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Astronomy 120
HOMEWORK - Chapter 6
Telescopes and Our Insight Into the Cosmos
Use a calculator whenever necessary.
For full credit, always show your work and explain how you got your answer.
Be careful about units!
Please CIRCLE or put a box around your final answer if it is numerical.
1. What is the most important function of a telescope?
2. Zeilik Study Exercise 6.5
Suppose you are on a TV quiz show. They show you optical, infrared, and radio telescopes
with the same size objective and ask you to list the devices in order of increasing resolving
power. What is the correct order?
3. Zeilik Study Exercise 6.4
What are the advantages and disadvantages of radio telescopes? How do they differ from
optical ones? How are they similar?
4. Zeilik Study Exercise 6.8
Describe one advantage of an infrared telescope over an optical one?
5. Zeilik Study Exercise 6.9
Why do x-ray telescopes have to be put above the earth’s atmosphere?
6. What are the justifications for spending $1.5 billion on a Space Telescope?
7. The f-ratio of a telescope is defined as:
f  ratio 
focal length
diameter of objective
a) the focal length of a typical reflecting telescope is 12 inches. If the diameter of the
objective is 8 inches, what is the f-ratio?
b) the focal length of a typical refracting telescope is about 92 cm. If the diameter of the
objective is 10 cm, what is its f-ratio?
The smaller the f-ratio, the brighter the image formed, but this cuts down on the detail or
resolving power.
8. Light-gathering power (LGP) is proportional to the diameter of the objective squared.
LGP  Dobj 2
The Hale telescope on Mt. Palomar has an objective of 5 meters. The objective of the Hubble
Space Telescope is 2.4 meters. How much more light can the Hale telescope gather as
compared to the Hubble Space Telescope?
9. Theoretical Resolving Power (TR) of a telescope is defined as:
TR (arcsec) = 2.52 x 105 ( (meters) / D (meters))
if D is the diameter of the objective and  is the wavelength of observed light and they are
both in the same units (the units cancel), then TR is in units of seconds of arc.
a) what is the theoretical resolving power of the 10 cm refractor in question #1 at a
wavelength of 5.5 x 10-7 m?
b) what is the theoretical resolving power of the 5 m Hale telescope at a wavelength of
5.5 x 10-7 m?
c) What size aperature telescope is needed to “split” a double star separated by 0.1 arcsec
at 5.5 x 10-7m?
10. Magnifying Power (MP) of a telescope is...
MP 
focal length of objective
focal length of eyepiece
a) what is the magnification power of a telescope having a focal length of 192 cm used
with an eyepiece having a focal length of 25 mm?
b) with an eyepiece of focal length 12.5 mm?