Telescopy

Summary

Telescopy is the science behind the creation and use of telescopes, devices that clearly render objects that are too dim or distant to be seen by the naked eye. The word "telescope" originated in two ancient Greek words, "to watch" and "from afar." By enabling people to study and accurately pinpoint stars, such as Polaris and Sirius, planets, such as Jupiter and Mars, and other structures in the sky, telescopy radically transformed humankind's ability to map locations and navigate from place to place. The use of telescopes in astronomy and cosmology has also profoundly deepened knowledge of the universe's complexities and its origins.

Definition and Basic Principles

Telescopy is the field concerned with the development, improvement, and practical application of telescopes. A telescope is any device that enables the viewing or photographing of objects that are either too dim or too far away to be seen without aid. Although they are built to operate in many different ways, telescopes make use of information gathered from various parts of the electromagnetic spectrum. The electromagnetic spectrum is made up of various forms of radiation—energy that comes from a particular source and travels through space or some other material in the form of a wave. Various types of radiation have different wavelengths and frequencies. The higher the frequency of a wave of radiation and the shorter its wavelength, the higher its energy as it travels.

The most common kinds of telescopes, known as refractors and reflectors, use systems of mirrors and lenses to gather and focus visible light. Both types of telescopes fall under the umbrella of optical telescopy. Radio telescopes gather information not from light but from radio waves, which have the longest wavelengths on the electromagnetic spectrum. X-ray telescopes and gamma-ray telescopes use the kinds of radiation with the shortest wavelengths. Specialized telescopes detect other types of electromagnetic radiation, including ultraviolet light and infrared light, and are used for specific purposes. For example, infrared telescopes are similar to reflecting optical telescopes in construction, but they are designed to collect radiation that is invisible to the naked eye. One reason these telescopes are so useful is that infrared radiation is able to travel through thick clouds of dust and gas in a way that visible light cannot. Thus, infrared telescopes allow scientists to gain insight into the phenomena taking place within hidden regions of space. With these tools and others, modern telescopy is able to detect every region of the electromagnetic spectrum. Researchers use telescopy to create clear images of stars, planets, galaxies, and other celestial objects.

No matter how it is constructed, a telescope is designed to serve three basic functions. First, it should effectively collect large amounts of electromagnetic radiation and focus, or concentrate, that radiation. This makes objects that would otherwise appear very dim seem much brighter and easier to see. Second, it should resolve, or clearly distinguish between, the small details of an image. This makes objects that would otherwise appear blurry seem sharp and focused. Third, it should magnify the image it creates, so that objects at a distance appear larger. Although many people think of magnification as being the primary purpose of a telescope, it is in fact the least important function—if an image is not bright or clearly resolved, no matter how much it is magnified, it will not be useful.

Background and History

Although it is difficult to trace the invention of the telescope to a single individual, the first person to have tried to patent a basic telescope (in 1608) was a German-Dutch lensmaker named Hans Lippershey. Lippershey used a combination of two lenses separated by a tube to magnify objects by about three or four times. At about the same time, Italian mathematician Galileo built a very similar instrument using a combination of a concave (inwardly curving) lens and a convex (outwardly curving) lens. Galileo promptly showed his version of the telescope to the chief magistrate of Venice and became famous for using it to conduct astronomical observations, showing, among other things, that the Earth revolved around the Sun rather than the other way around. As a result of these well-known activities, Galileo is often wrongly credited as the inventor of the telescope. While Galileo did not invent the telescope, he may have been the first to call the instrument a telescope.

The devices made by Lippershey and Galileo were both refractor telescopes, which rely primarily on lenses that gather and focus light. In the second half of the seventeenth century, English physicist Isaac Newton was among the early pioneers of the reflecting telescope, which relies primarily on curved mirrors that bend light.

Over the next three hundred years, optical telescopes underwent vast technological improvements. For example, achromatic lenses were invented to compensate for the errors in color caused by older lenses that failed to treat all the colors of visible light in the same way. Radio telescopes were first developed in the twentieth century, based on the discovery that faraway celestial bodies were constantly emitting faint amounts of radiation in the form of radio waves. This new form of telescopy was soon applied to both military radar operations and astronomical research.

Other technological advances have affected the field of telescopy. The development of photography enabled astronomers to create permanent still images of the celestial bodies they were observing and to use light-sensitive plates to gather, over long periods of time, even more light than could be collected by lenses or mirrors. Similarly, the invention of increasingly sensitive electronic devices for capturing light, such as the charge-coupled device (CCD), revolutionized modern telescopy. In addition, advances in computer technology allow astronomers and the military to constantly monitor selected portions of the sky using computers that alert human overseers if anything unusual is detected.

How It Works

Optical Telescopes. Optical telescopes are designed to gather and focus light that radiates from distant objects. The two main types of optical telescopes are reflectors and refractors. Each type is based on a different principle derived from the physics of light: reflection and refraction. A third type, a catadioptric telescope, is a hybrid of reflectors and refractors.

Refraction is a phenomenon by which light is bent as it travels from a medium of a certain density to a medium of a different density (such as moving from air into glass). Basic refracting telescopes use a combination of two lenses to refract light. As light rays from a distant object, such as a star, approach a telescope, they travel in nearly perfectly parallel lines. The first lens, known as the primary, bends or refracts these parallel rays of light so that they converge on a single point. This creates an intermediary image of the object that is both bright and in focus. The purpose of the second lens, known as the secondary, is to take that bright, focused image and magnify it by spreading the light rays once more, enabling them to form a larger image on the retina of the eye. In the course of this process, the rays of light cross (light from the top of the object is bent downward, and light from the bottom of the object is bent upward), so the image is upside down. Many refracting telescopes use another pair of lenses to render the image right side up.

Reflection telescopes are based on the principle that if light waves meet a surface that will not absorb them, they are redirected away from the surface at the same angle at which they were originally traveling. The angles at which light meets and is deflected from a surface are called the angles of incidence and reflection. Basic reflecting telescopes use a combination of two mirrors rather than lenses to reflect light. The mirrors are usually coated with a thin film of a shiny metal, such as aluminum, which makes them more reflective. As light enters the tube of a simple reflective telescope, it is reflected off the primary mirror and travels back in the direction from which it came to form a bright, focused image, just as in a refraction telescope. A secondary mirror in a reflection telescope functions similarly to the secondary lens in a refraction telescope, creating a magnified image focused comfortably on the retina.

Some optical telescopes use a combination of reflecting and refracting techniques; these are known as catadioptric telescopes. One of the most common types of catadioptric telescopes is the Schmidt-Cassegrain telescope, which takes its name from two scientists whose work informed its design. This type of telescope contains a deeply curved concave primary mirror at the back of the tube, which reflects light toward a convex secondary mirror at the front of the tube. Schmidt-Cassegrain telescopes also contain a corrective lens that helps counteract the optical aberrations caused by the mirrors, such as making points of light look like disks.

Radio Telescopes. Rather than manipulating light, radio telescopes collect and focus radio waves—the same electromagnetic radiation used to transmit radio, television, and cell phone signals. Radio telescopes are useful for astronomical observation because faraway celestial objects, including stars and quasars (incredibly bright, high-energy bodies resembling stars), are constantly emitting radio waves. There are many different kinds of radio telescopes, but each is made up of the same fundamental parts. The first is a radio antenna, which often looks like a huge, curved television satellite dish. The greater the surface area of the antenna, the more sensitive it can be to the relatively weak radio waves being transmitted from cosmic sources and the fainter and more distant the objects it can detect. The second basic part of a radio telescope is a radiometer, also known as an amplifier. This instrument is placed at the central focusing point of the antenna, and its purpose is to receive and amplify the signal produced by the antenna, convert it to a lower frequency, and transmit it via cable to an output device that charts or displays the information collected by the telescope.

Many radio telescopes use a technique known as interferometry to increase their angular resolving power, which is relatively weak compared with that of optical telescopes. (The reason for the relative weakness is that the angular resolving power of a telescope is defined by the wavelength of the radiation it measures divided by the telescope's diameter. Radio waves, with much longer wavelengths than those of visible light, require telescopes with very large diameters to achieve the same angular resolution as optical telescopes.) An interferometer is a device that takes advantage of the interference phenomenon to electronically combine the signals from multiple telescopes and create a single image. For instance, the National Radio Astronomy Observatory's Very Large Array (VLA), a prominent radio astronomy observatory in New Mexico, has nearly thirty radio antennae, each measuring 75 feet (23 meters) in diameter. By spacing the antennae far apart, the observatory has created an array that functions like a single telescope with a diameter as wide as the distance between the first and the last antenna. When the signals from the antennae are combined through interferometry, the array is able to resolve details in the sky at a much greater power.

Spectrographs. Spectrographs are important auxiliary instruments that are often attached to optical telescopes. Their primary function is to split up the light collected by a telescope and separate it into its individual wavelengths, thereby creating a spectrum. Spectrographs can be extremely complicated, but their basic construction involves an entrance slit, two lenses, a prism, and a charge-coupled device (CCD). The entrance slit is designed to reduce the interference of any background light not coming from the particular star being observed. The first lens is designed to direct the rays of light coming from the star into the prism, which then breaks up the light into its different wavelengths. After the light exits the prism as a spectrum, it is directed by the second lens onto the CCD, which produces a readout of how much light of each wavelength is coming from the star. This information can then be used to analyze various important characteristics of the object under observation. For example, spectrographs can help astronomers learn the chemical composition of a star as well as its temperature and rotation speed.

Applications and Products

Astronomical Observations. Perhaps the most important scientific application of telescopy is its use in facilitating astronomical observations. Telescopes are the fundamental tools used by astronomers and astrophysicists to further their understanding of space, celestial objects, and the universe as a whole. Measurements produced with the aid of telescopes, for example, revealed the shape of the Milky Way galaxy and the location of Earth within it. Decades of careful observations through the Hooker optical telescope at the Mount Wilson Observatory in Pasadena, California, enabled astronomer Edwin Hubble to prove not only that the galaxy is just one among many such systems in the universe but also that the universe itself is expanding as these galaxies move farther apart. Without the help of telescopes, the human eye would never have laid sight on such astonishing phenomena as the icy rings that surround Saturn, the gigantic high-pressure storm on Jupiter known as the Great Red Spot, the craggy craters on the far side of the Moon, or the brilliant azure of the atmosphere around Neptune caused by the reflection of blue light by methane gas.

The highest angular resolution achievable by ground-based telescopes is limited by the fact that radiation of some wavelengths does not travel well through the Earth's atmosphere but is absorbed by water vapor and carbon dioxide as it travels. Ground-based telescopes are also affected by atmospheric turbulence—small, irregularly moving air currents—which can cause blurry images. However, because space telescopes orbit the Earth at a high altitude, they are not affected by these problems. Therefore, some astronomical observations can be carried out only by telescopes located above the atmosphere of the Earth.

The Hubble Space Telescope, which was launched into orbit by the National Aeronautics and Space Administration (NASA) in 1990, was the most advanced space-based telescope system for decades. It is a large optical visible-ultraviolet reflecting telescope (its primary mirror is nearly 8 feet, or 2 1/2 meters, in diameter) that travels around the Earth several times a day, collecting images of star systems, planets, comets, galaxies, and other celestial bodies. The Hubble Space Telescope is also equipped with a wide-field planetary camera that can record images of space at resolutions several times higher than any telescope based on Earth. In addition, the telescope has a faint-object camera designed to detect extremely dim celestial objects, a faint-object spectrograph that collects information about the chemical composition of these objects, and a high-resolution spectrograph that gathers ultraviolet light from very distant objects.

NASA last serviced the Hubble in 2009, with the belief that the telescope could continue to operate until at least 2016. However, the Hubble continued to function well beyond that point. In 2021, NASA launched the successor to Hubble, the James Webb Space Telescope, the largest space telescope ever placed in orbit. The Webb Telescope is about the size of a small truck and has a mirror just over 21 feet (6 1/2 meters) in diameter.

Although telescopes are not generally used for navigation purposes, one of their most important early contributions was in helping sailors and explorers pinpoint their exact locations on the seas by finding the position of known stars or planets in the sky. Astronomers still rely on telescopes to pinpoint the positions of celestial bodies. In doing so, they are able to create detailed and systematic surveys, or maps, of the sky. For example, since 2000, the Sloan Digital Sky Survey has been using a large reflecting telescope with charge-coupled devices to pinpoint the location of distant galaxies and quasars.

Military Surveillance. For centuries, telescopes have provided army and navy surveillance teams with an invaluable tool by enabling military personnel to detect the movements of hostile forces from a distance. Initially, only ground-based telescopes were used, but in the late eighteenth century, it became possible to greatly increase the visual range of telescopes by placing them on board hot-air balloons. In World War I, European military forces used refractor telescopes mounted on airplanes to perform aerial surveillance, and in World War II, surveillance airplanes were equipped with sophisticated telescopes with powerful lenses and cameras that produced high-resolution images of military bases and enemy territories far below. Modern aerial surveillance techniques typically involve telescopes with extraordinarily good angular resolutions mounted on crewless aircraft systems (remotely-piloted aircraft), such as the United States Air Force's Global Hawk. In addition, ground-based telescopes are still used by countries around the world to keep an eye on objects in the sky, such as enemy aircraft, missiles and other weapons, and satellite surveillance equipment belonging to other nations. For example, there are two optical surveillance sites in the United States, one in Hawaii and one in New Mexico, equipped with the latest in adaptive optics telescopes.

Military surveillance is increasingly conducted in space, with telescopes mounted on satellites. Satellite telescopes can move at vastly greater speeds than airplanes and can navigate to any region above the desired surveillance target without having to contend with national airspace boundaries. Although detailed information about the tools used by military surveillance units is closely guarded, it is generally thought that most satellite-based telescopes travel in low-Earth-orbit altitudes, about 62 to 310 miles (100 to 500 meters) above sea level. They probably conduct observations by collecting electromagnetic radiation with short wavelengths, such as infrared light and green light and are likely to be about 20 to 26 feet (six to eight meters) in diameter. Based on these parameters, experts have calculated that military satellite telescopes are most likely capable of distinguishing details that are less than an inch apart on the Earth—enough resolving power to read a newspaper headline.

Environmental Applications. Governments and other organizations also use satellite-based telescopes in surveillance applications. One of the most common uses is to monitor changes in the environment. NASA's Earth-observing system satellite Terra carries multiple sophisticated telescopes onboard that are used to detect such phenomena as volcanic activity, emerging forest fires, and floods. Terra also provides scientists with images so that they may track the effects of climate change on the Earth's surface; for example, scientists track the melting of the ice sheets in the Arctic over time. Brazil, a country whose rain forests have been reduced by centuries of cattle ranching and other agricultural activity, uses satellite-based telescopes to closely monitor the extent of deforestation and to evaluate how well its efforts to preserve the rain forests are working.

Communications. One application of telescopy that is still largely in the research-and-development stage is its potential use in laser communications between deep space and the Earth. A laser is a device that uses excited atoms or molecules to emit a powerful beam of electromagnetic radiation in a single wavelength (called monochromatic light). The light produced by lasers is intense and directed, meaning that the light rays do not spread out very quickly. This makes lasers a useful tool for transmitting a secure information-carrying signal directly to a receiver in a specific location. NASA, for example, has been looking into using laser signals to transmit data (including photographs, radar images, and analyses of space dust) collected by space probes, such as the Cassini, which orbited Saturn. However, collection of the laser signal on Earth would require a huge telescope.

Recreational Applications. Telescopes are far from just a tool for scientists, military personnel, or government officials. Durable, lightweight general-purpose optical telescopes known as spotting scopes are used frequently in everyday life in a wide variety of recreational applications. These instruments have greater magnification and resolving powers than binoculars. Naturalists, for instance, use spotting scopes to identify plumage markings and observe the behavior of bird species at a distance without alerting the birds as to the presence of humans. Airplane and train spotters use them to distinguish fine details in faraway vehicles. Long-range game hunters use spotting scopes to view their prey as they take aim, and sharpshooters use them to check on the position of their targets.

Careers and Course Work

Preparation for a career in any of the major fields related to telescopy, namely astronomy, astrophysics, meteorology, or military surveillance, should begin with a complete course of high school mathematics, up to and including precalculus. In addition, chemistry and physics are important subjects to cover in high school. Outside the academic environment, astronomy clubs or observatories are excellent places to gain practical experience using telescopes and to learn about the details of their operation. At the undergraduate level, a student interested in a career involving telescopy should work toward a bachelor of science degree with a concentration in a field such as physics, astronomy, mathematics, or computer science. No matter what major is chosen, additional course work in optics, electromagnetism, thermodynamics, mechanics, atomic physics, cosmology, statistics, and calculus provides essential background knowledge for further study—an important consideration, because almost all research positions in astronomy or related sciences, as well as any job involving the development of telescope technology itself, require the completion of a graduate degree, preferably a doctorate.

The typical career path for a student interested in telescopy involves pursuing work as an astronomer either in an academic or a government-based observatory, such as the National Radio Astronomy Observatory or the Mauna Kea Observatories at the University of Hawaii. Most astronomers who work at universities teach in addition to conducting research. Because jobs for practicing astronomers can be relatively scarce, students may benefit from taking one or more short-term positions such as a paid internship or postdoctoral fellowship to gain experience and contacts in the field before seeking a more permanent appointment.

Another career option is to approach telescopy from the point of view of engineering rather than scientific research. Electrical engineers and other technicians are essential members of the teams at astronomical observatories. These jobs, which involve repairing, upgrading, testing, and maximizing the efficiency of high-powered telescopes, generally do not require graduate degrees. A bachelor's degree in engineering or electronics and a strong background in mathematics and physics are sufficient qualifications to pursue this kind of telescopy career.

Social Context and Future Prospects

Over the four hundred years of its existence, the telescope has enabled humankind to transcend not just visual limitations but also mental ones. The telescope has been the impetus for a flood of astonishingly deep revelations (and further questions) about the origin of matter itself, the place of the Earth within the universe, and the future of the universe.

For the entire length of recorded human history, humans have constructed stories and mythologies about how the world came into being. Telescopes have provided a way to approach that issue from a scientific point of view. With the help of ever larger telescopes, physicists hope to be able to see what the universe looked like just a few hundred million years after the big bang and thereby to gain an understanding of how the very first stars, planets, and galaxies were formed. For millennia, humankind believed that Earth held a central place in the universe. Telescopes have turned that worldview on its head by showing that, in fact, there may be billions of Earth-like planets in the Milky Way galaxy alone and countless more across the entire universe. Although scientists once believed that the universe was unchanging, they have come to know—because of telescopy—that it is dynamic and expanding.

In an age in which the devastating effects of environmental pollution and the growing impact of climate change dominate the headlines, telescopy—by giving humans a holistic view from afar of the beautiful, vulnerable planet they inhabit—has a particularly important role to play in inspiring those who live on Earth to preserve and protect it for future generations.

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