The Solar System

CHAPTER 6 | LIGHT AND TELESCOPES 121

7. Small telescopes are often advertised as “200 power” or “magnifi es
   200 times.” As someone knowledgeable about astronomical telescopes,
   how would you improve such advertisements?


8. Not too many years ago an astronomer said, “Some people think
   I should give up photographic plates.” Why might she change to
   something else?


9. What purpose do the colors in a false-color image or false-color radio
   map serve?


10. How is chromatic aberration related to a prism spectrograph?


11. Why would radio astronomers build identical radio telescopes in many
    different places around the world?


12. Why do radio telescopes have poor resolving power?


13. Why must telescopes observing in the far-infrared be cooled to low
    temperatures?


14. What might you detect with an X-ray telescope that you could not
    detect with an infrared telescope?


15. The moon has no atmosphere at all. What advantages would you have
    if you built an observatory on the lunar surface?


16. How Do We Know? How is the resolution of an astronomical image
    related to the precision of a measurement?

Discussion Questions

1. Why does the wavelength response of the human eye match so well the
   visual window of Earth’s atmosphere?


2. Most people like beautiful sunsets with brightly glowing clouds, bright
   moonlit nights, and twinkling stars. Astronomers don’t. Why?

Problems

1. The thickness of the plastic in plastic bags is about 0.001 mm. How
   many wavelengths of red light is this?


2. What is the wavelength of radio waves transmitted by a radio station
   with a frequency of 100 million cycles per second?


3. Compare the light-gathering powers of one of the 10-m Keck
   telescopes and a 0.5-m telescope.


4. How does the light-gathering power of one of the Keck telescopes
   compare with that of the human eye? (Hint: Assume that the pupil of
   your eye can open to about 0.8 cm.)


5. What is the resolving power of a 25-cm telescope? What do two stars
   1.5 arc seconds apart look like through this telescope?


6. Most of Galileo’s telescopes were only about 2 cm in diameter. Should
   he have been able to resolve the two stars mentioned in Problem 5?
   7. How does the resolving power of a 5-m telescope compare with that
   of the Hubble Space Telescope? Why does the HST outperform a 5-m
   telescope?
   8. If you build a telescope with a focal length of 1.3 m, what focal
   length should the eyepiece have to give a magnifi cation of 100 times?
   9. Astronauts observing from a space station need a telescope with a
   light-gathering power 15,000 times that of the human eye, capable of
   resolving detail as small as 0.1 arc seconds and having a magnifying
   power of 250. Design a telescope to meet their needs. Could you test
   your design by observing stars from Earth?
   10. A spy satellite orbiting 400 km above Earth is supposedly capable
   of counting individual people in a crowd. Roughly what minimum-
   diameter telescope must the satellite carry? (Hint: Use the small-angle
   formula.)

Learning to Look

1. The two images at the right
   show a star before and after
   an adaptive optics system was
   switched on. What causes the
   distortion in the fi rst image,
   and how does adaptive optics
   correct the image?


2. The star images in the photo at the
   right are tiny disks, but the diameter
   of these disks is not related to the
   diameter of the stars. Explain why the
   telescope can’t resolve the diameter of
   the stars.


3. The X-ray image at right shows the remains of
   an exploded star. Explain why images recorded
   by telescopes in space are often displayed in
   false color rather than in the “colors” received
   by the telescope.

ESO

NASA, ESA and G. Meylan

NASA/CXC/PSU/S. Park