Key Terms for Review

active optics

adaptive optics

angular resolution (resolution)

Cassegrain focus

charge-coupled device (CCD)

chromatic aberration

coudé focus

electromagnetic radiation

electromagnetic spectrum

eyepiece lens

focal length

focal plane

focal point

frequency

gamma ray

infrared radiation

interferometry

light-gathering power

magnification

Newtonian reflector

objective lens

photon

pixel

primary mirror

prime focus

radio telescope

radio wave

reflecting telescope (reflector)

reflection

refracting telescope

refraction

refractor

Schmidt corrector plate

secondary mirror

seeing disk

spectrum (plural spectra)

spherical aberration

twinkling

ultraviolet (UV) radiation

very-long-baseline interferometry (VLBI)

wavelength (λ)

X-ray

Review Questions

The answers to all computational problems, which are preceded by an asterisk (*), appear at the end of the book.

Question 3.1

Describe reflection and refraction. How do these processes enable astronomers to build telescopes?

Question 3.2

Give everyday examples of refraction and reflection.

Question 3.3

Which side of the secondary mirror in Figure 3-9 is coated with aluminum? Justify your answer.

Question 3.4

*How much more light does a 3-m-diameter telescope collect than a 1-m-diameter telescope?

Question 3.5

Explain some of the advantages of reflecting telescopes over refracting telescopes.

Question 3.6

What are the three major functions of a telescope?

Question 3.7

What is meant by the angular resolution of a telescope?

Question 3.8

What limits the ability of the 5-m telescope at Palomar Observatory to collect starlight? There are several correct answers to this question.

Question 3.9

Why will many of the very large telescopes of the future make use of multiple mirrors?

Question 3.10

What is meant by adaptive optics? What problem does adaptive optics overcome?

Question 3.11

Compare an optical reflecting telescope to a radio telescope. What do they have in common? How are they different?

Question 3.12

Why can radio astronomers observe at any time of the day or night, whereas optical astronomers are mostly limited to observing at night?

Question 3.13

Why must astronomers use satellites and Earth-orbiting observatories to study the heavens at X-ray wavelengths?

Question 3.14

What are NASA’s four Great Observatories, and in what parts of the electromagnetic spectrum do (or did) they observe?

Question 3.15

Why did Rømer’s observations of the eclipses of Jupiter’s moons support the heliocentric, but not the geocentric, cosmology?

Advanced Questions

Question 3.16

Advertisements for home telescopes frequently give a magnification for the instrument. Is this a good criterion for evaluating such telescopes? Explain your answer.

Question 3.17

*The observing cage in which an astronomer sits at the prime focus of the 5-m telescope at Palomar Observatory is about 1 m in diameter. What fraction of the incoming starlight is blocked by the cage? Hint: The area of a circle of diameter d is πd2/4, where π ≈ 3.14.

Question 3.18

*Compare the light-gathering power of the Palomar Observatory’s 5-m telescope to that of the fully dark-adapted human eye, which has a pupil diameter of about 5 mm.

Question 3.19

Show by means of a diagram why the image formed by a simple refracting telescope is “upside down.”

Question 3.20

*Suppose your Newtonian reflector has a mirror with a diameter of 20 cm and a focal length of 2 m. What magnification do you get with an eyepiece whose focal length is

  • a. 9 mm,
  • b. 20 mm, and
  • c. 55 mm?

Question 3.21

Why does no major observatory have a Newtonian reflector as its primary instrument, whereas Newtonian reflectors are popular among amateur astronomers?

Question 3.22

From the ground, how can astronomers detect gamma-ray sources in space?

Question 3.23

Why will many of the very large telescopes of the future have ultrathin primary mirrors?

Discussion Questions

Question 3.24

Discuss the advantages and disadvantages of using a relatively small visible-light telescope in Earth’s orbit (for example, the 2.4-m Hubble Space Telescope) versus a large visible-light telescope on a mountaintop (for example, the 8.3-m Subaru telescope on Mauna Kea, Hawaii).

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Question 3.25

If you were in charge of selecting a site for a new observatory, what factors would you consider?

Question 3.26

Consider two identical Cassegrain telescope mirrors. One is set up as a prime focus telescope, whereas the other is used in a Cassegrain telescope.

  • a. Sketch both telescopes.
  • b. What are the differences between the two that make each useful in different observing situations?

What If…

Question 3.27

Telescopes were first invented today? What objects or areas of the sky would you recommend that astronomers explore first? Why?

Question 3.28

We had eyes sensitive to radio waves? How would our bodies be different, and how would our visual perceptions of the world be different?

Question 3.29

Humans were unable to detect any electromagnetic radiation? How would that change our lives, and what alternatives might evolve (some species indeed have done this) to provide information about distant objects?

Web Questions

Question 3.30

Several telescope manufacturers build Schmidt-Cassegrain telescopes. These devices use a correcting lens in an arrangement like that shown in Figure 3-23c. Consult advertisements on the Web and list the dimensions, weights, and costs of some of these telescopes. Why are they popular among amateur astronomers?

Question 3.31

Discuss the advantages and disadvantages of setting up an observatory on the Moon. Hint: To get a broad perspective on this question, you might find it useful to explore the Web for the challenges of living on the Moon.

Question 3.32

The Large Zenith Telescope (LZT) in British Columbia, Canada, uses a 6-m liquid mirror made of mercury. Use the Web to investigate this technology. How can a liquid metal be formed into the necessary shape of a telescope mirror? What are the advantages and disadvantages of a liquid mirror?

Got It?

Question 3.33

Why do stars twinkle?

Question 3.34

Why do all research telescopes use primary mirrors rather than objective lenses?

Question 3.35

For the purpose of observing very faint objects, which of the following features of a telescope is most important? Explain your answer.

  • a. its maximum magnification
  • b. its ability to resolve colors
  • c. the size of its objective lens or primary mirror
  • d. the type of mount it has (if necessary, please see Appendix H for information on mounts)
  • e. its weight

Question 3.36

Of the following types of electromagnetic radiation, which is most dangerous to life?

  • a. radio waves
  • b. X-rays
  • c. ultraviolet radiation
  • d. infrared radiation
  • e. visible light

Observing Projects

Question 3.37

During the daytime, obtain a telescope and several eyepieces of differing focal lengths. If you can determine the focal length of the telescope’s objective lens or mirror (often printed on it), calculate and record the magnifying power for each eyepiece. Focus the telescope on some familiar object, such as a distant lamppost or tree. DO NOT FOCUS ON THE SUN! Looking directly at the Sun through a telescope will cause blindness. Describe the image you see through the telescope. Is it upside down? How does the image move as you slowly and gently shift the telescope left and right or up and down? Examine the distant objects under different magnifications. How does the field of view change as you go from low magnification to high magnification?

Question 3.38

You can use Starry Night™ to help you to determine the brightness of the faintest stars that are likely to be visible to the naked eye from your location under your present sky conditions and to estimate the fraction of possible stars that you can see with the unaided eye. This exercise is best done outdoors with a laptop computer, or through a window from a very dark room, on a dark, clear night. Click on Home to restore the view to your present location and time, if you have not already done so. You can now adjust the amount of light pollution on the view to match your present sky, where only stars above a limiting brightness are discerible. On the Menu, select View > Hide Daylight. Open the Options side pane and expand the Local View layer. Place the cursor over the words Local Light Pollution and click the Local Light Pollution Options… button that appears. This will open the Local View Options dialog window. Move this dialog window to the side of the view. Click the checkbox to the left of the Local Light Pollution option to turn this feature on. You can now slide the bar to adjust the local light pollution level until the view matches your night sky. When you are satisfied that the view matches your sky, click on OK to dismiss the Local View Options dialog window. Move the cursor over some of the stars that appear on your screen to display their properties in the HUD (Heads-up Display). (If necessary, open the Preferences dialog from the File (Windows) or Starry Night (Macintosh) menu and add the Apparent magnitude option to the Cursor Tracking (HUD) options.) (a) Make a note of the apparent magnitudes of some of the faintest stars in this view and compare these values to the faintest apparent magnitude of about +6 that can be seen under ideal conditions by the human eye. (b) The second goal is to estimate what fraction of the visible stars you can see under these conditions, compared to the total number of stars you might see under ideal conditions. Carefully do the following on the screen: Select a small, square section of the sky on the screen (maybe 8 cm [about 3 in., or roughly half the length of a typical pen] on each side) and count and record the number of stars in this square with your present setting. (To help with this, you might consider cutting a suitable hole in a sheet of paper to use as a mask). Now open the Local View Options dialog window again and set the Local Light Pollution to Less (which essentially gives you ideal conditions) and repeat the count of visible stars. Divide the first number you count by the second. What fraction of the stars that would be visible under ideal conditions were you seeing? (c) To see how much light pollution occurs in large cities, adjust the slide bar for Local Light Pollution in the Local View Options dialog window to the far right for maximum light pollution and repeat the star counting within the same limited sky region. Compared to viewing under ideal conditions, what fraction of the stars are observers in large urban centers seeing?

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Question 3.39

Observe the stars when the Moon is either full, new, or in a quarter phase. Record the phase of the Moon and note, qualitatively, whether you see many more stars than just those visible in relatively bright asterisms such as the Big Dipper or Orion. Compare these qualitative observations of the number of visible stars with those made on a clear night about a week later, when the Moon’s phase has changed and its contribution to the light of the night sky is different, noting the phase of the Moon again. (a) Compare the numbers of stars on the two nights. On which night did you see more stars? Why? (b) During which three phases of the Moon do you expect that astronomers prefer to make observations of faint objects?

Question 3.40

On a clear night, view the Moon, a planet, and a star through a telescope using eyepieces of various focal lengths. (You can use Starry Night™ or consult a source on the Internet or such magazines as Sky & Telescope or Astronomy to determine the phase of the Moon and the locations of the planets.) (a) How do the images change as you view with increasing magnification? (b) Do they become distorted at any level of magnification?

Question 3.41

In this exercise, you can use Starry Night™ to explore the appearance of a nearby galaxy, M31, the Andromeda Galaxy, under conditions that simulate the use of binoculars or a telescope to enhance the unaided-eye view. This will allow you to evaluate the effects of greater light-gathering power and improved resolution that come from the use of these optical aids. Select Favourites > Discovering the Universe > Andromeda to open a view of the sky as seen from New York City at just after midnight on the first day of summer, June 21, 2011, looking toward the northwest. This 100° field of view of the sky over Central Park is centered upon M31. You can use this view to show you where to look for this faint object in the sky by selecting View > Constellations > Astronomical to show this distant galaxy with respect to the Square of Pegasus and the V-shaped constellation of Andromeda. You can label these constellations by clicking on Labels > Constellations if you like. This is the approximate view that one has of this galaxy with the unaided eye from a dark-sky site. (a) Sketch the constellation patterns of bright stars around M31 and include its position for reference when you view the real sky. Remove the constellations and labels. (b) Now use the Zoom facility on the toolbar to adjust the field of view to about 24° to match that of a pair of ordinary binoculars. (To display a more precise indication of the field of view of your own binoculars, click on the FOV tab and the Edit button to the right of Binoculars, open the From List box, and select the appropriate binoculars from this list. You can now click on your binocular characteristics in this FOV > Binoculars list). Again, sketch and describe the galaxy’s appearance. (c) Finally, adjust the field of view to about 2°, to match that of a small telescope. Here, the galaxy will extend across the full field. Sketch and describe this galaxy again, noting particularly any details now apparent at this higher magnification. Locate and identify two other galaxies, M32 and M110, in this image. (d) Is the resolution of this telescope image better, the same, or worse than that seen through binoculars or with the unaided eye?

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WHAT IF…: Humans Had Infrared-Sensitive Eyes?

Our eyes are sensitive to less than a trillionth of 1% of the electromagnetic spectrum—what we call visible light. But this minuscule resource provides an awe-inspiring amount of information about the universe. We interpret visible-light photons as the six colors of the rainbow—red, orange, yellow, green, blue, and violet. These colors combine to form all of the others that make our visual world so rich. But the Sun actually emits photons of all wavelengths. So, what would happen if our eyes had evolved to sense another part of the spectrum?

A Darker Vision? Gamma rays, X-rays, and most ultraviolet radiation do not pass through Earth’s atmosphere. Because the world is illuminated by sunlight, Earth and the sky would look dark, indeed, if our eyes were sensitive only to these wavelengths. Radio waves, in contrast, easily pass through our atmosphere. But to see the same detail from radio waves that we now see from visible-light photons, our eyes would require a diameter 10,000 times larger. Each would be the size of a baseball infield!

What about infrared radiation? Although not all incoming infrared photons get through our atmosphere, short-wavelength (“near”) infrared radiation passes easily through air. Depending on wavelength, the Sun emits between one-half and a ten-billionth as many infrared photons as red-light photons. Fortunately, most of these are in the near infrared.

Heat-Sensitive Vision? To see infrared photons, human eyes would need to be only 5 to 10 times larger. Some snakes have evolved infrared vision. Portable infrared “night vision” cameras and goggles are available to us humans. Because everything that emits heat emits infrared photons, infrared sight would be very useful. Also, not everything we see with infrared sight would be due just to reflected sunlight—hotter objects would be intrinsically brighter than cool ones. For example, seeing infrared would allow us to observe changes in a person’s emotional state. Someone who is excited or angry often has more blood near the skin and, thus, releases more infrared radiation (heat) than normal. Conversely, someone who is scared has less blood near the skin and, thus, emits less heat.

Night Vision? The night sky would be a spectacular sight through infrared-sensitive eyes. Gas and dust clouds in the Milky Way absorb visible light, thus preventing the light of distant stars from getting to Earth. However, because most infrared radiation passes through these clouds, unaided, we would be able to see distant stars that we cannot see today. On the other hand, the white glow of the Milky Way, which is caused by the scattering of starlight by interstellar clouds, would be dimmer, because the gas and dust clouds do not scatter infrared light as much as they do visible light. (The haze created by the Milky Way would not vanish, however, because when gas and dust clouds are heated by starlight, they emit their own infrared radiation.)

Our concept of stars would be different, too. Many stars, especially young, hot ones, are surrounded by cocoons of gas and dust that emit infrared radiation. This dust is heated by the nearby stars. Instead of appearing as pinpoints, many stars would appear to be surrounded by wild strokes of color, and we would have an impressionist sky.

Kissing Is Hot The infrared (heat) from this kissing couple has been converted into visible light colors so that we can interpret the invisible radiation. The hottest regions are white, with successively cooler areas shown in yellow, orange, red, green, sky blue, dark blue, and violet.