Download Ch. 5

Astronomy 120
HOMEWORK - Chapter 5
Telescopes
Use a calculator whenever necessary.
For full credit, always show your work and explain how you got your answer in full, complete
sentences on a separate sheet of paper.
Be careful about units!
Please CIRCLE or put a box around your final answer if it is numerical.
If you wish, you may discuss the questions with friends, but please turn in your own hand-written
solutions, with questions answered in your own way.
1. Chaisson Review and Discussion 5.1
Cite two reasons that astronomers are continually building larger and larger telescopes.
The two major reasons why telescopes with larger apertures are better than smaller
telescopes are greater collecting area and better angular resolution. The primary purpose
of a telescope is to make faint objects bright enough to detect. Larger telescope mirrors
have a larger surface area and can therefore collect more light and bring it to a focus. It is
also necessary to see fine detail in an image formed by the telescope, to resolve two objects
that appear close to each other. Larger telescope mirrors produce less scattering of light
due to diffraction, which blurs an image and limits resolution.
2. Chaisson Review and Discussion 5.2
List three advantages of reflecting telescopes over refractors.
Today, all the major research-grade telescopes use mirrors to gather light. Lenses have
three significant disadvantages over mirrors. First, when light passes through a lens, light
waves of different wavelengths focus at slightly different places. This is known as
chromatic aberration and can produce “rainbow” effects that keep us from seeing clearly.
It is not easy to correct when making large lenses, although small lenses, such as those in a
camera, do not suffer as much. A second problem is the glass in a lens absorbs certain
wavelengths of light that astronomers need to observe. For instance, glass is not
transparent to infrared light as it is for visible light. Therefore, the glass lens blocks
infrared light from entering the telescope. Lastly, it is difficult to keep a lens bigger than
about one meter from warping under its own weight. Glass is flexible and lenses can only
be supported around their edge. When the curvature of a lens changes, so does the focus,
thus ruining the image.
3. Chaisson Review and Discussion 5.4
How does Earth’s atmosphere affect what is seen through an optical telescope?
The atmosphere of the Earth is in almost constant turbulent motion and contains layers of
varying temperatures and density. Because of this, light passing through these layers is
refracted into many slightly different paths. This has the effect of “smearing” the image of
star, preventing it from appearing as a small point. This atmospheric blurring is called
“seeing.” Fortunately for astronomers, the Earth’s atmosphere is really rather thin and so
the images are not completely blurred to uselessness.
4. Chaisson Review and Discussion 5.5
What advantages does the Hubble Space Telescope (HST) have over ground-based
telescopes? List some disadvantages.
The Hubble Space Telescope is not affected by blurring in the Earth’s atmosphere because
it orbits above the atmosphere. It can also observe at wavelengths that are absorbed by
Earth’s atmosphere. However, it is a very complex telescope to operate and astronomers
must use it remotely. If something goes wrong, a space shuttle mission is usually required
to fix it. Because the Hubble orbits close to Earth, half the sky is always blocked by Earth.
Therefore, objects may only be observable for a brief time, before being blocked by Earth.
5. Chaisson Review and Discussion 5.6
What are the advantages of a CCD over a photograph?
A charge-coupled device, or CCD, has thousands of individual electronic light detectors
arranged in a grid. Each detector is much more sensitive than a photographic plate, and
the light level of each detector can be read out by a computer. A CCD’s primary
advantages are its high sensitivity to light, its linear response to light (twice as much light
produces twice as much signal, unlike a photographic plate, which is highly non-linear),
and the ease with which the image can be processed by computer software.
6. Chaisson Review and Discussion 5.9
How do astronomers use active optics to improve the resolution of the telescope?
A telescope with active optics can continuously adjust its optical system to compensate for
problems like mirror distortion, temperature changes, and bad seeing.
7. Chaisson Review and Discussion 5.13
What is interferometry, and what problem in radio astronomy does it address?
Since resolution degrades as wavelength increases, radio telescopes would have to be many
kilometers in diameter to achieve the same resolution as optical telescopes. The technique of
interferometry creates a telescope of this effective size by separating several radio telescopes
by great distances and simultaneously observing the same object. Using high-speed
computers, the individual images are combined to synthesize what would have been observed
by a telescope the size of the separation between the telescopes. Radio interferometry can
now reach resolutions that are far better than optical telescopes.
8. Chaisson Review and Discussion 5.16
Why do infrared satellites have to be cooled?
Any object at about room temperature or higher emits a great deal of infrared radiation, so
infrared-sensitive instruments must be cooled to low temperatures to reduce the amount of
interference from their surroundings.
9. Chaisson Review and Discussion 5.19
What are the main advantages of studying objects at different wavelengths of radiation?
Many objects emit most of their radiation at wavelengths other than visible. When the
Universe is observed at new wavelengths, these objects can become visible, or at least look
very different, and astronomers are then able to study them. Generally, observing at many
different wavelengths increases the total amount of information available to astronomers.
10. 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? (3 points)
LGPHale
(5 m)2
25 m2
=
=
= 4.34
LGPHST
(2.4 m)2
5.76 m2
or
LGPHale = 4.34 LGPHST
11. 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? (3 points)
TR (arcsec) = 2.52 x 105
TR = 1.386”
5.5 x 10-7 m
0.1 m
b) what is the theoretical resolving power of the 5 m Hale telescope at a wavelength of
5.5 x 10-7 m? (3 points)
TR (arcsec) = 2.52 x 105
5.5 x 10-7 m
5m
TR = 0.028”
c) What size aperature telescope is needed to “split” a double star separated by 0.1 arcsec
at 5.5 x 10-7m? (3 points)
D (meters) = 2.52 x 105 ( (meters) / TR (arcsec))
D (meters) = 2.52 x 105
5.5 x 10-7 m
0.1"
D = 1.386 meters
12. Magnifying Power (MP) of a telescope is...
MP 
a)
focal length of objective
focal length of eyepiece
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? (2 points)
Be careful with units!
192 cm
MP = 2.5 cm = 76.8 x
b) with an eyepiece of focal length 12.5 mm? (2 points)
192 cm
MP = 1.25 cm = 153.6 x