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21st Century Astronomy

Laura Kay, Stacy Palen, George Blumenthal

Chapter 6

The Tools of the Astronomer - all with Video Answers

Educators

Chapter Questions

02:30

Problem 1

You are shopping for telescopes online. You find two in your price range. One of these has an aperture of $20 \mathrm{cm},$ and the other has an aperture of $30 \mathrm{cm} .$ If aperture size is the only difference, which should you choose, and why?
a. The $20 \mathrm{cm}$, because the light-gathering power will be better.
b. The $20 \mathrm{cm}$, because the image size will be larger.
c. The $30 \mathrm{cm}$, because the light-gathering power will be better.
d. The $30 \mathrm{cm}$, because the image size will be larger.

Matthew Miranda
Matthew Miranda
Numerade Educator
04:22

Problem 2

Which of the following can be observed from Earth's surface? (Choose all that apply.)
a. radio waves
b. gamma radiation
c. far UV light
d. X-ray light
e. visible light

Matthew Miranda
Matthew Miranda
Numerade Educator
04:19

Problem 3

Match the following properties of telescopes (lettered) with their corresponding definitions (numbered).
a. aperture
b. resolution
c. focal length
d. chromatic aberration
e. diffraction
f. interferometer
g. adaptive optics
(1) two or more telescopes connected
to act as one
(2) distance from lens to focal plane
(3) diameter
(4) ability to distinguish close objects
(5) computer-controlled atmospheric distortion correction
(6) color-separating effect
(7) smearing effect due to sharp edge

Matthew Miranda
Matthew Miranda
Numerade Educator
04:32

Problem 4

The two Keck 10 -meter telescopes, separated by a distance of 85 meters, can operate as an optical interferometer. What is its resolution when it observes in the infrared at a wavelength of 2 microns?
a. 0.01 aresec
b. 0.005 arcsec
c. 0.2 arsec
d. 0.05 arcsec

Matthew Miranda
Matthew Miranda
Numerade Educator
01:45

Problem 5

Arrays of radio telescopes can produce much better resolution than single-dish telescopes because they work based on the principle of
a. reflection.
b. refraction.
c. diffraction.
d. interference.

Matthew Miranda
Matthew Miranda
Numerade Educator
00:55

Problem 6

Refraction is caused by
a. light bouncing off a surface.
b. light changing colors as it enters a new medium.
c. light changing speed as it enters a new medium.
d. two light beams interfering.

Matthew Miranda
Matthew Miranda
Numerade Educator
02:56

Problem 7

The light-gathering power of a 4 -meter telescope is ________________ than that of a 2 -meter telescope.
a. 8 times larger
b. 4 times larger
c. 16 times smaller
d. 2 times smaller

Matthew Miranda
Matthew Miranda
Numerade Educator
02:07

Problem 8

Improved resolution is helpful to astronomers because
a. they often want to look in detail at small features of an object.
b. they often want to look at very distant objects.
c. they often want to look at many objects close together.
d. all of the above

Matthew Miranda
Matthew Miranda
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03:01

Problem 9

The part of the human eye that acts as the detector is the
a. retina.
b. pupil.
c. lens.
d. iris.

Matthew Miranda
Matthew Miranda
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01:41

Problem 10

Cameras that use adaptive optics provide higher-spatialresolution images primarily because
a. they operate above Earth's atmosphere.
b. deformable mirrors are used to correct the blurring due to Earth's atmosphere.
c. composite lenses correct for chromatic aberration.
d. they simulate a much larger telescope.

Matthew Miranda
Matthew Miranda
Numerade Educator
01:27

Problem 11

The advantage of an interferometer is that
a. the resolution is dramatically improved.
b. the focal length is dramatically increased.
c. the light-gathering power is dramatically increased.
d. diffraction effects are dramatically decreased.
e. chromatic aberration is dramatically decreased.

Matthew Miranda
Matthew Miranda
Numerade Educator
01:45

Problem 12

The angular resolution of a ground-based telescope is usually determined by
a. diffraction.
b. the focal length.
c. refraction.
d. atmospheric seeing.

Matthew Miranda
Matthew Miranda
Numerade Educator
01:49

Problem 13

A grating is able to spread white light out into a spectrum of colors because of the property of
a. reflection.
b. diffraction.
c. dispersion.
d. interference.

Matthew Miranda
Matthew Miranda
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01:08

Problem 14

Why would astronomers put telescopes in airplanes?
a. to get the telescopes closer to the stars
b. to get the telescopes above the majority of the water vapor in Earth's atmosphere
c. to be able to observe one object for more than 24 hours without stopping
d. to allow the telescopes to observe the full spectrum of light

Matthew Miranda
Matthew Miranda
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01:17

Problem 15

If we could increase the quantum efficiency of the human eye, it would
a. allow humans to see a larger range of wavelengths.
b. allow humans to see better at night or in other low-light conditions.
c. increase the resolution of the human eye.
d. decrease the resolution of the human eye.

Matthew Miranda
Matthew Miranda
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02:02

Problem 16

Galileo's telescope used simple lenses. What is the primary disadvantage of using a simple lens in a refracting telescope?

Matthew Miranda
Matthew Miranda
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03:57

Problem 17

The largest astronomical refractor has an aperture of 1 meter. List several reasons why it would be impractical to build a larger refractor with twice this aperture.

Matthew Miranda
Matthew Miranda
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01:48

Problem 18

Your camera may have a zoom lens, ranging between wide angle (short focal length) and telephoto (long focal length). How does the size of an object in the camera's focal plane differ between wide angle and telephoto?

Matthew Miranda
Matthew Miranda
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03:25

Problem 19

Optical telescopes reveal much about the nature of astronomical objects. Why do astronomers also need information provided by gamma-ray, X-ray, infrared, and radio telescopes?

Matthew Miranda
Matthew Miranda
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02:11

Problem 20

For light reflecting from a flat surface, the angles of incidence and reflection are the same. This is also true for light reflecting from the curved surface of a reflecting telescope's primary mirror. Sketch a curved mirror and several of these reflecting
rays.

Matthew Miranda
Matthew Miranda
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04:22

Problem 21

Consider two optically perfect telescopes having different diameters but the same focal length. Is the image of a star larger or smaller in the focal plane of the larger telescope? Explain your answer.

Matthew Miranda
Matthew Miranda
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06:39

Problem 22

Study the Process of Science Figure. Make a flowchart for the symbiosis between technology and science that led to the development of the CCD camera as discussed in Section 6.2.

Matthew Miranda
Matthew Miranda
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03:01

Problem 23

Explain adaptive optics and how they improve a telescope's image quality.

Matthew Miranda
Matthew Miranda
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02:58

Problem 24

Explain integration time and quantum efficiency and how each contributes to the detection of faint astronomical
objects.

Matthew Miranda
Matthew Miranda
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06:20

Problem 25

Some people believe that we put astronomical telescopes on high mountaintops or in orbit because doing so gets them closer to the objects they are observing. Explain what is wrong with this popular misconception, and give the actual reason telescopes are located in these places.

Matthew Miranda
Matthew Miranda
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05:14

Problem 26

Humans have sent various kinds of spacecraft-including flybys, orbiters, and landers-to all of the planets in our Solar System. Explain the advantages and disadvantages of each of these types of spacecraft.

Matthew Miranda
Matthew Miranda
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01:48

Problem 27

If there are meteorites that are pieces of Mars on Earth, why is it so important to go to Mars and bring back samples of the martian surface?

Matthew Miranda
Matthew Miranda
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02:03

Problem 28

Humans had a first look at the far side of the Moon as recently as $1959 .$ Why had we not seen it earlier-when Galileo first observed the Moon with his telescope in $1610 ?$

Matthew Miranda
Matthew Miranda
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03:39

Problem 29

Where are neutrino detectors located? Why are neutrinos so difficult to detect?

Matthew Miranda
Matthew Miranda
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03:33

Problem 30

Why do telescopes in space give a better picture of the leftover radiation from the Big Bang?

Matthew Miranda
Matthew Miranda
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05:23

Problem 31

Compare the light-gathering power of the Thirty Meter Telescope with that of the dark-adapted human eye (aperture $8 \mathrm{mm}$ and with that of one of the 10 -meter Keck telescopes.

Matthew Miranda
Matthew Miranda
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02:41

Problem 32

Study the photograph of light entering and leaving a block of refractive material in Figure $6.2 \mathrm{b}$. Use a protractor to measure the angles of the green light as it enters the block and as it leaves the block. How are these angles related?

Matthew Miranda
Matthew Miranda
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03:29

Problem 33

Many amateur astronomers start out with a 4 -inch (aperture) telescope and then graduate to a 16 -inch telescope. By what factor does the light-gathering power of the telescope increase with this upgrade? How much fainter are the faintest stars that can be seen in the larger telescope?

Matthew Miranda
Matthew Miranda
Numerade Educator
05:22

Problem 34

The resolution of the human eye is about 1.5 arcmin. What would the aperture of a radio telescope (observing at $21 \mathrm{cm}$ ) have to be to have this resolution? Even though the atmosphere is transparent at radio wavelengths, humans do not see light in the radio range. Using your calculations and logic, explain why.

Matthew Miranda
Matthew Miranda
Numerade Educator
05:06

Problem 35

Assume that you have a telescope with an aperture of 1 meter. Compare the telescope's theoretical resolution when you are observing in the near-infrared region of the spectrum $(\lambda=1,000 \mathrm{nm})$ with that when you are observing in the violet region of the spectrum $(\lambda=400 \mathrm{nm})$.

Matthew Miranda
Matthew Miranda
Numerade Educator
05:08

Problem 36

Assume that the maximum aperture of the human eye, $D$, is approximately $8 \mathrm{mm}$ and the average wavelength of visible light, $\lambda,$ is $5.5 \times 10^{-4} \mathrm{mm}$.
a. Calculate the diffraction limit of the human eye in visible light.
b. How does the diffraction limit compare with the actual resolution of $1-2$ arcmin $(60-120 \text { arcsec }) ?$
c. To what do you attribute the difference?

Matthew Miranda
Matthew Miranda
Numerade Educator
03:23

Problem 37

The diameter of the full Moon in the focal plane of an average amateur's telescope (focal length 1.5 meters) is $13.8 \mathrm{mm}$ How big would the Moon be in the focal plane of a very large astronomical telescope (focal length 250 meters)?

Matthew Miranda
Matthew Miranda
Numerade Educator
03:11

Problem 38

One of the earliest astronomical CCDs had 160,000 pixels, each recording 8 bits ( 256 levels of brightness). A new generation of astronomical CCDs may contain a billion pixels, each recording 15 bits $(32,768$ levels of brightness). Compare the number of bits of data that each of these two CCD types produces in a single image.

Matthew Miranda
Matthew Miranda
Numerade Educator
02:10

Problem 39

Consider a CCD with a quantum efficiency of 80 percent and a photographic plate with a quantum efficiency of 1 percent. If an exposure time of 1 hour is required to photograph a celestial object with a given telescope, how much observing time would be saved by substituting a CCD for the photographic plate?

Matthew Miranda
Matthew Miranda
Numerade Educator
07:22

Problem 40

The VLBA uses an array of radio telescopes ranging across $8,000 \mathrm{km}$ of Earth's surface from the Virgin Islands to Hawaii.
a. Calculate the angular resolution of the array when radio astronomers are observing interstellar water molecules at a microwave wavelength of $1.35 \mathrm{cm}$.
b. How does this resolution compare with the angular resolution of two large optical telescopes separated by 100 meters and operating as an interferometer at a visible wavelength of $550 \mathrm{nm} ?$

Matthew Miranda
Matthew Miranda
Numerade Educator
03:52

Problem 41

When operational, the SVLBI may have a baseline of $100,000 \mathrm{km} .$ What will be the angular resolution when studying interstellar molecules emitting at a wavelength of $17 \mathrm{mm}$ from a distant galaxy?

Matthew Miranda
Matthew Miranda
Numerade Educator
03:45

Problem 42

The Mars Reconnaissance Orbiter (MRO) flies at an average altitude of $280 \mathrm{km}$ above the martian surface. If its cameras have an angular resolution of 0.2 arcsec, what is the size of the smallest objects that the $M R O$ can detect on the martian surface?

Matthew Miranda
Matthew Miranda
Numerade Educator
05:48

Problem 43

Voyager 1 is now about 125 astronomical units (AU) from Earth, continuing to record its environment as it approaches the limits of our Solar System.
a. How far away is Voyager 1 , in kilometers?
b. How long does it take observational data to come back to us from Voyager 1 ?
c. How does Voyager I's distance from Earth compare with that of the nearest star (other than the Sun)?

Matthew Miranda
Matthew Miranda
Numerade Educator
02:24

Problem 44

Gravitational waves travel at the speed of light. Their speed, wavelength, and frequency are related as $c=\lambda \times f$. If we were to observe a gravitational wave from a distant cosmic event with a frequency of 10 hertz $(\mathrm{Hz}),$ what would be the wavelength of the gravitational wave?

Matthew Miranda
Matthew Miranda
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03:00

Problem 45

Compute the peak of the blackbody spectrum with a temperature of $2.73 \mathrm{K}$. What region of the spectrum is this?

Matthew Miranda
Matthew Miranda
Numerade Educator
05:55

Problem 46

A webcast for the International Year of Astronomy 2009 called "Around the World in 80 Telescopes" can be accessed at http: $/ /$ eso.org/public/events/special-evt/100ha.html. The 80 telescopes are situated all over, including Antarctica and space. Pick two of the telescopes and watch the videos. Do you think these videos are effective for public outreach for the observatory in question or for astronomy in general? For each telescope you choose, answer the following questions: Does the telescope observe in the Northern Hemisphere or the Southern Hemisphere? What wavelengths does the telescope observe? What are some of the key science projects at the telescope?

Donald Albin
Donald Albin
Numerade Educator
05:02

Problem 47

Most major observatories have their own websites. Use the link in question 46 to find a master list of telescopes, and click on a telescope name to link to an observatory website (or run a search on names from Tables 6.1 and 6.2 ). For the telescope you choose, answer the following questions: (a) What is this telescope's "claim to fame"-is it the largest? at the highest altitude? at the driest location? with the darkest skies? the newest? (b) Does the observatory website have news releases? What is a recent discovery from this telescope?

Donald Albin
Donald Albin
Numerade Educator
03:43

Problem 48

Go to the website for the International Dark Sky Association (http://www.darksky.org/). Click on "Night Sky Conservation" and then "Do you live under light pollution?" Is your location dark? From the menu on the left, is there a "dark sky park" near you? What are the ecological arguments against too much light at night?

Donald Albin
Donald Albin
Numerade Educator
09:22

Problem 49

What is the current status of the James Webb Space Telescope (http://jwst.nasa.gov)? How will this telescope be different from the Hubble Space Telescope? What are some of the instruments for the JWST and its planned projects? What is the current estimated cost of the JWST?

Donald Albin
Donald Albin
Numerade Educator
09:22

Problem 50

What is the current status of the James Webb Space Telescope (http://jwst.nasa.gov)? How will this telescope be different from the Hubble Space Telescope? What are some of the instruments for the JWST and its planned projects? What is the current estimated cost of the JWST?

Donald Albin
Donald Albin
Numerade Educator