Name: ___________________________________ Date: ______________
Astro 130 Fall 2011, Homework 2, Chapter 6
1. A typical refracting telescope is made up of
A) a long-focal-length lens at the front and a short-focal-length lens at the rear (next to your
eye as you look through the telescope).
B) a short-focal-length lens at the front and a long-focal-length lens at the rear (next to your
eye as you look through the telescope).
C) a mirror which gathers and focuses the light, and a lens next to your eye to examine the
image.
D) two mirrors, one concave in shape and the second one convex in shape.
2. By what factor is the amount of light gathered by the 10-m diameter Keck telescope on
Mauna Kea, Hawaii, greater than that gathered by the 2.5-m diameter Mount Wilson
telescope?
A) 4
B) 16
C) 256
D) 2
3. A particular reflecting telescope has an objective mirror with a focal length of 1.2 m and an
eyepiece lens of focal length 6 mm. What is the magnifying power of this telescope?
A) 5×
B) 2000×
C) 20×
D) 200×
4. A Cassegrain reflecting telescope is constructed from a
A) concave primary mirror and a convex secondary mirror that reflects light back through a
hole in the primary mirror.
B) series of mirrors that channel the light to a remote location.
C) concave primary mirror and a concave secondary mirror that reflects light back through
a hole in the primary mirror.
D) concave primary mirror and a flat, diagonal secondary mirror.
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5. A spherical mirror suffers from spherical aberration because
A) the starlight is distorted by turbulence in Earth's atmosphere.
B) the mirror sags under its own weight, distorting the image.
C) different parts of the mirror focus the light at different distances from the mirror.
D) different colors are focused at different distances from the mirror.
6. If all effects caused by Earth's atmospheric variations (seeing) could be removed from the
visible image of a star on one of the 10-m diameter Keck telescopes on Hawaii, what would
be the angular resolution achievable by this telescope in arcseconds for light of wavelength
500 nm? (1 nm = 10– 9 m)
A) 0.00125 arcsec
B) 0.125 arcsec
C) 12.5 arcsec
D) 0.0125 arcsec
7. To reduce the effects of bad seeing on astronomical images, the best Earth-based sites for
modern large astronomical telescopes are
A) at sea level. The air is less turbulent and the presence of water stabilizes the air.
B) the tops of high mountains.
C) the downwind side of mountain ranges. Smooth airflow produces clear air and stable
images.
D) near large cities. The warm air from human activity serves to stabilize the overlying
atmosphere.
8. Why was adaptive optics developed?
A) to prevent distortion of mirrors by the vacuum of space
B) to prevent distortion by sagging in very thin, lightweight mirrors
C) to compensate for spherical aberration
D) to compensate for image distortion caused by Earth's atmosphere
9. What is a charge-coupled device (CCD)?
A) a very fine grained photographic film which can be “read” electronically
B) an extremely sensitive thermocouple which records the temperature increase every time
a photon hits it
C) a spectrometer which has been made especially sensitive by using a grating with over
one million lines per centimeter
D) an array of electronic sensors which record the charge buildup due to photon
absorption
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10. A spectrograph is usually used in astronomy to measure the
A) variation of the mass of an object as it moves through space.
B) distribution of light intensity among the various colors.
C) vibration of Earth following an earthquake.
D) brightness of light at one specific color.
11. The first extraterrestrial radio source detected was
A) the Sun.
B) the Moon.
C) the center of our galaxy.
D) Jupiter.
12. The angular resolution attainable with a radio telescope, compared to that attainable with an
optical telescope of the same diameter, is significantly inferior because
A) the wavelength of radio waves is larger than that of visible light.
B) it is difficult to make a reflector for radio waves since these waves penetrate most
materials.
C) Earth's atmosphere disturbs radio waves from space much more than it does visible
light.
D) radio wavelengths are smaller than visible wavelengths, making it difficult to produce a
reflector sufficiently smooth to produce images.
13. The primary reason for spreading many radio telescopes across a large area and combining
the signals at a central station is to
A) ensure that cloudy weather only affects a few of the telescopes, leaving the others to
continue observing.
B) avoid interference between signals from the separate telescopes.
C) collect more radiation than would be possible with the same telescopes clustered
together.
D) produce much sharper images of radio sources.
14. An optical telescope using 650 nm light has a diameter of 1 m. A radio telescope using
radio waves of wavelength 6.5 cm has a diameter of 100 m. The resolution of the optical
telescope is
A) better than that of the radio telescope by a factor of 1000.
B) worse than that of the radio telescope by a factor of 1000.
C) better than that of the radio telescope by a factor of 100,000.
D) worse than that of the radio telescope by a factor of 100,000.
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15. The two ranges of electromagnetic radiation for which Earth's atmosphere is reasonably
transparent are
A) UV and radio waves.
B) visible and far infrared radiation.
C) X-rays and visible radiation.
D) visible and radio radiation.
16. The main reason for placing astronomical telescopes and detectors on satellites is to
A) avoid dust and haze in Earth's atmosphere.
B) avoid light pollution from cities and other built-up areas.
C) get closer to the objects being viewed.
D) get above the absorption in Earth's atmosphere.
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