily and required repeated polishing. In the mid-1800's, the German chemist Justus von Liebig learned how to deposit a thin coating of silver on glass to produce a brilliantly reflecting surface. When the surface tarnished or dulled, the mirror could be recoated without having to polish it. Today, all reflecting telescopes have glass mirrors, most of which are coated with aluminum.

The first telescope to use a large glass mirror was built in 1908 on Mount Wilson, near Pasadena, Calif. It has a Cassegrain design, and its mirror measures 60 inches (1.5 meters) in diameter. Most large telescopes today are modeled after this telescope. One of the largest is the Hale Telescope, an instrument with a 200-inch (5-meter) mirror on Palomar Mountain in southwestern California. Another telescope of the same type at Zelenchukskaya, near Mineralnyye Vody in southwestern Russia, has a mirror 6 meters (20 feet) in diameter.

The Multiple Mirror Telescope on Mount Hopkins, near Tucson, Ariz., has a much different design. Completed in 1979, the telescope has six 72-inch (1.8-meter) mirrors that work together. For a single mirror to collect as much light, it would have to be 176 inches (4.5 meters) in diameter.

Refracting-reflecting telescopes, also called catadioptric telescopes, have a large lens at the front end of the tube and a large mirror at the rear. These telescopes use spherical mirrors instead of parabolic mirrors. The lens, however, refracts light rays slightly to correct the reflective errors caused by a spherical mirror.

Bernhard Schmidt, a German optician, invented the catadioptric telescope in 1930. The telescope forms images of a larger region of the sky than is possible with any other telescope design. Astronomers have used large Schmidt telescopes to photograph the entire sky.

Recording images produced by optical telescopes. Astronomers often use photographic plates or film to record the images formed by an optical telescope. If film is exposed to a dim star or other object for a long time, a bright picture results. For this reason, photographs of the sky taken through a telescope reveal many details that cannot be seen with the eye. In the mid-1970's, electronic detectors called charge-coupled devices (CCD's) began to replace film. CCD's convert light into an electric charge, which is used to form images on a computer screen. CCD's produce better pictures than photographic plates or film because they are far more sensitive to light. Using CCD's, astronomers can see extremely faint galaxies in almost any part of the sky. Some of these galaxies are so far away that their light began its journey toward the earth long before the earth formed about 41/2 billion years ago.

Limitations of optical telescopes. Astronomers can see galaxies across the universe with big optical telescopes. The images are blurred, however, by the earth's atmosphere. Wind and daily heating and cooling of the atmosphere create pockets and swirls of warm and cool air. These differences in air temperature affect the direction and speed of light through the air. As a result, the waves of starlight arrive at the focus at slightly different times, spoiling the image.

Since the late 1970's, astronomers have discovered that they can improve seeing by insulating and cooling observatory domes. But only a telescope operating above the atmosphere can escape all blurring. One such telescope, an orbiting observatory called the Hubble Space Telescope, was launched into orbit in 1990. Although a flawed mirror prevented the Hubble from working as well as scientists hoped, it produced more finely detailed images than any telescope on earth. In 1993, astronauts corrected the telescope's flaw. It now produces significantly sharper images and observes objects 50 times fainter than telescopes on the earth can.

Ground-based telescopes may someday overcome atmospheric blurring with instruments called adaptive optics. These devices will measure and correct the timing error of light arriving across a mirror to restore a sharp focus. Adaptive optics are extremely difficult to make because air currents move and change so quickly. Computers must calculate and correct the timing error several hundred times a second. Astronomers developed the concept of adaptive optics in the 1950's. But they did not begin building such systems until the early 1990's, after scientists developed the advanced technology the systems required.

How radio telescopes work. Most radio telescopes use a large parabolic reflector, often called a dish antenna or simply a dish, to collect radio waves from space. The dish has the same shape as the parabolic mirror of a reflecting telescope. Radio waves, however, are much longer than light waves. As a result, a radio telescope's dish need not be polished or shaped as accurately as the mirror of a reflecting telescope. But it must be much larger in diameter to focus the long radio waves. The reflector focuses the waves onto an antenna that changes them into electric signals. A radio receiver amplifies these signals and records their strength at different frequencies and from different directions as data on a tape. The data are analyzed by a computer, which combines the signals from the receiver. The computer then uses the signals to draw a picture of the source of the radio waves or to analyze the radio spectrum and chemical composition of the source.

Large radio telescopes are also used as giant radar systems to map the surfaces of the moon and the planets. Astronomers send powerful radio waves to the moon or planet and then record the radio echoes that bounce back. Astronomers call this technique radar mapping.

Types of radio telescopes. In most radio telescopes, motors turn the reflector toward any source of radio waves in the sky. The largest moving dish measures 330 feet (100 meters) across. Astronomers can use a single large fixed dish to study radio signals from a faint object. The world's largest radio telescope is a fixed dish built into a bowl-shaped valley near Arecibo, Puerto Rico. The dish measures 1,000 feet (305 meters) in diameter. It is often used for locating and measuring pulsars.

Astronomers produce extremely sharp radio images by combining signals from many radio dishes spread over large distances. At a central station, computers electronically combine the radio signals from various locations, introducing time delays between the signals from the different dishes. These delays cause the signals from a radio wave to come together at the same time and reinforce each other, just as a light wave is concentrated at the focus by a mirror or lens. Radio telescopes connected in this way are called a radio interferometer. The longer the baseline (distance) between the telescopes, the better the resolution of the interferometer. Astronomers use interferometers to make radio maps of the sky.

The most powerful radio interferometer is called the Very Large Array (VLA). It stands on a high plain near Socorro, N. Mex. It has 27 dishes, each measuring 82 feet (25 meters) in diameter. Another important interferometer, the Very Long Baseline Array (VLBA), began operating in 1993. The system consists of 10 reflectors located across the United States from Hawaii to the Virgin Islands. The VLBA provides the sharpest radio images ever produced.

In 1961, American physicist Frank J. Low built the first infrared detector sensitive enough for use in astronomy. The device, called a bolometer, was an extremely cold electronic thermometer in a vacuum. When infrared rays hit the bolometer, it warmed up and gave off an electric signal. Today, infrared telescopes use electronic devices called array detectors, similar to CCD's, to form infrared images on a computer screen.

An infrared telescope operated in orbit aboard the Infrared Astronomical Satellite (IRAS) from January to November 1983. Liquid helium cooled the entire telescope--its mirrors, detectors, and tube--to a temperature only a few degrees above absolute zero (-273.15 ºC). IRAS detected rings of dust around the star Vega and other nearby stars that might be solar systems in the process of formation. Astronomers believe a similar ring of dust around the sun developed into the planets.

Ultraviolet telescopes. Astronomers use reflecting telescopes in space with electronic detectors to study most wavelengths of ultraviolet rays, which can be reflected just like visible light. But the shortest wavelengths, called extreme ultraviolet rays, are harder to reflect. Extreme ultraviolet rays can only be reflected off a mirror at a small angle called a grazing incidence. This characteristic of the rays resembles how stones can be skipped along the surface of a pond.

Ultraviolet telescopes enable astronomers to study extremely hot objects in space, including quasars and stars called white dwarfs. Astronomers also use ultraviolet telescopes to study how stars form and the composition of gas between stars and galaxies.

X-ray telescopes. X rays have shorter wavelengths and higher energy than ultraviolet rays. X rays that are not absorbed or scattered by matter pass straight through many materials. But longer wavelength X rays, like extreme ultraviolet rays, can be reflected at a grazing incidence. Astronomers have found that some objects in the universe give off much of their energy in the form of X rays. These X-ray sources include the centers of galaxies and clouds of extremely hot gas that lie between galaxies.

An X-ray telescope aboard Rosat, a satellite launched in 1990, reflects X rays off curved mirrors at a slight angle called a grazing incidence. This method of focusing X rays resembles how stones can be skipped along the surface of a pond. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustrations by Oxford Illustrators Limited.

The simplest X-ray telescopes use an arrangement of iron or lead slats instead of mirrors. The slats block all X rays except those from one line across the sky. The X-ray photons then enter a detector filled with an X-ray absorbing gas, where they are counted. By scanning the sky, these telescopes can locate X-ray sources.

During the 1970's, slatted X-ray telescopes discovered many sources of X rays in space. Astronomers now know that many of the brightest X-ray sources are certain double stars--that is, a pair of stars orbiting each other. In these pairs, one of the stars has collapsed and become a small, dense star called a neutron star or a black hole--an invisible object with such powerful gravitational force that not even light can escape its surface. X rays occur when gas from a star falls into the neutron star or black hole.

Gamma ray telescopes. Gamma rays have the shortest wavelength and highest energy of any electromagnetic radiation. When a gamma ray photon collides with an atom while passing through matter, it may knock electrons loose from the atom or even break up the nucleus of the atom. These collisions can produce a shower of subatomic particles and low-energy radiation. The shower travels in the same direction as the original gamma ray and is detected with devices called scintillators. When radiation or particles from a shower hit a scintillator, the instrument produces a flash of light that can be recorded. By measuring the shower, scientists can calculate the energy level of the gamma ray and the direction of its source.

Gamma ray telescopes on the Compton Gamma Ray Observatory, a satellite launched in 1991, have enabled scientists to learn more about some of the least understood objects in the universe, including pulsars and quasars. Many of these high-energy objects are strong sources of gamma rays.

One new design is the segmented mirror, used in two identical telescopes called Keck I and Keck II. The light-gathering mirror of each telescope consists of 36 hexagonal (six-sided) mirrors mounted close together. The mirrors form a reflecting surface 33 feet (10 meters) in diameter. The two telescopes are located on the island of Hawaii. Keck I was completed in 1992; Keck II, in 1996.

Some projects involve linking two or more telescopes to collect more light. A project called the Very Large Telescope (VLT) will consist of four telescopes with mirrors 27 feet (8.2 meters) in diameter. Used together, the four telescopes will have the light-gathering power of a single mirror with a diameter of 52 feet (16 meters). The VLT's mirrors will consist of thin disks of glass supported by hundreds of computer-controlled devices. The devices, called actuators, will make continuous adjustments to maintain the mirrors' proper shape. The European Southern Observatory, led by astronomers from several European nations, is building the VLT near Antofagasta, Chile. The project is scheduled to start operations in the late 1990's.

Astronomers at the University of Arizona have made huge glass honeycomb mirrors using a mold filled with hundreds of hexagonal blocks. Melted glass covers the blocks and fills spaces between them. The blocks are removed after the glass cools, leaving a stiff glass structure that is light enough to float on water.

The Columbus Telescope, scheduled for completion in 1997, will also use honeycomb mirrors. The instrument will consist of two telescopes, each with a mirror 271/2 feet (8.4 meters) in diameter, mounted side by side like a giant pair of binoculars. The Columbus Telescope is a joint project of Italian and American astronomers. It will stand on Mount Graham in southeastern Arizona. Several other telescopes with honeycomb mirrors were under construction in the mid-1990's.

Both honeycomb mirrors and the VLT's thin disk mirrors are made by a new technique called spin-casting, developed in the mid-1980's. Spin-casting replaces the costly, laborious process of grinding a mirror to the proper parabolic shape. Instead, a huge rotating oven spins molten glass at a carefully controlled rate. The liquid glass flows into a shape that is nearly perfect for a telescope mirror.

Contributor: J. Roger P. Angel, D.Phil., Prof. of Astronomy, Univ. of Arizona.

See also: Aberration; Binoculars; Lens; Light; Newton, Sir Isaac; Spectrometer; Sun.

Why are parts of infrared telescopes cooled to low temperatures?

How does an optical telescope focus light?

Why must astronomers make radio telescopes larger than optical telescopes?

What is a grazing incidence? Why is it important in the design of some ultraviolet and X-ray telescopes?

Why do astronomers build infrared telescopes on mountaintops?

What is a radio interferometer?

Who first used a telescope to observe objects in space?

Why do astronomers usually prefer reflecting telescopes to refracting telescopes?

What causes star images to blur? How do astronomers correct blurring in optical telescopes?

Chaple, Glenn F., Jr. Exploring with a Telescope. Watts, 1988.

Dickinson, Terence, and Dyer, Alan. The Backyard Astronomer's Guide. Camden Hse., 1991.

Muirden, James. How to Use an Astronomical Telescope. 1985. Reprint. Simon & Schuster, 1988.

Schultz, Ron. Looking Inside Telescopes and the Night Sky. John Muir, 1993. Younger readers.

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