Telescope

Background

A telescope is a device used to form images of distant objects. The most familiar kind of telescope is an optical telescope, which uses a series of lenses or a curved mirror to focus visible light. An optical telescope which uses lenses is known as a refracting telescope or a refractor; one which uses a mirror is known as a reflecting telescope or a reflector. Besides optical telescopes, astronomers also use telescopes that focus radio waves, X-rays, and other forms of electromagnetic radiation. Telescopes vary in size and sophistication from homemade spyglasses built from cardboard tubes to arrays of house-sized radio telescopes stretching over many miles.

The earliest known telescope was a refractor built by the Dutch eyeglass maker Hans Lippershey in 1608 after he accidentally viewed objects through two different eyeglass lenses held a distance apart. He called his invention a kijker, "looker" in Dutch, and intended it for military use. In 1609, the Italian scientist Galileo Galilei built his own telescopes and was the first person to make astronomical observations using them. These early telescopes consisted of two glass lenses set within a hollow lead tube and were rather small; Galileo's largest instrument was about 47 inches (120 cm) long and 2 inches (5 cm) in diameter. Astronomers such as Johannes Kepler in Germany and Christian Huygens in Holland built larger, more powerful telescopes throughout the 1600s. Soon these telescopes got too large to be easily controlled by hand and required permanent mounts. Some were more than 197 feet (60 m) long.

The ability to construct enormous telescopes outpaced the ability of glassmakers to manufacture appropriate lenses for them. In particular, the problems caused by chromatic aberration (the tendency for a lens to focus each color of light at a different point, leading to a blurred image) became acute for very large telescopes. Scientists of the time knew of no way to avoid this problem with lenses, so they designed telescopes using curved mirrors instead.

In 1663, the Scottish mathematician James Gregory designed the first reflecting telescope. Alternate designs for reflectors were invented by the English scientist Isaac Newton in 1668 and the French scientist N. Cassegrain in 1672. All three designs are still in use today. In the 1600s, there was no good way to coat glass with a thin reflective film, as is done today to make mirrors, so these early reflectors used mirrors made out of polished metal. Newton used a mixture of copper, tin, and arsenic to produce a mirror which could only reflect 16% of the light it received; today's mirrors reflect nearly 100% of the light that hits them.

It had been known as early as 1730 that chromatic aberration could be minimized by replacing the main lens of the telescope with two properly shaped lenses made from two different kinds of glass, but it was not until the early 1800s that the science of glassmaking was advanced enough to make this technique practical. By the end of the 19th century, refracting telescopes with lenses up to a meter in diameter were constructed, and these are still the largest refracting telescopes in operation.

Reflectors once again dominated refractors in the 20th century, when techniques for constructing very large, very accurate mirrors were developed. The world's largest optical telescopes are all reflectors, with mirrors up to 19 feet (6 m) in diameter.

Raw Materials

A telescope consists of an optical system (the lenses and/or mirrors) and hardware components to hold the optical system in place and allow it to be maneuvered and focused. Lenses must be made from optical glass, a special kind of glass which is much purer and more uniform than ordinary glass. The most important raw material used to make optical glass is silicon dioxide, which must not contain more than one-tenth of one percent (0.1%) of impurities.

Optical glasses are generally divided into crown glasses and flint glasses. Crown glasses contain varying amounts of boron oxide, sodium oxide, potassium oxide, barium oxide, and zinc oxide. Flint glasses contain lead oxide. The antireflective coating on telescope lenses is usually composed of magnesium fluoride.

A telescope mirror can be made from glass that is somewhat less pure than that used to make a lens, since light does not pass through it. Often a strong, temperature-resistant glass such as Pyrex is used. Pyrex is a brand name for glass composed of silicon dioxide, boron oxide, and aluminum oxide. The reflective coating for telescope mirrors is usually made from aluminum, and the protective coating on top of the reflective coating is usually composed of silicon dioxide.

Hardware components that are directly involved with the optical system are usually manufactured from steel or steel and zinc alloys. Less critical parts can be made from light, inexpensive materials such as aluminum or acrylonitrile-butadiene-styrene plastic, commonly called ABS.

The Manufacturing
Process

Making the hardware components

• 1 Metal hardware components are manufactured using standard metalworking machines such as lathes and drill presses.
• 2 Components made from ABS plastics (usually the external body of the telescope) are produced using a technique known as injection molding. In this process the plastic is melted and forced under pressure into a mold in the shape of the final product. The plastic is allowed to cool back into a solid, and the mold is opened to allow the component to be removed.

Making optical glass

• 3 The glass manufacturer mixes the proper raw materials with waste glass of the same type as the glass to be made. This waste glass, known as cullet, acts as a flux; that is, it causes the raw materials to react together at a lower temperature than they would without it.
• 4 This mixture is heated in a glass furnace until it has melted into a liquid. The temperature needed to form molten glass varies with the type of glass being made, but it is typically about 2550°F (1400°C).
• 5 The temperature of the molten glass is raised to about 2820°F (1550°C) to force air bubbles to come to the surface. It is then allowed to cool while being stirred constantly until it has reached about 1830°F (1000°C), at which point it is an extremely thick fluid. This viscous, molten glass is poured into molds with roughly the same shape as the lenses required.
• 6 After the glass has cooled to about 570°F (300°C), it must be reheated to about 1020°F (550°C) to remove internal stresses that form during the initial cooling period and which weaken the glass. It is then allowed to cool slowly to room temperature. This process is known as annealing. The final lens-shaped chunks of glass are known as blanks.

Making the lenses

The blanks are processed by the telescope manufacturer in three steps: cutting, grinding, and polishing. A mirror is formed in exactly the same way as a lens until the reflective coating is applied.

• 7 First a high-speed, rotating cylindrical cutter with a round diamond blade, known as a curve generator, shaves the surface of the lens until a close approximation of the desired curve is achieved. The cut lens is inspected with a spherometer to check the curvature and is recut if necessary. The time required for cutting varies greatly with the type of glass being cut and the kind of lens being shaped. A lens may require several cuttings, each of which may take anywhere from a few minutes to more than half an hour.
• 8 Several cut blanks are placed on a curved block in such a way that their surfaces line up as if they were all part of one large spherical curve. This is necessary so that the grinding machine can grind them all in the same way. A cast iron grinding surface known as a tool is pressed onto them. During grinding, the block of lenses rotates while the tool is free to move at random on top of it. Between the tool and the block flows a slurry containing water, an abrasive to do the grinding (usually silicon carbide), a coolant to prevent the lenses from being damaged by overheating, and a surfactant to keep the abrasive from settling out. The speed at which the block rotates, the force placed on the lenses, the exact contents of the slurry, and other variables are controlled by experienced opticians to produce the exact type of lens desired. Each lens is once again inspected with a spherometer and reground if necessary. The total grinding process may take anywhere from one hour to eight hours. The ground lenses are cleaned and moved to the polishing room.
• 9 The polishing machine is similar to the grinding machine, but the tool is made from pitch—a thick, soft, resinous substance derived from coal tar or wood tar. A pitch tool is made by placing tape around the circumference of a curved dish, pouring in hot, liquid pitch with other ingredients such as beeswax and jeweler's rouge, and letting it cool back into a solid. A pitch tool can polish about 50 lenses before it must be reshaped. Polishing proceeds in the same manner as grinding, but instead of an abrasive the slurry contains a polishing substance, usually cerium dioxide, in the form of a very fine pink powder. The polished lenses are optically inspected and repolished if necessary. The polishing procedure may take anywhere from half an hour to four or five hours. The lenses are cleaned and are ready for coating.

Applying coatings

• 10 To make a lens into a mirror, a very thin, very smooth coating of aluminum is applied. Aluminum is heated in a vacuum to form a vapor. A negative electro-static charge is applied to the surface of the lens so that the positively charged aluminum ions are attracted to it. Similar procedures are followed to apply a coating of silicon dioxide to protect the fragile surface of a mirror or to apply an antireflective coating of magnesium fluoride to the surface of a lens. The finished lens or mirror is inspected, labeled with a date of manufacture and a serial number, and stored until needed.

Assembling and shipping the telescope

• 11 The hardware components, lenses, and mirrors required to make a particular model of telescope are assembled by hand in an assembly line process. The completed telescope is packed with close-fitting expanded polystyrene foam to protect it from damage during shipping. The telescope is packed in a cardboard box and shipped to the retailer or consumer.

Quality Control

The most critical aspect of quality control for an optical telescope is the accuracy of the lenses and mirrors. During the cutting and grinding stages, the physical dimensions of the lens are measured very carefully. The thickness and the diameter of the lens are measured with a vernier caliper, an instrument which looks something like a monkey wrench. The outer, fixed jaw of the caliper is placed against one side of the lens and the inner, sliding jaw is gently moved until it meets the other side of the lens. In a classic vernier caliper, the dimensions of the lens are read very accurately using a scale which moves along with the inner jaw and which is compared with a stationary scale attached to the outer jaw. This type of caliper works much like a slide rule. There also exist electronic versions of this instrument, in which the measured dimension automatically appears on a digital display.

The curvature of a lens is measured with a spherometer, a device which resembles a pocket watch with three small pins protruding from its base. The outer two pins are fixed in place while the inner pin is free to move in and out. The spherometer is gently placed on the surface of the lens. Depending on the type of curve, the middle pin will either be higher than the other two pins or lower than the other two pins. The movement of the inner pin moves a needle on a calibrated dial on the face of the spherometer. This value is compared with the standard value that should be obtained for the desired curvature.

Tolerances vary with the type of lens being manufactured, but a typical acceptable variation might be plus or minus 0.0008 inches (20 micrometers). For a flat lens, generally one destined to become a flat mirror, the tolerance is much smaller, usually about plus or minus 0.00004 inches (1.0 micrometer).

During the polishing stage, these instruments are not accurate enough to ensure that the lens will work properly. Optical tests, which measure the way light is affected by the lens, must be used. One common test is known as an autocollimation test. The lens is placed in a dark room and is illuminated with a low intensity pinpoint light source. A diffraction grating (a surface containing thousands of microscopic parallel grooves per inch) is placed at the point where the lens should focus light. The grating causes an interference pattern of dark and light lines to form in front of and behind the focal point. The true focal point can thus be found precisely and compared with the theoretical focal point for the type of lens desired.

In order to test a flat lens, a lens that is known to be flat is placed face down on the lens that is to be tested, which rests on a piece of black felt. The microscopic gaps between the two lenses cause an interference pattern to appear when gentle pressure is applied. The light and dark lines are known as Newton's rings. If the lens being tested is flat, the lines should be straight and regular. If the lens is not flat, the lines will be curved.

The Future

The techniques used to produce excellent lenses and mirrors have been well under-stood for many years, and major innovations in this area are unlikely. One area of active research is in coating technology. New coating substances may be developed to provide better protection for mirrors and better prevention of loss of light through reflection for lenses.

A more dramatic area of progress is in the electronic accessories that accompany telescopes. Amateur astronomers will soon be able to obtain telescopes with built-in computer guidance systems that will enable them to automatically point the telescope at a selected celestial object and to track it night by night. They will also be able to attach video cameras to their telescopes and film such astronomical phenomena as lunar eclipses and the movements of planets and moons.

Where To Learn More

Books

Asimov, Isaac. Eyes on the Universe: A History of the Telescope. Houghton Mifflin, 1975.

Bell, Louis. The Telescope. Dover, 1981.

Manly, Peter L. Unusual Telescopes. Cambridge University Press, 1991.

Periodicals

Mullins, Mark. "A Truly Economical Telescope." Sky and Telescope, December 1993, pp. 91-92.

Nash, J. Madeleine. "Shoot for the Stars." Time, April 27, 1992, pp. 56-57.

Nelson, Ray. "Reinventing the Telescope." Popular Science, January 1995, pp. 57-59, 85.

—Rose Secrest

Telescope

There is much confusion and debate concerning the origin of the telescope. Many notable individuals appear to have simultaneously and independently discovered how to make a telescope during the last months of 1608 and the early part of 1609. Regardless of its origins, the invention of the telescope has led to great progress in the field of astronomy.

The Origin of the Telescope

Contrary to popular belief, Galileo Galilei (1564–1642) did not invent the telescope, and he was probably not even the first person to use this instrument in astronomy. Instead, the latter honor may be attributed to Thomas Harriot (1560–1621). Harriot developed a map of the Moon several months before Galileo began observations. Nevertheless, Galileo distinguished himself in the field through his patience, dedication, insight, and skill.

The actual inventor of the telescope may never be known with certainty. Its invention may have been by a fortuitous occurrence when some spectacle maker happened to look through two lenses at the same time. Several accounts report that Hans Lipperhey of Middelburg in the Netherlands had two lenses set up in his spectacle shop to allow visitors to look through them and see the steeple of a distant church. However, this story cannot be verified.

It is known that the first telescopes were shown in the Netherlands. Records show that in October 1608, the national government of the Netherlands examined the patent application of Lipperhey and a separate application by Jacob Metius of Alkmaar. Their devices consisted of a convex and concave lens mounted in a tube. The combination of the two lenses magnified objects by 3 or 4 times. However, the government of the Netherlands considered the devices too easy to copy to justify awarding a patent. The government did vote a small award to Metius and employed Lipperhey to devise binocular versions of his telescope. Another citizen of Middelburg, Zacharias Janssen, had also made a telescope at about the same time but was out of town when Lipperhey and Matius made their applications.

News of the invention of the telescope spread rapidly throughout Europe. Within a few months, simple telescopes, called "spyglasses," could be purchased at spectacle-maker's shops in Paris. By early 1609, four or five telescopes had made it to Italy. By August of 1609, Thomas Harriot had observed and mapped the Moon with a six-power telescope.

What Galileo Discovered

Despite Harriot's honor as the first telescopic astronomer, it was Galileo who made the telescope famous. At the time, Galileo was Professor of Mathematics at the University of Padua in Italy. Somehow, he learned of the new instrument that had been invented in Holland, although there is no evidence that he actually saw one of the telescopes known to be in Italy. Over the next several months in 1609 and 1610, Galileo made several progressively more powerful and optically superior telescopes using lenses he ground himself. Galileo used these instruments for a systematic study of the night sky. He saw mountains and craters on the Moon, discovered four satellites of Jupiter, viewed sunspots, observed and recorded the phases of Venus, and found that the Milky Way galaxy consisted of clouds of individual stars.

Galileo summarized his discoveries in the book Sidereus Nuncius (The Starry Messenger ) published in March of 1610. Others working at around the same time claimed to have made similar discoveries—others certainly observed sunspots—but Galileo gathered all of his observations together and wrote about them first. Consequently, he is generally credited with their discovery.

The observation of Venus's phases was especially important to Galileo. According to Ptolemaic theory , Venus would show crescent and "new" phases, but it would not go through a complete cycle of phases. The Ptolemaic model never placed Venus on the opposite side of the Sun as seen from Earth, so Venus would never appear "full." Yet Galileo clearly observed a nearly full Venus. He also observed that the four satellites of Jupiter orbited the planet, conclusively demonstrating that there was at least one instance of an orbital center other than Earth, a clear contradiction to the Ptolemaic model.

Adapting the Telescope

Galileo apparently had no real knowledge of how the telescope worked but he immediately recognized its military value, as well as its entertainment value. He set about building a version that is commonly known as a "Galilean telescope." It had a convex lens as a primary objective (the lens in front) and a concave lens as the eyepiece. The focal point of the objective lens was behind the eyepiece, and the eyepiece served primarily to form the upright image desired for terrestrial observation.

Johannes Kepler was arguably the first person to give a concise theory of how light passed through the telescope and formed an image. Kepler also discussed the various ways in which the lenses could be combined in different optical systems, improving on Galileo's design. Kepler's design used convex lenses for both the primary objective and the eyepiece. However, in spite of his theoretical understanding, there is no evidence that Kepler ever actually tried to put together a telescope.

Telescopes built following Kepler's design were not practical for military applications or everyday use because they inverted and reversed the images and showed people upside-down. However, their greater magnification, brighter image, and wider angle of view made them best for astronomical observations where the inverted image made no difference. The telescope rapidly came into common astronomical use during the 20 years after it was invented.

Unfortunately, it soon became evident that the refracting telescope had a great disadvantage. The main problem with early telescopes was the low quality of their glass and the poor manner in which the lenses were ground. However, even the best lenses had two inherent defects. One defect resulted because the objective lens did not bend all wavelengths equally, and this resulted in the red part of the light-beam being brought to a focus at a greater distance from the objective. An image of a star viewed through an astronomical telescope from this period seemed to be surrounded by colored fringes. This defect is known as "chromatic aberration."

The other problem resulted when the surface of the lens was ground to a spherical shape. Rays passing through the edge of the lens were brought to a focus at a different distance than rays passing near the center of the lens. This defect is called "spherical aberration." A lens can be ground to a different shape (all modern optical instruments use "aspheric" lenses) but the lens grinders of Galileo's time did not possess the technology to do this.

One remedy for both chromatic and spherical aberrations was to make telescopes with extremely long focal length lenses (so that the lenses did not have much curvature), but this required telescopes several meters long. These telescopes were cumbersome and difficult to use. Another solution for chromatic aberration, unknown at the time, was to use an objective lens made from two different kinds of glass glued together. This method greatly reduces chromatic aberration.

Development of the Reflecting Telescope

During the 1680s, Cambridge University was often closed for fear of the Plague. When this occurred, physicist Isaac Newton would retreat to his country home in Lincolnshire. During one of these intervals, Newton began trying to unravel the problem of chromatic aberration.

Newton allowed a beam of sunlight to pass through a glass prism and observed that the beam was split into a rainbow of colors. On the basis of this and other experiments, he decided (incorrectly, it turns out) that the refractor could never be cured of chromatic aberration. Newton consequently developed a new type of telescope, "the reflector," in which there is no objective lens. The light from the object under observation is collected by a curved mirror, which reflects all wavelengths equally.

Newton likely did not originate the idea of a reflecting telescope. Earlier, in 1663, Scottish mathematician James Gregory had suggested the possibility of a reflector. However, Newton was apparently the first person to actually build a working reflector.

Newton created a reflecting telescope with a 2.5-cm (centimeter) metal mirror and presented it to the Royal Society in 1671. But using a reflecting mirror instead of a reflecting lens created another problem. The light beam is reflected back up the tube but it cannot be observed without blocking the light entering the tube. Gregory had suggested inserting a curved secondary mirror that would reflect the light back through a hole in the center of the primary mirror. However, the technology needed to grind the complex mirror surfaces required for Gregory's design did not exist at the time. Newton solved this problem by introducing a second flat mirror in the middle of the tube mounted at a 45-degree angle so that the light beam is reflected out the side of the tube. The eyepiece was then mounted to the side of the tube. This design is still known as a "Newtonian reflector." Since Newton's time, several other reflecting telescope designs have been developed.

Newtonian reflectors were not free of problems. Metal mirrors were hard to grind. The mirror surface tarnished quickly and had to be polished every few months. These problems kept the Newtonian reflector from being widely accepted until after 1774, when new designs, polishing techniques, the use of silvered glass, and other innovations were developed by William Herschel. Herschel discovered the planet Uranus in 1781 using a telescope he had made. He continued to build reflecting telescopes over the next several years, culminating in an enormous 122-cm instrument completed in 1789.

Herschel's 122-cm telescope remained the largest in the world until 1845, when the Irish astronomer, William Parsons, the third Earl of Rosse, completed an instrument known as the Leviathan, which had a mirror diameter of 180 cm. Lord Rosse used this instrument to observe "spiral nebulae," which are now known to be other galaxies. Throughout the eighteenth and nineteenth centuries, telescopes of ever-increasing size were built.

Modern Telescopes

The twentieth century saw continued improvement in telescope size and design. Larger telescopes are preferred for two reasons. Larger instruments gather more light. Astronomical distances are so great that most objects are not visible to the unaided eye. The Andromeda galaxy (M31) is generally considered the most distant object that can be seen with the naked eye, and it is the closest galaxy to Earth outside of the Milky Way. To see very far out into space requires large telescope objectives. This is another reason for the general preference of astronomers for reflecting telescopes. It is easier to build large mirrors than it is to build large lenses.

A second reason for the general trend toward large instruments is resolving power. The ability of a telescope to separate two closely spaced stars (or see fine detail in general) is known as resolving power. If R is the resolving power (in arc seconds), is the wavelength (in micrometers) and d is the diameter of the objective (in meters) then: R = 0.25 . As d gets larger, R gets smaller (smaller is better).

During most of the twentieth century, astronomical images were recorded on photographic film. Later in the century, most observatories and research astronomers switched to solid state devices called CCDs (chargecoupled devices). These devices are much more sensitive to low light levels, and they also have the advantage of creating an electronic image that can be fed directly into a computer where it can be immediately processed.

Earth's atmosphere continues to challenge the progress of astronomy. Infrared and ultraviolet wavelengths do not pass through the atmosphere, so astronomy in those parts of the spectrum is generally done by balloon-based telescopes or by satellites. A bigger problem with the atmosphere is its inherent instability. Even on the clearest of nights, images jiggle and quiver due to small variations in the atmosphere. One way to get around this problem is to get above the atmosphere.

The Hubble Space Telescope (named after Edwin Hubble, who discovered the galactic redshift ) is a satellite based optical and infrared observatory. The spectacular images from "Hubble" have pushed back the frontiers of astronomical knowledge, while raising many new questions.

Ground-based large telescope design also continues to evolve. Very large mirrors suffer from many problems. To make them stiff, they must be very thick, which makes them very heavy. Modern telescopes use thin mirrors with many different supports that can be independently controlled by a computer. As the mirror is moved, the supports continually adjust to compensate for gravity, keeping the shape of the mirror within precise tolerances.

As of 2001, the largest telescope in the world is the twin mirror Keck Telescope. The Keck consists of two matched mirrors housed in separate buildings. Each telescope housing stands eight stories tall and each mirror weighs 300 tons. The mirrors are not made of one solid piece of glass. Instead, each mirror combines thirty-six 1.8-m (meter) hexagonal cells combined to form a collecting area equivalent to one 10-m mirror. Since the twin mirrors are separated by a wide distance, the Keck telescope has a much greater resolving power than either mirror alone would have.

The other advantage of the Keck telescope is its position on the top of an extinct volcano, Mauna Kea, in Hawaii. The atmosphere is very stable and the mountaintop, at 4 km (kilometers), is so high that the telescopes are above most of Earth's atmosphere. The European Southern Observatory in Chile is constructing a similar telescope that will combine light from four8.2-m mirrors working as a single instrument. These and similar instruments around the world promise to reveal even more about our universe.

see also Astronomer.

Elliot Richmond

Bibliography

Chaisson, Eric, and Steve McMillan. Astronomy Today, 3rd ed. Upper Saddle River, NJ: Prentice Hall, 1993.

Giancoli, Douglas C. Physics, 3^rd ed., Englewood Cliffs, NJ: Prentice Hall, 1991

Pannekoek, Anton. A History of Astronomy. New York: Dover Publications, 1961.

Sagan, Carl. Cosmos. New York: Random House, 1980.

telescope traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy; other types of astronomical telescopes gather radio waves (see radio astronomy ), X rays (see X-ray astronomy ), or infrared or ultraviolet radiation.

Types of Optical Telescopes

There are three major types of optical telescopes, classified according to the element that gathers and focuses the incoming light. In the refracting telescope, or refractor, light is bent, or refracted, as it passes through an objective lens. The objective lens is convex, i.e., thicker at the middle than the edges. Parallel light passing through the lens is refracted so that it converges to a point behind the lens, called the focus. The distance from the lens to the focus is called the focal length. In a reflecting telescope, or reflector, light is reflected by a concave mirror and brought to a focus in front of the mirror. If parallel light rays are to be reflected so that they converge to a single point, the mirror must be paraboloid in shape. Typically, a glass disk is ground to this shape and then coated with a thin layer of silver or aluminum to make it highly reflecting. The third type of telescope, the catadioptric system, focuses light by a combination of lenses and mirrors.

Images Produced by Optical Telescopes

The properties of the image produced by a telescope are similar, whether formed by lenses or mirrors. The real image produced is inverted; i.e., top and bottom are reversed, as are left and right. In a terrestrial refracting telescope used to view objects on the earth, an additional lens is used to invert the image a second time, so that objects appear as they do when viewed with the unaided eye; in an astronomical telescope, image inversion is unimportant and no lens is added to invert the image a second time. The angular size of an object as seen from the position of the telescope may be expressed in degrees or in radians (1 radian equals about 57°). The angle in radians determined by the object is given by the ratio of the object's diameter to its distance from the telescope. The size of the object's image is the product of this and the focal length of the image-forming lens or mirror. For example, the angular size of the moon's diameter is about 1/2 °, or roughly 1/100 radian; a telescope with a focal length of 60 in. (152 cm) would produce an image of the moon 0.6 in. (1.52 cm) in diameter. The brightness of the image depends on the total light gathered and hence is proportional to the area of the objective or the square of the diameter of the telescope.

Resolving and Magnifying Power

The resolution of the telescope is a measure of how sharply defined the details of the image can be. The laws of diffraction make a certain amount of blurring unavoidable, because of the wave nature of light. If two stars are very close, a given telescope may not be able to separate them into two distinct points. The smallest angular separation that can be unambiguously distinguished is called the resolving power of the telescope and is proportional to the ratio of the wavelength of light being observed to the diameter of the telescope. Thus, the larger the diameter, the smaller the minimum angular separation and the higher the resolving power.

The magnification, or power, of the telescope is relevant only when an eyepiece, or ocular, is used to magnify the image for visual inspection. The angular size of the virtual image seen by the observer will be larger than the actual angular size of the object. The ratio of these two sizes is the magnifying power and is equal to the ratio of the focal lengths of the objective and ocular. Any desired magnification can be obtained with a given telescope by the use of an appropriate ocular, but beyond a point determined by the resolving power, higher magnification will reveal no further details.

In addition to diffraction, other defects limit the performance of real optical systems. The most serious of these for lenses is chromatic aberration . Other defects include coma, astigmatism, distortion, and curvature of field. In general, it is easier to eliminate these faults in the reflector than in the refractor.

Arrangement of Mirrors in a Reflector

The prime focus of the reflector is inside the main tube of the telescope and thus the image cannot be observed there without blocking part of the incoming light. A variety of schemes are employed to divert the image to a more convenient location. The simplest of these, constituting the Newtonian reflector, is the placement of a flat secondary mirror in the path of the converging light just before the prime focus. The small secondary mirror, which blocks a negligible portion of the primary mirror, is tilted at an angle of 45° in order to reflect the convergent light at right angles and bring it to a focus outside the telescope tube. In the Cassegrain system, the secondary mirror is convex and reflects the convergent light directly back along the axis of the telescope through a hole in the center of the primary mirror. By causing light to traverse a longer path, the effective focal length is increased and a larger image is formed. The Gregorian system is similar to the Cassegrain, except that the secondary mirror is concave. The Coudé system uses both a convex secondary mirror and one or more diagonal flat mirrors to produce a focus outside the tube. The secondaries are arranged so that the position of the focus remains stationary as the telescope rotates, allowing the use of image-recording and analyzing devices that would be too heavy to mount directly on a moving telescope.

The Schmidt Telescope and Other Innovations

The Schmidt camera telescope, invented in 1930 by Bernard Schmidt, is a catadioptric system used for wide-angle photography of star fields. The primary mirror is spherical instead of paraboloidal, which requires that a special correcting lens be used on the front of the tube. The Maksutov telescope, invented by D. D. Maksutov in 1941, is similar in design and purpose to the Schmidt telescope but has a spherical meniscus in place of the correcting plate of the Schmidt.

Mounting the Telescope

Equal in importance to the mirrors and lenses constituting the optics of a telescope is the mounting of the telescope. The mounting must be massive, in order to minimize mechanical vibration that would blur the image, especially at high magnification or during long-exposure photography. At the same time, motion of the telescope must be precise and smooth. To allow the telescope to be pointed in any direction in the sky, the mounting must provide rotation about two perpendicular axes. In the altazimuth mounting, one axis points to the zenith and allows rotation along the horizon and the other allows changes in altitude, or distance above the horizon. This mounting is used for small terrestrial telescopes and, since the 1970s, most new astronomical telescopes use altazimuth mountings that are computer-driven in both axes. Before the 1970s, most astronomical telescopes used the equatorial mounting, in which one axis points at the celestial pole and hence is parallel to the earth's axis.

Evolution of Telescopes

Refracting Telescopes

The first practical telescopes were refracting telescopes produced at the beginning of the 17th cent. By 1610, Galileo had made extensive astronomical use of the simple refractor. The best telescopes of this period had very long focal lengths to minimize the chromatic aberration inherent in the single-element objective. The multielement objective, invented in 1733, allowed the construction of telescopes of large aperture. The art of building refracting telescopes reached a high point in the 19th cent. The largest refractor in existence, with an objective lens 40 in. (102 cm) in diameter, is located at the Yerkes Observatory in Williams Bay, Wis. A 36-in. (91-cm) refractor is located at the Lick Observatory in California and a 33-in. (84-cm) refractor is located at Meudon, France. These telescopes represent the practical limit on the size of a refractor.

Reflecting Telescopes

Because a lens can be supported only at its edge, the weight of the lens itself produces unavoidable distortion in the shape. Because a mirror can be supported from behind, it can be much more massive without incurring distortion, and mirrors many feet in diameter have been constructed. The first reflecting telescope, built by Isaac Newton in 1672, had a mirror made of a metal alloy. When techniques for depositing metal films on glass surfaces were developed, reflecting telescopes became comparable in precision to refractors. An important advantage of the reflecting telescope is the absence of chromatic aberration. Because only one surface must be ground to an exact shape, the reflector is also easier to manufacture. Although increasingly larger mirrors provide increasingly greater light-gathering ability, the cost increases even more rapidly. Several innovations were introduced toward the end of the 20th cent. to achieve the goal of increasing light gathering more economically.

One of these innovations is the use of segmented or multimirror reflectors. The largest of these are the twin W. M. Keck telescopes at the Mauna Kea Observatories , Hawaii. Each has a segmented primary mirror, composed of 36 separate hexagonal pieces. Each segment is about 72 in. (1.8 m) across but only 3 in. (76 mm) thick, creating a 394-in. (10-m) diameter primary mirror. The position of each 880-lb (400-kg) segment is computer controlled to a tolerance of less than one millionth of an inch. The 430-in. (11-m) primary mirror array of the Hobby-Eberly telescope at the McDonald Observatory, Tex., is made of 91 250-lb (113-kg) hexagonal segments. The revolutionary design, which resulted in an effective aperture of 362 in (9.2 m), enabled it to be constructed at 20% the cost of other 350-in (9-m class) telescopes.

Another technique for compensating for smaller mirrors is called optical interferometry. The signals from two or more smaller telescopes at separate locations are combined so that the resulting image is equal to that which would be received from a very large telescope, or virtual telescope . The largest of these installations is at the European Southern Observatory in Chile. Completed in 2003, it comprises four 315-in. (8-m) fixed telescopes and several movable 72-in. (1.8-m) auxiliary telescopes, the images from which can be combined to provide the total resolving capability of a 630-in. (16-m) conventional reflecting telescope. Similar approaches to solving the problem of building a large reflector were the multiple-mirror telescope (MMT) at the Fred Lawrence Whipple Observatory , Ariz., the COAST ( C ambridge O ptical A perture S ynthesis T elescope) system at the Univ. of Cambridge observatory, England, and the CHARA ( C enter for H igh A ngular R esolution A stronomy) Array at the Mount Wilson Observatory , Calif. The MMT, which became operational in 1979, consisted of six 72-in. (1.8-m) telescopes on a common mounting and having a resolving capability equal to that of a 176-in. (4.5-m) reflector of conventional design. It was replaced by a conventional 256-in. (6.5-m) single-mirror telescope in 1999. The COAST system, which became operational in 1996, combines light from a trio of small telescopes spaced about 20 ft (6 m) apart. The twin Keck telescopes, in domes a few hundred feet apart, have adaptive optics that make them equivalent in resolving power to a telescope with a mirror 280 ft (85 m) across. The CHARA Array, fully operational in 2002, consists of six 39-in. (1-m) aperture telescopes arranged in a Y-shape and contained in a 1,300-ft (400-m) diameter circle; the combined signals from the six telescopes provide the equivalent of the resolving capability of a telescope 1,080 ft (330 m) wide.

The largest single-mirror reflecting optical telescopes are the 327-in. (8.3-m) Subaru telescope, formerly called the Japanese National Large Telescope, at the Mauna Kea Observatories and the Gemini North telescope (320 in./8.1 m), also at Mauna Kea, and its twin, the Gemini South telescope, at Cerro Pachon, Chile. Other large conventional optical telescopes include those at the Special Astrophysical Observatory near Zelenchukskaya, in the Caucasus (236 in./6 m), the world's largest solid-mirror optical telescope; Palomar Observatory (see under Palomar Mountain ), Calif. (200 in./5 m); the Cerro Tololo Inter-American Observatory , Chile, and the Kitt Peak National Observatory , Ariz. (158 in./4 m each); the European Southern Observatory , Chile (142 in./3.6 m); Lick Observatory , Calif. (120 in./3 m); and McDonald Observatory , Tex. (107 in./2.7 m). Large Schmidt telescopes are at Palomar, Siding Spring Observatory , Australia, and the European Southern Observatory. The Hubble Space Telescope is a 94.5-in. (2.4-m) reflector.

Bibliography

See H. G. J. Rutten and M. A. M. Van Venrooij, Telescope Optics: Evaluation and Design (1988); R. N. Wilson, Reflecting Telescope Optics: Basic Design Theory and Its Historical Development (1996); R. Moore, Eyes on the Universe: The Story of the Telescope (1997); S. F. Tonkin et al., Amateur Telescope Making (1998); J. B. Zirker, An Acre of Glass: A History and Forecast of the Telescope (2005).

Telescope

The telescope is an instrument that gathers light or some other form of electromagnetic radiation (from radio waves to gamma rays) emitted by distant sources. The most common type is the optical telescope, which uses a collection of lenses or mirrors to magnify the visible light emitted by a distant object. There are two basic types of optical telescopes—the refractor and the reflector. The one characteristic all telescopes have in common is the ability to make distant objects appear to be closer.

The first optical telescope was constructed in 1608 by Dutch spectacle-maker Hans Lippershey (1570–1619). He used his telescope to view distant objects on the ground, not distant objects in space. The following year, Italian physicist and astronomer Galileo Galilei (1564–1642) built the first astronomical telescope. With this telescope and several following versions, Galileo made the first telescopic observations of the sky and discovered lunar mountains, four of Jupiter's moons, sunspots, and the starry nature of our Milky Way galaxy.

Words to Know

Black holes: Remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.

Chromatic aberration: Blurred coloring of the edge of an image when white light passes through a lens, caused by the bending of the different wavelengths of the light at different angles.

Electromagnetic radiation: Radiation that transmits energy through the interaction of electricity and magnetism.

Gamma ray: Short-wavelength, high-energy radiation formed either by the decay of radioactive elements or by nuclear reactions.

Interferometry: In astronomy, the precise combining of light or radio waves collected by two or more instruments from one single celestial object.

Radiation: Energy transmitted in the form of subatomic particles or waves.

Radio wave: Longest form of electromagnetic radiation, measuring up to 6 miles (9.6 kilometers) from peak to peak.

Reflector telescope: Telescope that directs light from an opening at one end to a concave mirror at the far end, which reflects the light back to a smaller mirror that directs it to an eyepiece on the side of the tube.

Refractor telescope: Telescope that directs light through a glass lens, which bends the light waves and brings them to a focus at an eyepiece that acts as a magnifying glass.

Ultraviolet radiation: Electromagnetic radiation of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum.

X ray: Electromagnetic radiation of a wavelength shorter than ultraviolet radiation but longer than gamma rays that can penetrate solids and produce an electrical charge in gases.

Refractor telescopes

In a refractor telescope, light waves from a distant object enter the top of the telescope through a lens called an objective lens. This lens is convex—thicker at the middle than the edges. As light waves pass through it, they are bent (refracted) so that they converge (come together) at a single point, known as the focus, behind the objective lens. The distance between the objective lens and the focus is called the focal length. A second lens, the eyepiece, at the focus then magnifies the image for viewing. This is the type of telescope Galileo developed and used.

As refractor telescopes came into wider use, observers realized the instruments had a slight imperfection. Since, like a prism, a lens bends the different wavelengths (colors) that make up light through different angles, refractor telescopes produced a false color around any bright object. This defect is called chromatic aberration. Early astronomers tried to correct this problem by increasing the focal length, but the new instruments were very clumsy to use.

A solution to this problem came in 1729 when English scientist Chester Moore Hall (1703–1771) devised the achromatic lens: two lenses, made of different kinds of glass and shape, set close together. As light passes through the lenses, the false color brought about by the first lens is canceled out by the second lens. Hall went on to create the achromatic telescope in 1733. The lens itself was further developed by English optician John Dollard in 1758. His lens combined two or more lenses with varying chemical compositions to minimize the effects of aberration.

Reflector telescopes

In a reflector telescope, light waves from a distant object enter the open top end and travel down the tube until they hit a mirror at the bottom. This mirror is concave—thicker at the edges than in the middle. Because of this primary mirror's shape, the light waves are reflected back up the tube to a focus, where a small, flat secondary mirror reflects the image to an eyepiece on the side of the telescope. English physicist and mathematician Isaac Newton (1642–1727) developed the reflector telescope in 1668. English astronomer William Herschel (1738–1822) used an updated version when he discovered Uranus in 1781.

Even today, reflectors are perhaps the most prominent type of telescope. They are relatively inexpensive to build and maintain, produce little false color, and maintain a high resolution. The mirrors used in larger reflectors, however, often cause distortion due to the weight on the instrument. Newer reflectors incorporate mirrors of varying shapes (hexagonal glass segments, for example) and of lighter, more durable materials (such as Pyrex™).

Limits to ground-based telescopes

Earth's atmosphere provides an effective filter for many types of cosmic radiation. This fact is crucial for the survival of humans and other life-forms. However, since the atmosphere only allows visible light and radio waves to pass through it, celestial objects that emit other types of electromagnetic radiation cannot be viewed through telescopes on the ground. Many observatories have been constructed at high altitudes where the atmosphere is thinner—and where the glare of urban artificial light interferes less with viewing—but this improves the situation only slightly.

Space-based telescopes

One way astronomers have sought to overcome the distortion caused by the atmosphere and by city lights is by placing telescopes in space. The first of these instruments, placed in orbit around Earth during the 1970s, were small telescopes that could detect X rays, gamma rays, and

ultraviolet radiation. They discovered hundreds of previously unknown entities, including one likely black hole.

While many other space telescopes have been placed in orbit, the most well known is the Hubble Space Telescope (HST). Launched in April 1990 aboard the space shuttle Discovery, the HST has an 8 foot (2.4 meter) primary mirror and five major instruments for examining various characteristics of distant celestial bodies. Shortly after the HST began orbiting Earth, scientists learned that the curve in its primary mirror was off by just a fraction of a hair's width. This flaw caused light to reflect away from the center of the mirror. As a result, the HST produced blurry pictures.

In 1993, astronauts aboard the space shuttle Endeavor caught up with the HST and installed a group of three coin-sized mirrors around the primary mirror, which brought the light into proper focus. In 1997, another space shuttle crew conducted general repairs to the HST. Then in November 1999, the HST stopped working after its gyroscopes broke down. Without the gyroscopes, the telescope could not hold steady while focusing on stars, galaxies, and other cosmic targets. A month later, during an eight-day mission, astronauts aboard the space shuttle Discovery installed almost $70 million worth of new equipment on the HST, including a computer 20 times faster than the telescope's old one; new gyroscopes; batteries with voltage regulators to prevent overheating; a new guidance unit, data recorder, and radio transmitter; and steel sunshades to protect the telescope from solar damage.

Despite the need for repairs, the HST has proven to be the finest of all telescopes ever produced. The thousands of images it has captured—a comet hitting Jupiter, a nursery where stars are born, stars that belong to no galaxy, galaxies that house quasars, galaxies violently colliding—have amazed astronomers.

The future on the ground

Technological advances in the 1990s began to return astronomy to the ground. New observatories have sprung up on every continent, including Antarctica, housing telescopes that are able to capture celestial images almost as clearly as the HST. These new ground-based telescopes are far more advanced than previous ones. The cost of producing their light-gathering mirrors has been reduced, so the mirrors and the telescopes can be built even larger. Advances in photographic devices allow these telescopes to capture images in minutes instead of hours or entire nights.

The twin domes at the Keck Observatory complex on Mauna Kea in Hawaii house telescopes with mirrors roughly 32 feet (9.8 meters) in diameter. The Hobby-Eberly Telescope at the University of Texas McDonald Observatory near Fort Davis, Texas, was completed on December 12, 1996. Its mirror, made up of 91 hexagonal segments, measures 36 feet (11 meters) in diameter.

Perhaps the greatest advancement is the development of interferometry. Astronomical interferometry is the art of combining light or radio waves collected by two or more telescopes from a single celestial object. The information is fed into a computer, which precisely matches up the light-wave images gathered from the telescopes, peak for peak and

trough for trough. After distortion is removed through mathematical analysis, the resulting image is equal in sharpness to what a single telescope of enormous size can produce.

Sometime early in the twenty-first century, the world's largest telescope will be completed. On the summit of Cerro Paranal, an 8, 645-foot (2,635-meter) mountain in the Atacama Desert in northern Chile (an area considered to be the driest on Earth), stands the Paranal Observatory. The observatory houses the Very Large Telescope, which will consist of four telescopes, each containing a mirror almost 27 feet (8.2 meters) in diameter. In the language of the Mapuche, indigenous people who live in the area, the four unit telescopes are known as Antu ("Sun"), Kueyen ("Moon"), Melipal ("Southern Cross"), and Yepun ("Venus"). The four units were each operational as of the beginning of 2001, but had yet to be combined as an interferometer. Through interferometry (the precise combining of light or radio waves collected by two or more instruments from one single celestial object), the teamed instruments will have a light-gathering capacity greater than a single telescope with a mirror more than 52 feet (16 meters) in diameter.

[See also Gamma ray; Infrared astronomy; Interferometry; Radio astronomy; Ultraviolet astronomy; X-ray astronomy ]

telescope Instrument for enlarging a distant object or studying electromagnetic radiation from a distant source. Optical telescopes can use lenses (refracting telescopes) or mirrors (reflecting telescopes); catadioptric telescopes use both in combination. The lens or mirror is the telescope's main light-gathering part (objective), and its diameter, known as the aperture of the telescope, determines its magnifying power. The point at which the objective concentrates the light from the source is its focus, and the distance from the focus to the objective is its focal length. Refracting telescopes were invented by Dutch optician Hans Lippershey (1608) and Italian astronomer Galileo (1609). The main disadvantage with refracting telescopes was chromatic aberration. This problem was solved in reflecting telescopes by combining lenses so their aberrations cancelled each other out. In 1668, English scientist Sir Isaac Newton built an early astronomical reflector. Modern observatories are built on mountain peaks in order to improve ‘seeing’ and to observe infrared waves from celestial bodies. Earth-bound telescopes have limitations because the incoming radiation has to pass through the Earth's atmosphere. This ceases to be a problem with telescopes in Earth orbit, such as the Hubble Space Telescope (HST). Orbiting telescopes can also detect other types of electromagnetic radiation more easily, such as ultraviolet radiation, X-rays and gamma radiation. Radio telescopes are complex electronic systems that detect and analyse radio waves from beyond the Earth. In 1937, the first radio telescope was built by US radio engineer Grote Reber. Radio interferometers are arrays of smaller dishes that permit the investigation of even more distant radio sources.

telescope An optical instrument that collects light from faint and distant objects and magnifies their images. Telescopes have an objective lens or mirror which gathers light and focuses it at a focal plane. The resulting image may be observed with an eyepiece or recorded by a photographic emulsion, a charge-coupled device, or some other detector. In a refracting telescope the objective is a lens, and in a reflecting telescope it is a mirror. A telescope that uses both lenses and mirrors is called a catadioptric system. The larger the aperture of a telescope the more light it collects, making it possible to observe fainter stars and to see finer details. The highest magnification that can be used on a telescope of given aperture is restricted by the effects of diffraction. In practice, the finest detail visible with a telescope is set by the Dawes limit. Telescope apertures range from about 25 mm for the smallest refractors up to many metres for the largest reflectors.

tel·e·scope / ˈteləˌskōp/ • n. an optical instrument designed to make distant objects appear nearer, containing an arrangement of lenses, or of curved mirrors and lenses, by which rays of light are collected and focused and the resulting image magnified. ∎  short for radio telescope. • v. [tr.] cause (an object made of concentric tubular parts) to slide into itself, so that it becomes smaller. ∎  [intr.] be capable of sliding together in this way: five steel sections that telescope into one another. ∎  crush (a vehicle) by the force of an impact. ∎ fig. condense or conflate so as to occupy less space or time: a way of telescoping many events into a relatively brief period.

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