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17.3: Telescopes

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    Lesson Objectives

    • Explain how astronomers use the whole electromagnetic spectrum to study the universe beyond Earth.
    • Identify different types of telescopes.
    • Describe historical and modern observations made with telescopes.


    • astronomer
    • catadioptric telescope
    • constellation
    • electromagnetic (EM) radiation
    • electromagnetic spectrum
    • frequency
    • gamma ray
    • infrared light
    • light-year
    • microwave
    • planet
    • radio telescope
    • radio wave
    • reflecting telescope
    • refracting telescope
    • space telescope
    • spectrometer
    • ultraviolet (UV)
    • visible light
    • wavelength
    • X-ray


    Many scientists interact directly with what they are studying. Biologists can collect cells, seeds, or sea urchins and put them in a controlled laboratory environment. Physicists can subject metals to stress or smash atoms into each other. Geologists can chip away at rocks to see what is inside. But astronomers, scientists who study the universe beyond Earth, rarely have a chance for direct contact with their subject. Astronomers observe their subjects at a distance, usually a very large distance!

    Electromagnetic Radiation

    Earth is separated from the rest of the universe by very large expanses of space. Very rarely matter from outside Earth’s environment reaches us, such as when a meteorite makes it through the atmosphere from elsewhere in the solar system. But for the most part, astronomers have one main source for their data — light. Light can travel across empty space, and as it does, so it carries both energy and information. Light is one type ofelectromagnetic (EM) radiation, energy that is transmitted through space as a wave.

    These videos discuss infrared, ultraviolet, and radio telescopes, as well as telescopes that detect visible light, and reveal tremendous features of the stars and galaxies around the Universe (2d): (2:19), (5:30).

    An interactive or media element has been excluded from this version of the text. You can view it online here:

    An interactive or media element has been excluded from this version of the text. You can view it online here:

    The Speed of Light

    Light travels faster than anything else in the universe. In the almost completely empty vacuum of space, light travels at a speed of approximately 300,000,000 meters per second (670,000,000 miles per hour). To give you an idea of how fast that is, a beam of light could travel from New York to Los Angeles and back again nearly 40 times in just one second. Even though light travels extremely fast, objects in space are so far away that it takes a significant amount of time for light from those objects to reach us. For example, light from the Sun takes about 8 minutes to reach Earth.


    Since astronomical distances are so large, it helps to have a unit of measurement that is good for expressing those large distances. A light-year is a unit of distance that is defined as the distance that light travels in one year. One light-year is approximately equal to 9,500,000,000,000 (9.5 trillion) kilometers, or 5,900,000,000,000 (5.9 trillion) miles (Figure below). That’s a long way! But by astronomical standards, it’s actually a pretty short distance.

    Proxima Centauri, the closest star to us after the Sun, is 4.22 light-years away. That means the light from Proxima Centauri takes 4.22 years to reach us. The galaxy we live in, the Milky Way Galaxy, is about 100,000 light-years across. How long does it take light to travel from one side of the galaxy to the other? 100,000 years! If an astronomer looks through a telescope at a star that is 1,000 light years away, is she seeing the star as it is now?


    The smallest red-shifted galaxies are among the most distant galaxies we have detected so far at 13 billion light-years away. That’s over a hundred-billion-trillion (100,000,000,000,000,000,000,000) kilometers!

    Looking Back in Time

    When we look at astronomical objects such as stars and galaxies, we are not just seeing over great distances—we are also seeing back in time. Because light takes time to travel, the image we see of a distant galaxy is an image of how the galaxy used to look. For example, the Andromeda Galaxy, shown in (Figure below), is about 2.5 million light years from Earth. If you look at the Andromeda Galaxy through a telescope, what are you seeing? You are seeing the galaxy as it was 2.5 million years ago. If the galaxy ceased to exist 1 million years ago, when would you know that? If you want to see the galaxy as it is now, you will have to wait and look again 2.5 million years into the future.


    This recent picture of the Andromeda Galaxy actually shows the galaxy as it was about 2.5 million years ago.

    Electromagnetic Waves

    Light is one type of EM radiation; light is energy that travels in the form of an electromagnetic wave. (Figure below) shows a diagram of an electromagnetic wave. An EM wave has two components: an electric field and a magnetic field. Each of these components oscillates between positive and negative values, which is what makes the “wavy” shape in the diagram.

    The distance between two adjacent oscillations is called wavelength. A related value is frequency, which measures the number of wavelengths that pass a given point every second. Wavelength and frequency are reciprocal, which means that as one increases, the other decreases.


    An electromagnetic wave consists of oscillating electric and magnetic fields.

    The Electromagnetic Spectrum

    Visible light — the light that human eyes can see — comes in a variety of colors. The color of visible light is determined by its wavelength. Visible light ranges from wavelengths of 400 nm to 700 nm, corresponding to the colors violet through red. EM radiation with wavelengths shorter than 400 nm or longer than 700 nm exists all around you — you just can’t see it. The full range of electromagnetic radiation, or theelectromagnetic spectrum, is shown in Figure below.


    (a) Visible light is part of the electromagnetic spectrum, which ranges from gamma rays with very short wavelengths, to radio waves with very long wavelengths. (b) These are images of the same scene. In the top, only the wavelengths of visible light show. In the bottom, a layer of thick clouds appears in the infrared wavelengths.

    Like our Sun, every star emits light at a wide range of wavelengths, all across the visible spectrum and even outside the visible spectrum. Astronomers can learn a lot from studying the details of the spectrum of light from a star.

    Some very hot stars emit light primarily at ultraviolet (UV) wavelengths, while some very cool stars emit mostly in the infrared. There are extremely hot objects that emit X-rays and even gamma rays. Light from some of the faintest, most distant objects is in the form of radio waves. In fact, a lot of the objects most interesting to astronomers today can’t even be seen with the naked eye. Astronomers use telescopes to detect the faint light from distant objects and to see objects at wavelengths all across the electromagnetic spectrum.

    To learn more about star’s spectra, check out

    Types of Telescopes

    Optical Telescopes

    People have been making and using lenses for magnification for thousands of years. However, the first true telescopes were made in Europe in the late 16th century. These telescopes used a combination of two lenses to make distant objects appear both nearer and larger. The term telescopewas coined by the Italian scientist and mathematician Galileo Galilei (1564–1642). Galileo built his first telescope in 1608 and subsequently made many improvements to telescope design.

    Telescopes that rely on the refraction, or bending, of light by lenses are called refracting telescopes, or simply refractors. The earliest telescopes, including Galileo’s, were all refractors. Many of the small telescopes used by amateur astronomers today are refractors. Refractors are particularly good for viewing details within our solar system, such as the surface of Earth’s moon or the rings around Saturn (Figure below).


    The largest refracting telescope in the world is at the University of Chicago’s Yerkes Observatory in Wisconsin and was built in 1897. Its largest lens has a diameter of 102 cm.

    Around 1670, another famous scientist and mathematician — Sir Isaac Newton (1643–1727) — built a different kind of telescope. Newton used curved mirrors to focus light and so created the first reflecting telescopes, or reflectors (Figure below). The mirrors in a reflecting telescope are much lighter than the heavy glass lenses in a refractor. This is significant, because:

    • To support the thick glass lenses a refractor must be strong and heavy.
    • Mirrors are easier to make precisely than it is to make glass lenses.
    • Because they do not need to be as heavy to support the same size lens, reflectors can be made larger than refractors.

    Larger telescopes can collect more light and so they can study dimmer or more distant objects. The largest optical telescopes in the world today are reflectors.


    (a) Reflecting telescopes used by amateur astronomers today are similar to the one designed by Isaac Newton in the 17th century. (b) The South African Large Telescope (SALT) is one of the largest reflecting telescopes on Earth. SALT’s primary mirror consists of 91 smaller hexagonal mirrors, each with sides 1 m long. (c) Many amateur astronomers today use catadioptric telescopes.

    Catadioptric telescopes have a combination of mirrors and lenses to focus light. Catadioptric telescopes have large mirrors to collect a lot of light, but short tubes for portability.

    KQED: Amateur Astronomers

    Amateur astronomers enjoy observing and studying stars and other celestial objects. Both professional and amateur astronomers use telescopes. A telescope is an instrument that makes faraway objects look closer. Learn more at:

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    Radio Telescopes

    Notice it says above that the largest optical telescopes in the world are reflectors. Optical telescopes collect visible light. Even larger telescopes are built to collect light at longer wavelengths — radio waves. What do you think these telescopes are called? Radio telescopes look a lot like satellite dishes because both are designed to do the same thing — to collect and focus radio waves or microwaves (the shortest wavelength waves) from space.

    The largest single telescope in the world is at the Arecibo Observatory in Puerto Rico (Figure below). This telescope is located in a naturally occurring sinkhole that formed when water flowing underground dissolved the limestone rock. If this telescope were not supported by the ground, it would collapse under its own weight. Since the telescope is set into the ground it cannot be aimed to different parts of the sky and so can only observe the part of the sky that happens to be overhead at a given time.


    The radio telescope at the Arecibo Observatory has a diameter of 305 m.

    A group of radio telescopes can be linked together with a computer so that they are all observing the same object (Figure below). The computer combines the data, making the group function like one single telescope.

    For more on radio telescopes and radio astronomy in general, go to


    The Very Large Array in New Mexico has 27 radio dishes, each 25 m in diameter. When all the dishes are pointed at the same object, they are like a single telescope with a diameter of 22.3 mi.

    KQED: SETI: The New Search for ET

    Scientists have upped their search for extraterrestrial intelligence with the Allen Telescope Array, a string of 350 radio telescopes, located 300 miles north of San Francisco. Find out why SETI scientists now say we might be hearing from ET sooner than you think. Learn more at:

    An interactive or media element has been excluded from this version of the text. You can view it online here:

    KQED: Interview with Astronomer Jill Tartar

    SETI listens for signs of other civilization’s technology. Dr. Jill Tartar explains the program: What it’s looking for; what the problems are; what the potential benefits are. Learn more at:

    An interactive or media element has been excluded from this version of the text. You can view it online here:

    Space Telescopes

    Telescopes on Earth all have one significant limitation: the electromagnetic radiation they gather must pass through Earth’s atmosphere. The atmosphere blocks some radiation in the infrared part of the spectrum and almost all radiation in the ultraviolet and higher frequency ranges. Furthermore, motion in the atmosphere distorts light. That distortion is why stars twinkle in the night sky. To minimize these problems, many observatories are built on high mountains, where there is less atmosphere above the telescope. Even better, space telescopes avoid such problems completely because they orbit outside Earth’s atmosphere in space. Space telescopes can carry instruments to observe objects emitting various types of electromagnetic radiation such as visible, infrared or ultraviolet light; gamma rays; or x-rays. X-ray telescopes, such as the Chandra X-ray Observatory, use X-ray optics to observe remote objects in the X-ray spectrum.

    The Hubble Space Telescope (HST), shown in (Figure below), is perhaps the best known space telescope. The Hubble was put into orbit by the Space Shuttle Atlantis in 1990. Once it was in orbit, scientists discovered that there was a flaw in the shape of the mirror. A servicing mission to the Hubble by the Space Shuttle Endeavor in 1994 corrected the problem. Since that time, the Hubble has provided huge amounts of data that have helped to answer many of the biggest questions in astronomy.

    Find out more by visiting the Hubble Space Telescope website at


    (a) The Hubble Space Telescope orbits Earth at an altitude of 589 km (366 mi). It collects data in visible, infrared, and ultraviolet wavelengths. (b) This starburst cluster is one of the many fantastic images taken by the HST over the past two decades.

    The National Aeronautics and Space Administration (NASA) has placed three other major space telescopes in orbit, comprising what NASA calls the ‘Great Observatories’. Each of these telescopes specializes in a different part of the electromagnetic spectrum (Figure below). NASA is planning for another telescope, the James Webb Space Telescope, to serve as a replacement for the aging Hubble. The James Webb is scheduled to launch in 2018.

    To learn more about NASA’s great observatories, check out


    NASA’s four space-based Great Observatories were designed to view the universe in different ranges of the electromagnetic spectrum. A. Hubble Space Telescope: visible, infrared and ultraviolet light; B. Compton Gamma Ray Observatory (inactive): gamma ray; C. Spitzer Space Telescope: infrared; D. Chandra X-ray Observatory: X-ray.

    Observations with Telescopes

    Ancient Astronomers

    Humans have been studying the night sky for thousands of years. Observing the patterns and motions in the sky helped ancient peoples keep track of time (Figure below). By understanding annual rhythms, people could know when to plant crops. They also timed many of their religious ceremonies to coincide with events in the heavens.


    Many archaeologists think that Stonehenge was used to observe the movement of the moon and the sun.

    The ancient Greeks made careful observations of the locations of stars in the sky. They noticed that some of the ‘stars’ moved against the background of other stars. They called these bright and odd bodies in the sky planets, which in Greek means “wanderers.” Today we know that the planets are not stars, but members of our solar system that orbit the Sun. Greeks also identified constellations, patterns of stars in the sky (Figure below). They associated the constellations with stories and myths from their culture.


    Stars in the constellation Orion. Constellations help astronomers today identify different regions of the night sky.

    Galileo’s Observations

    Ancient astronomers knew a lot about the patterns of stars and the movement of objects in the sky, but they did not know what these objects actually were. That understanding began in the year 1610, when Galileo turned a telescope toward the heavens. With his telescope, Galileo made the following discoveries (among others):

    • There are more stars in the night sky than the naked eye can see.
    • The band of stars called the Milky Way consists of many stars.
    • The Moon has craters (Figure below).
    • Venus has phases like the Moon.
    • Jupiter has orbiting moons.
    • There are dark spots that move across the surface of the Sun.


    Galileo was the first person known to look at the Moon through a telescope. Galileo made the drawing on the left in 1610; on the right is a modern photograph of the Moon.

    Galileo’s observations challenged people to think in new ways about the universe and Earth’s place in it. About 100 years before Galileo, Nicolaus Copernicus had proposed a controversial new model of the universe in which Earth and the other planets revolve around the Sun. In Galileo’s time, most people still believed that the Sun and planets revolved around Earth. Galileo’s observations provided direct evidence to support Copernicus’ model.

    Observations with Modern Telescopes

    Equipped with no more than a good pair of binoculars, you can see all of the things Galileo saw, and more. You can even see sunspots, but be sure to use special filters on the lenses to protect your eyes. With a basic telescope like those used by many amateur astronomers, you can see more than Galileo saw, such as polar caps on Mars, the rings of Saturn, and bands in the atmosphere of Jupiter.

    All of the objects mentioned above are within our solar system. With a telescope you can also see many times more stars than without a telescope. However, because they are so far away, the stars will appear as points of light. This is true even of the most powerful professional telescopes, with one rare exception (Figure below).


    This ultraviolet image of the red supergiant star Betelgeuse taken with the Hubble Space Telescope in 1996 was the first direct image taken of the disk of a star other than the Sun.

    Very few professional astronomers today look directly through the eyepiece of a telescope. Instead, they attach to the telescopes sophisticated instruments that capture and process the light. The astronomers then look at the images or data shown on these instruments. Most often the instruments then pass the data on to a computer where the data can be stored for later use. An astronomer may take weeks or months to analyze all the data collected from just a single night.

    Astronomers use spectrometers to study the light from a telescope. A spectrometer uses a prism or other device to break light down into its component colors. The spectrum produced can be observed directly, captured on film, or stored digitally on a computer (Figure below).


    A simplified example of starlight after it passes through a spectrometer. The dark lines in a star’s spectrum are caused by gases absorbing light in the star’s outer atmosphere.

    From a single spectrum of a star, an astronomer can tell:

    • How hot the star is (by the relative brightness of different colors).
    • What elements the star contains (by the pattern of dark lines).
    • Whether and how fast the star is moving toward or away from Earth (by how far the dark lines are shifted from their normal positions).

    Using telescopes, astronomers can also learn how stars evolve, what kind of matter is found throughout the universe, how that matter is distributed, and even how the universe might have formed.

    Lesson Summary

    • Astronomers study light from distant objects.
    • Light travels at 300,000,000 meters per second — faster than anything else in the universe.
    • A light-year is equal to the distance light travels in one year, 9.5 trillion kilometers.
    • When we see distant objects, we see them as they were in the past, because their light has been traveling to us for many years.
    • Visible light is part of the electromagnetic spectrum.
    • Telescopes make distant objects appear both nearer and larger.
    • Optical telescopes collect visible light. The three main types are reflecting telescopes, refracting telescopes, and catadioptric telescopes.
    • Radio telescopes collect and focus radio waves from distant objects.
    • Space telescopes orbit Earth, collecting wavelengths of light that are normally blocked by the atmosphere.
    • Galileo was the first person known to use a telescope to study the sky. His discoveries helped change the way humans think about the universe.
    • Modern telescopes collect data that can be stored on a computer.
    • Astronomers can learn a lot about a star by studying its spectrum.

    Review Questions

    1. Betelgeuse is around 640 light-years from Earth. Light travels 9.5 trillion kilometers in one year. How far away is Betelgeuse in kilometers?
    2. Identify four regions of the electromagnetic spectrum that astronomers use when observing objects in space.
    3. List the three main types of optical telescopes, and describe their differences.
    4. Explain the advantages of putting a telescope into orbit around Earth.
    5. Describe two observations that Galileo was the first to make with his telescope.
    6. List three things that an astronomer can learn about a star by studying its spectrum.

    Further Reading / Supplemental Links

    Points to Consider

    • Radio waves are used for communicating with spacecraft. A round-trip communication from Earth to Mars takes anywhere from 6 to 42 minutes. What challenges does this present for sending unmanned spacecraft and probes to Mars?
    • The Hubble Space Telescope is a very important source of data for astronomers. The fascinating and beautiful images from the Hubble also help to maintain public support for science. However, the Hubble is growing old. Missions to service and maintain the telescope are extremely expensive and put the lives of astronauts at risk. Do you think there should be another servicing mission to the Hubble?
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