Chapter 5 Telescopes Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

How do humans observe and explore space?

Observing the Universe
How do scientists use the electromagnetic spectrum to study the universe? What types of telescopes and technology are used to explore space?

Observing the Sky Telescopes enable astronomers to observe many more stars than they could with their eyes alone. telescope from Greek tele, means “far”; and Greek skopos, means “seeing”

Upon completing this chapter you should be able to:
Learning Objectives Upon completing this chapter you should be able to: 1. Classify the common types of telescope designs. 2. Compare the advantages and disadvantages of reflectors and refractors. 3. Describe what causes refraction and how lenses focus light. 4. Identify the important aspects for determining a telescope's sensitivity. 5. Compare the light gathering power of different telescopes. 6. Describe the factors affecting telescope resolution, and calculate the diffraction limit for a telescope. 7. Describe the idea of interferometry and how astronomers use it to improve resolution. 8. Describe the methods used for detecting visible light and other wavelengths of electromagnetic radiation. 9. Discuss the problems caused by observing through the Earth's atmosphere, and describe the methods astronomers use to overcome these problems. 10. Identify the wavelength ranges in which telescopes cannot operate from the ground and the reasons for this. 11. Describe the causes and remedies for light pollution.

Ancient peoples often gazed up in wonder at the many points of light in the night sky. But they could see few details with their eyes alone. It was not until the invention of the telescope in 1608 that people could observe objects in the sky more closely. Recall that a telescope is a device that makes distant objects appear to be closer. The telescope revolutionized astronomy. Scientists now had a tool that allowed them to see many objects in space for the first time.

A telescope enables the astronomer to observe things not visible to the naked eye. Although our eyes are superb detectors, they cannot see extremely faint objects or fine details on distant sources. A telescope overcomes these difficulties—first, by collecting more light than the eye can collect “light-gathering power”, and second, by increasing the detail discernible “resolving power.”

Electromagnetic Radiation
To understand how telescopes work, it's useful to understand the nature of electromagnetic radiation. Light is a form of electromagnetic radiation (ih lek troh mag NET ik), or energy ‘ that can travel through space in the form of waves. You can see stars when the light that they produce reaches your eyes.

Forms of Radiation Scientists call the light you can see visible light
Forms of Radiation Scientists call the light you can see visible light. Visible light is just one of many types of electro-magnetic radiation. Many objects give off radiation that you can't see. Objects in space give off all types of electromagnetic radiation.

The electromagnetic spectrum includes the entire range of radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. Telescopes are instruments that collect and focus light and other forms of electromagnetic radiation.

Observing the Sky (cont.)
Astronomers use many kinds of telescopes to study the light energy emitted by stars and other objects in space. Michael Matisse/Getty Images

Electromagnetic Waves
Stars radiate energy into space. This energy travels as electromagnetic waves. The entire range of radiant energy carried by electromagnetic waves is the electromagnetic spectrum.

Most wavelengths of the electromagnetic spectrum are not visible to the human eye.

Electromagnetic Waves (cont.)
The type of radiant energy a star emits depends on the star’s temperature. Some stars are so far away that it takes billions of years for their radiant energy to reach Earth.

Telescopes make distant objects appear larger and brighter.
A telescope that uses lenses or mirrors to collect and focus visible light is called an optical telescope. Modern astronomy is based on the detection of many forms of electromagnetic radiation besides visible light. Non-optical telescopes collect and focus different types of electromagnetic radiation, just as optical telescopes collect visible light.

Telescopes There are many kinds of telescopes, but they all have the same primary purpose--to collect light. By light, we mean any part of the electromagnetic spectrum, not only visible light. Thus, we have radio telescopes, and X-ray telescopes, and even gamma-ray telescopes. As we will find, the shape, size, and internal workings of telescopes depends a great deal on what part of the electromagnetic spectrum it is designed to work for. We begin the discussion with the most familiar type of telescope, the optical telescope.

The two main types of optical telescope:
Reflecting Telescope -- uses a curved mirror to focus the light (like a shaving mirror). Refracting Telescope -- uses a lens to focus the light (like eye-glasses). Refraction means light-bending. The light bends (changes direction) because it slows down on passing through the glass. Red light slows more than blue. Classify the common types of telescope designs

Compare the advantages and disadvantages of reflectors and refractors
Advantages of reflecting telescope over a refracting telescope: A refracting telescope uses a lens, which bends red light more than blue light, so the image has color halos. This is called chromatic aberration. A reflecting telescope reflects all wavelengths of light at the same angle, so there are no color halos. A lens has two surfaces to be figured, which is more difficult to control its shape. A mirror has only one surface to be figured, so it is easier to control the shape. A lens passes light through, so the glass has to be very transparent and pure, and some wavelengths (infrared and ultraviolet) are absorbed by glass. A mirror reflects the light, so the material that it is made from does not have to be transparent, and infrared and ultraviolet light reflects equally well. A lens can only be mounted by its edges, so a large lens can sag under its own weight. A mirror can be supported from the back, so it is less subject to sagging. Also, a mirror can be hollow, to reduce weight. Compare the advantages and disadvantages of reflectors and refractors

Earth-Based Telescopes
Optical telescopes gather visible light. Refracting telescopes and reflecting telescopes are the two types of optical telescopes.

A telescope that uses a curved mirror to concentrate light from a distant object is a reflecting telescope.

A telescope that uses a convex lens to concentrate light from a distant object is a refracting telescope.

Tools of the Trade: Telescopes
Stars and other celestial objects are too far away to test directly Astronomers passively collect radiation emitted from distant objects Extremely faint objects make collection of radiation difficult Specialized Instruments Required Need to measure brightness, spectra, and positions with high precision Astronomers use mirrored telescopes and observatories Modern Astronomers are rarely at the eyepiece, more often they are at a computer terminal!

The Powers of a Telescope
Collecting Power Bigger telescope, more light collected! Focusing Power Use mirrors or lenses to bend the path of light rays to create images Resolving Power Picking out the details in an image To double the resolving power of a telescope, you must increase the diameter by a factor of two

Telescopes described by lens or mirror diameter (inches)
The two most important properties of any telescope are the light gathering power and the resolving power. Light collected proportional to “collector” area Pupil for the eye Mirror or lens for a telescope Telescope “funnels” light to our eyes for a brighter image Small changes in “collector” radius give large change in number of photons caught How bright that object appears to us depends on the number of its photons that enter our eye per second, a number limited by the size of our eyes. Astronomers overcome that limit by “collecting” photons with a telescope, which then “funnels” the photons to our eyes. The bigger the telescope's collecting area, be it a lens or mirror, the more photons it collects. Light Gathering Power Telescopes described by lens or mirror diameter (inches)

The lens of a refractor focuses the light by bending the rays
Refraction Light moving at an angle from one material to another will bend due to a process called refraction Refraction occurs because the speed of light is different in different materials The lens of a refractor focuses the light by bending the rays Once light has been gathered, it must be focused to form an image or to concentrate it on a detector. Telescopes in which light is gathered and focused by a lens are called “refracting telescopes,” or refractors for short. A larger-diameter mirror or lens gives a telescope a greater light-gathering power. The result is a brighter image, which allows us to see dim objects that are invisible in telescopes with smaller gathering areas

Compare the light gathering power of different telescopes
The light gathering power of telescopes depends on how big the collecting area is on the telescope. So the telescope with the greater collecting area has a greater light gathering power.

Describe what causes refraction and how lenses focus light
Due to the refraction caused by atmospheric irregularities stars twinkle. Describe what causes refraction and how lenses focus light Refraction The Sun looks flattened near the horizon because the larger refraction near the horizon lifts the lower edge of the Sun more than the upper edge and makes the Sun look flattened.

Disadvantages to Refractors
Lenses have many disadvantages in large telescopes! Large lenses are extremely expensive to fabricate A large lens will sag in the center since it can only be supported on the edges Dispersion causes images to have colored fringes Many lens materials absorb short-wavelength light

Reflecting Telescopes
Reflectors Used almost exclusively by astronomers today Twin Keck telescopes, located on the 14,000 foot volcanic peak Mauna Kea in Hawaii, have 10-meter collector mirrors! Light is focused in front of the mirror

Reflecting Telescopes
A secondary mirror may be used to deflect the light to the side or through a hole in the primary mirror Multi-mirror instruments and extremely thin mirrors are two modern approaches to dealing with large pieces of glass in a telescope system

Styles of Refractors

Infrared telescope Infrared telescope, instrument designed to detect and resolve infrared radiation from sources outside Earth’s atmosphere such as nebulae, young stars, and gas and dust in other galaxies. Infrared telescopes do not differ significantly from reflecting telescopes designed to observe in the visible region of the electromagnetic spectrum. The main difference between the two types is in the physical location of the infrared telescope…

Waves are diffracted as they pass through narrow openings
The resolving power of a telescope is affected by the property of light called diffraction. Resolving Power A telescope’s ability to discern detail is referred to as its resolving power Resolving power is limited by the wave nature of light through a phenomenon called diffraction Waves are diffracted as they pass through narrow openings A diffracted point source of light appears as a point surrounded by rings of light If you mark two black dots close together on a piece of paper and look at them from the other side of the room, your eye may see them as a single dark mark, not as separate spots. Likewise, stars that lie close together or markings on planets may not be distinguishable. A telescope's ability to discern such detail depends on its resolving power.

Identify the important aspects for determining a telescopes sensitivity
The greater the light gathering power Quality of mirror or lens Build quality A telescope of larger primary mirror or lens is more sensitive than one with a smaller one. The light-gathering power of a telescope depends on its collecting area, which is proportional to the square of the mirror diameter. To study the faintest sources of radiation, astronomers must use large telescopes. Another important aspect of a telescope is its angular resolution, the ability to distinguish between light sources lying close together on the sky.

Resolving Power and Aperture
Diffraction seriously limits the detail visible through a telescope. In fact, diffraction theory shows that if two points of light that are separated by an angle α (measured in arc seconds*) are observed at a wavelength λ through a telescope whose diameter is D, the points cannot be seen as separate sources unless D is greater than (>) Two points of light separated by an angle a (in arcsec) can be seen at a wavelength l (in nm) only if the telescope diameter D (in cm) satisfies: D > 0.02 l/a

The Atmosphere, weather, light pollution
Describe the factors affecting telescope resolution, and calculate the diffraction limit for a telescope The Atmosphere, weather, light pollution D>2.5x10^5λ/a is how to calculate the diffraction limit

Describe the idea of interferometry and how astronomers use it to improve resolution
Interferometry is when multiple telescopes are used to interfere the wave with each other, and when that crests of two waves arrive together, they create a bright region. Where the crest of one wave arrives simultaneously with the trough of another, they cancel and create a dark patch. The result is a complex pattern of altering light and dark regions, which can be analyzed bay a computer to create an image of the object observed.

Increasing Resolving Power: Interferometers
For a given wavelength, resolution is increased for a larger telescope diameter An interferometer accomplishes this by simultaneously combining observations from two or more widely-spaced telescopes Diffraction effects can never be totally eliminated, but they can be reduced by enlarging the opening through which the light passes, so that its waves do not mix as severely. Astronomers sometimes accomplish this with a device called an interferometer. Increasing Resolving Power: Interferometers Using interferometry, scientists can use a few smaller telescopes to take images with the same resolution as a much larger telescope.

Interferometers The resolution is determined by the individual telescope separations and not the individual diameters of the telescopes themselves Key to the process is the wave nature of interference and the electronic processing of the waves from the various telescopes The interferometer is so-named because when it mixes the separate beams, the light waves of one “interfere” with the waves from the other. Where the crests of two waves arrive together, they create a bright region. Where the crest of one wave arrives simultaneously with the trough of another, they cancel and create a dark patch. The result is a complex pattern of alternating light and dark regions, which can be analyzed by a computer to create an image of the object observed.

Once light has been collected, it must be detected and recorded
Once light has been collected, it must be detected and recorded. In olden days, the detector was the eye of an astronomer who sat at the telescope eyepiece and wrote down data or made sketches of the object being observed. The human eye, marvel that it is, has difficulty seeing very faint light. Many astronomical bodies are too distant or too dim for their few photons to create a sensible effect on the eye.

From the late 1800s until the 1980s, astronomers generally used photographic film to record the light from the bodies they were studying. Film forms an image by absorbing photons that cause a chemical change, making the film dark where light has fallen on it. This process, however, is very inefficient: fewer than 4% of the photons striking the film produce a useful image. The result of such low efficiency is that many hours are needed to accumulate enough light to create an image of faint objects. Moreover, the film must be developed, thereby delaying the observing process even further.

Astronomers today use many kinds of electronic detectors.
One of the main types is the CCD (charge-coupled device). Modern CCDs can make pictures with a sensitivity to faint light approximately 20 times greater than photographic film.

The Moon appears bigger near the horizon due to an optical illusion.
Detecting the Light The Human Eye Once used with a telescope to record observations or make sketches Not good at detecting faint light, even with the 10-meter Keck telescopes Photographic Film Chemically stores data to increase sensitivity to dim light Very inefficient: Only 4% of striking photons recorded on film Electronic Detectors Incoming photons strike an array of semiconductor pixels that are coupled to a computer Efficiencies of 75% possible CCD (Charged-coupled Device) for pictures

Nonvisible Wavelengths
Many astronomical objects radiate in wavelengths other than visible Cold gas clouds radiate in the radio Dust clouds radiate in the infrared Hot gases around black holes emit x-rays Visible light, which we can see because its wavelengths are detectable by our eyes, is just one of many wave bands of electromagnetic radiation, as discussed in chapter 4. Many astronomical objects, however, radiate at wavelengths that our eyes cannot see, and so astronomers have devised ways to observe such objects.

Earth-Based Telescopes (cont.)
Radio telescopes collect invisible radio waves and some microwaves. They look like TV satellite dishes. Steve Allen/Brand X Pictures

Describe the methods used for detecting visible light and other wavelengths of electromagnetic radiation CCD, or charge-coupled device and its 20 times more sensitive than photographic film Radio telescopes that pick up on the radio energy emitting form cold clouds of gas in interstellar space

Telescopes in space collect energy of all wavelengths, including those absorbed by Earth’s atmosphere.

Radio Observations (A) A false-color picture of a radio galaxy. We can't see radio waves, so colors are used to represent their brightness—red brightest, blue dimmest. (B) A false-color X-ray picture of Cas A, an exploding star. In this case colors represent different wavelengths of X-ray photons (bluer colors corresponding to more energetic photons) False color images are typically used to depict wavelength distributions in non-visible observations In a false color image, colors can represent photon energies or the intensity of electromagnetic radiation. A galaxy with almost no starlight but plenty of cool clouds of hydrogen gas would be best observed with a radio telescope.

Radio telescopes vary widely, but they all have two basic components:
a large radio antenna and (2) a sensitive radiometer, or radio receiver. The sensitivity of a radio telescope—the ability to measure weak sources of radio emission—depends both on the area and efficiency of the antenna and on the sensitivity of the radio receiver used to amplify and to detect the signals. For broadband continuum emission over a range of wavelengths, the sensitivity also depends on the bandwidth of the receiver.

Astronomers have turned to a method called adaptive optics.
Discuss the problems caused by observing through the earths atmosphere, and describe the methods astronomers use to overcome these problems When astronomers try to look into space from the ground, they have to look through the atmosphere, which can distort and blur images through the telescope. Astronomers have turned to a method called adaptive optics. Sophisticated, deformable mirrors controlled by computers can correct in real-time for the distortion caused by the turbulence of the Earth's atmosphere, making the images obtained almost as sharp as those taken in space. Adaptive optics allows the corrected optical system to observe finer details of much fainter astronomical objects than is otherwise possible from the ground.

Identify the wavelength ranges in which telescopes cannot operate from the ground and the reasons for this The only wavelengths that can get through our atmosphere that are useful to astronomers are visible light, and radio waves. Ultraviolet rays can get through but most gets absorbed by water vapor and carbon dioxide. The other wavelengths are filtered out by our atmosphere because they are harmful to earth, so we launch telescopes into space so they can receive those wavelengths.

The 305-metre (1,000-foot) radio telescope at the Arecibo Observatory, Puerto Rico.

Arecibo Observatory, Puerto Rico.

Designing a telescope for observing X rays presents different challenges.
X rays entering a normal telescope would strike the mirror surface and be absorbed, making the telescope no more effective for observing than a slab of concrete. Astronomers have found, however, that X rays can be reflected if they strike a smooth surface at a very shallow angle, somewhat as a rock can skip over the surface of water if thrown nearly horizontally.

An X-ray telescope (XRT) is a telescope that is designed to observe remote objects in the X-ray spectrum. In order to get above the Earth's atmosphere, which is opaque to X-rays, X-ray telescopes must be mounted on high altitude rockets, balloons or artificial satellites.

Major Space Observatories
Why put them in space?

Artificial Satellites
Any small object that orbits a larger object is a satellite. satellite from Latin satellitem, means “attendant” or “bodyguard”

Artificial Satellites (cont.)
Rockets place satellites into orbit around Earth or other objects in space. Satellites send information back to Earth. CORBIS

Artificial Satellites (cont.)
Today, Earth-orbiting satellites are used to transmit television and telephone signals and to monitor weather and climate. An array of satellites called the Global Positioning System (GPS) is used for navigation in cars, boats, airplanes, and even for hiking.

Space vs. Ground-Based Observatories
Space-Based Advantages Freedom from atmospheric blurring Freedom of atmospheric absorption Ground-Based Advantages Larger collecting power Equipment easily fixed Ground-Based Considerations Weather, humidity, and haze Light pollution

The best site for placing a ground-based optical telescope is a mountain top.
Observatories The immense telescopes and their associated equipment require observatories to facilitate their use and protection from the elements Thousands of observatories are scattered throughout the world and are on every continent including Antarctica Some observatories: Twin 10-meter Keck telescopes are largest in U.S. The Hobby-Eberly Telescope uses 91 1-meter mirrors set in an 11-meter disk Largest optical telescope, VLT (Very Large Telescope) in Chile, is an array of four 8-meter mirrors

Computers and Astronomy
For many astronomers, operating a computer and being able to program are more important than knowing how to use a telescope Computers accomplish several tasks: Solve equations Move telescopes and feed information to detectors Convert data into useful form Networks for communication and data exchange

Scintillation Twinkling of stars AKA Scintillation
Refraction is also responsible for seeing Twinkling of stars AKA Scintillation Temperature and density differences in pockets of air shift the image of the star Scintillation

Twinkling of stars in sky, called scintillation, is caused by moving atmospheric irregularities refracting star light into a blend of paths to the eye The condition of the sky for viewing is referred to as the seeing Distorted seeing can be improved by adaptive optics, which employs a powerful laser and correcting mirrors to offset scintillation Atmospheric Blurring The distortion of an image due to an atmospheric turbulence is “seeing”.

Describe the causes and remedies for light pollution
One of the biggest problems for ground based astronomy today is light pollution makes it difficult to observe faint objects. Artificial light pollutes the view of 83% of Earth’s population according to recent data.

Figure 1. The effect of light pollution on the night sky
Figure 1. The effect of light pollution on the night sky. This split image shows how artificial light washes out most of the faint detail in the constellation Orion.

Light Pollution Unwanted light can travel directly into your eyes ruining the dark adaption they need to observe faint celestial objects. It can also invade telescopes causing washed out images and unwanted glare. This form of light pollution involves light traveling directly from an unwanted light source (such as a street lamp) to your eye/telescope. Light pollution is excessive, misdirected or inappropriate outdoor lighting. Too much of light pollution washes out view of the Universe, result in increase in the energy consumption, interferes with astronomical research…

Describe the causes and remedies for light pollution
Urban Sky glow- artificial lights pointing upward, making the sky glow and blocks the view of stars. It can be minimized if cities would dim, or turn lights off at night.

Gamma-ray telescope Gamma-ray telescope, instrument designed to detect and resolve gamma rays from sources outside Earth’s atmosphere. Gamma rays are the shortest waves (about 0.1 angstrom or less) and therefore have the highest energy in the electromagnetic spectrum. Since gamma rays have so much energy, they pass right through the mirror of a standard optical telescope. Instead, gamma rays are detected by the optical flashes they produce when interacting with the material in a specially designed instrument such as a scintillation detector. Earth’s atmosphere blocks most gamma rays, so most gamma-ray telescopes are carried on satellites and balloons. However, some ground-based telescopes can observe the Cherenkov radiation produced when a gamma ray strikes Earth’s upper atmosphere.

The Compton Gamma Ray Observatory as seen through the space shuttle window during deployment in … NASA

X-ray telescope, instrument designed to detect and resolve X-rays from sources outside Earth’s atmosphere. Because of atmospheric absorption, X-ray telescopes must be carried to high altitudes by rockets or balloons or placed in orbit outside the atmosphere. Balloon-borne telescopes can detect the more penetrating (harder) X-rays, whereas those carried aloft by rockets or in satellites are used to detect softer radiation. X-rays are blocked by ozone and oxygen present in the Earth's atmosphere.

Ultraviolet telescope, telescope used to examine the ultraviolet portion of the electromagnetic spectrum, between the portion seen as visible light and the portion occupied by X-rays. Ultraviolet radiation has wavelengths of about 400 nanometres (nm) on the visible-light side and about 10 nm on the X-ray side. Earth’s stratospheric ozone layer blocks all wavelengths shorter than 300 nm from reaching ground-based telescopes. As this ozone layer lies at an altitude of 20–40 km (12–25 miles), astronomers have to resort to rockets and satellites to make observations from above it.

Telescopes Review

Using ___________, scientists can use a few smaller telescopes to take images with the same resolution as a much larger telescope. A. Satellite telescopes B. Charge-coupled devices (CCDs) C. Interferometry D. Adaptive optics

A warm dust cloud in which stars are forming would be best observed with a(n) _________ telescope. A. Radio B. Gamma-Ray C. Infrared D. Optical (visible light)

In a false color image, A. Only one color is used. B
In a false color image, A. Only one color is used. B. Color information from optical telescopes is combined with information from a non-visible wavelength telescope to make the image. C. Stars appear black and the background appears white. D. Colors can represent photon energies or the intensity of electromagnetic radiation.

If your pupils have a diameter of about 5mm, about how many times more light gathering power does a telescope with a diameter of about 20 cm (8 inches) have than your pupils? A. 4 times B. 16 times C. 40 times D. 160 times E times

If your pupils have a diameter of about 5mm, about how many times more light gathering power does a telescope with a diameter of about 20 cm (8 inches) have than your pupils? A. 4 times B. 16 times C. 40 times D. 160 times E. 1600 times

One of the biggest problems for ground based astronomy today is A
One of the biggest problems for ground based astronomy today is A. All the best mountains already have telescopes on them. B. Atmospheric pollution has significantly worsened the seeing. C. Space-based observatories are making most of the observations. D. Light pollution makes it difficult to observe faint objects.

A pencil inside a water glass appears to be bent because of A
. A pencil inside a water glass appears to be bent because of A. Reflection of light. B. Refraction of light. C. Diffraction of light. D. None of the above.

A pencil inside a water glass appears to be bent because of A
A pencil inside a water glass appears to be bent because of A. Reflection of light. B. Refraction of light. C. Diffraction of light. D. None of the above.

Why does the Moon appear bigger near the horizon. A
Why does the Moon appear bigger near the horizon? A. Due to an optical illusion. B. Due to refraction. C. Due to reflection. D. Due to the compression produced by the Earth's atmosphere.

The ability of a telescope to show two very close objects separately is called its ________. A. Light gathering power B. Resolving power C. Magnification D. None of the above

Tom has a 4-inch refracting telescope and Steve has a 3-inch reflecting telescope. Whose telescope has a higher resolving power? A. Tom's, because lenses are more efficient in showing the objects separately. B. Steve's, because mirrors are more efficient in showing the objects separately. C. Tom's, because the larger the diameter, the better the resolution. D. Steve's, because the smaller the diameter, the better the resolution.

The best site for placing an X-ray telescope is _________. A
The best site for placing an X-ray telescope is _________. A. A mountain top B. A valley C. Near an ocean D. A desert E. Above the Earth's atmosphere

An interstellar gas is emitting 10-centimeter wavelength radiation and a nearby star is emitting 10-micrometer wavelength radiation. Which of these can you observe through an Earth-based telescope? A. The interstellar gas B. The star C. Both of them D. Neither of them

Why do stars twinkle? A. Due to rapid changes occurring on their surfaces. B. Due to their movement across the sky. C. Due to the refraction caused by atmospheric irregularities. D. Due to imperfections in the human eye.

Which space telescope is observing X-rays. A
Which space telescope is observing X-rays? A. The Hubble Space Telescope (HST) B. The Chandra Observatory C. The Spitzer Space Telescope D. The Extreme Ultraviolet Explorer (EUVE)

Which of the following telescopes is most suitable for observing cool gas clouds? A. X-ray telescope. B. Radio telescope. C. Visible light telescope. D. All of the above. E. Cool gas clouds cannot be observed by telescopes.

If a 3 meter diameter telescope is doubled in size, then its new light collecting power would A. Not change. B. Double. C. Increase by a factor of four. D. Reduce by half.

______ is the most important quality of an astronomical telescope. A
______ is the most important quality of an astronomical telescope. A. Magnification B. Resolving power C. Ability to see at night D. Rigidity

Which of the following is an inherent disadvantage of radio telescopes
Which of the following is an inherent disadvantage of radio telescopes? A. Radio telescopes cannot detect visible light. B. Radio telescopes have low magnification. C. Radio signals are very weak, and their photons do not penetrate the atmosphere easily. D. The long wavelength of radio waves results in lower resolving power, compared to other telescopes of the same size. E. They only work at night.

Which is the most efficient means of recording light. A
Which is the most efficient means of recording light? A. Photographic films B. Photomultiplier tubes C. Charge-coupled device (CCD) D. The human eye

X-rays are blocked by ________ and _________ present in the Earth's atmosphere. A. Water molecules; carbon dioxide B. Ozone; oxygen C. Nitrogen; helium D. Electric charges; clouds

The best site for placing a ground-based optical telescope is _________. A. A mountain top B. A valley C. In an urban setting D. in a location that has few sunny days throughout the year.

What is "seeing"? A. The ability of a telescope to show two very close objects separately. B. The capacity of a telescope to gather more light. C. The ability of a telescope to see in the night. D. The distortion of an image due to an atmospheric turbulence.

Carl has a 5-inch refracting telescope and Jim has a 3-inch reflecting telescope. Whose telescope has a higher light-gathering power? A. Carl's, because lenses gather more light. B. Jim's, because mirrors gather more light. C. Carl's, because the larger the diameter, the more light to be collected. D. Jim's, because the smaller the diameter, the more light to be collected.

What is a disadvantage of using a single, large lens in a telescope. A
What is a disadvantage of using a single, large lens in a telescope? A. Large lenses are expensive to fabricate. B. A lens has to be supported only at its edges, so the lens can sag in the middle. C. Different colors of white light on passing through a lens focus at different points and result in a blurred image. D. Some lens materials completely absorb short wavelengths. E. All of the above.

Why does the Sun look flattened near the horizon. A
Why does the Sun look flattened near the horizon? A. The Sun's light is reflected off the horizon, making the Sun appear compressed. B. The Earth's dense atmosphere compresses the gaseous Sun. C. The larger refraction near the horizon lifts the lower edge of the Sun more than the upper edge and makes the Sun look flattened. D. The Sun is cooler on the horizon, so it looks flattened.

The two most important properties of any telescope are A
The two most important properties of any telescope are A. The light gathering power and the length of the tube. B. The length of the tube and the magnification. C. The magnification and the light gathering power. D. The resolving power and the magnification. E. The light gathering power and the resolving power.

The resolving power of a telescope is affected by the property of light called A. Refraction. B. Diffraction. C. Reflection. D. Seeing. E. Attenuation.

Ignoring the effects of the atmosphere, what is the theoretical resolution of an 8 inch telescope (about 20 cm) if you are looking at visible light with a wavelength of about 500 nm? A. About half an arcsecond. B. About half a degree. C. About 1/100 of an arcsecond. D. About 25 arcseconds.

A galaxy with almost no starlight but plenty of cool clouds of hydrogen gas would be best observed with a(n) _________ telescope. A. X-ray B. Radio C. Infrared D. Optical (visible light)

To double the resolving power of a telescope, you must _________. A
To double the resolving power of a telescope, you must _________. A. Increase the diameter by a factor of two B. Increase the collecting area by a factor of two C. Decrease the diameter by half D. Decrease the collecting area by half

T or F An optical telescope is able to resolve blue objects better than red objects.

T or F Gamma ray telescopes, such as the Fermi Gamma Ray Space Telescope, are launched into space because they are dangerous to operate on Earth.

T or F To double the light gathering power of a telescope, we need to double the diameter.

T or F Most of the modern large optical telescopes are refractors.

T or F In a reflecting telescope, the secondary mirror causes a hole in the center of the image.

How do astronomers calculate a telescope's light collecting area (A)
How do astronomers calculate a telescope's light collecting area (A)? (where r = radius of the telescopes mirror or lens, and B is "pi" or ) A) A = 2Br B) A = Br2 C) A = 4Br2 D) A = 4/3 Br3

Telescope A has a mirror twice the diameter of telescope B's mirror
Telescope A has a mirror twice the diameter of telescope B's mirror. How does A's light-gathering power compare to B's? A) A gathers 1/2 the light that B does. B) A gathers twice the light that B does. C) A gathers 4 times the light that B does. D) A gathers 1/4th as much light as B does.

A telescope's resolving power measures its ability to see A) fainter sources. B) more distant sources. C) finer details in sources. D) larger sources.

One way to increase the resolving power of a telescope is to A) make its mirror bigger. B) make its mirror smaller. C) replace its mirror with a lens of the same diameter. D) observe objects using longer wavelengths.

What causes the image of a star to "twinkle" or scintillate
What causes the image of a star to "twinkle" or scintillate? A) irregularities in the density of layers of Earth's atmosphere, rapidly refracting the light. B) the flickering fusion process in the stars. C) a purely physiological reaction in the eye. D) none of these answers are correct.

What process limits a telescope's resolving power
What process limits a telescope's resolving power? A) Reflection B) Refraction C) Diffraction D) Distraction

What is the difference between a reflecting and a refracting telescope
What is the difference between a reflecting and a refracting telescope? A) A reflecting telescope uses a lens to focus light; a refracting telescope uses a mirror. B) A reflecting telescope uses a mirror to focus light; a refracting telescope uses a lens. C) Both use a mirror to focus light, but a reflecting telescope uses mirrors elsewhere to bend light. D) There is no difference. "Refracting" is an outmoded way of saying "reflecting."

Light travels __ in water than in air A) faster B) slower C) at the same speed D) perpendicular to

Which of the these is a reason for using mirrors rather than lenses in telescopes? A) Lenses are more expensive to make than mirrors. B) Most transparent materials focus light of different colors to different spots. C) Lenses can only be supported by their edges, making them sag in the middle (mirrors can be supported from behind). D) All of the above.

Why use an interferometer
Why use an interferometer? A) Its two widely-spaced mirrors act like one giant telescope with increased collecting area. B) Its two widely-spaced mirrors act like one giant telescope with increased resolving power. C) By putting one mirror above the other you can make the instrument much smaller. D) It can detect light at wavelengths not available to single telescopes.

Which of the following astronomical objects emit radiation mostly at non-visible wavelengths? A) dust clouds in space B) hot gas surrounding black holes C) cold interstellar gas clouds D) all of the above

Do astronomers use ground-based X-ray telescopes
Do astronomers use ground-based X-ray telescopes? A) Yes, because they can penetrate gas clouds. B) No, because no astronomical objects emit x- rays. C) No, because x-rays cannot get through the Earth's atmosphere. D) No, because astronomers have not yet devised detectors for x-rays.

Why does the useful resolving power of a ground-based telescope not match it's theoretical value? A) Mirrors can't be built accurately enough. B) The atmosphere blurs the image, decreasing the resolving power. C) Mirrors cannot collect enough light to reach their theoretical expectations. D) The theoretical value can only be reached when there is a Full Moon.

On some telescopes, actuators on the mirror change its shape to match distortions in the atmosphere. What is this technique called? A) Actuary observing B) Interferometry. C) Refraction D) Adaptive Optics

Which of the following is a reason to build an observatory in space
Which of the following is a reason to build an observatory in space? A) They are much less expensive than ground-based observatories. B) They can last forever. C) To avoid atmospheric blurring. D) There is no good reason to build a space-based observatory.

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Chapter 5 Telescopes Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Presentation on theme: "Chapter 5 Telescopes Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display."— Presentation transcript:

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