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2. Chapter 6
Light and Telescopes

3. Guidepost
Previous chapters have described the sky as it appears to our unaided eyes, but modern astronomers turn powerful telescopes on the sky. Chapter 6 introduces us to the modern astronomical telescope and its delicate instruments.
The study of the universe is so challenging, astronomers cannot ignore any source of information; that is why they use the entire spectrum, from gamma rays to radio waves. This chapter shows how critical it is for astronomers to understand the nature of light.
In each of the chapters that follow, we will study the universe using information gathered by the telescopes and instruments described in this chapter.

4. Outline I. Radiation: Information from Space
A. Light as a Wave and a Particle
B. The Electromagnetic Spectrum
II. Optical Telescopes
A. Two Kinds of Telescopes
B. The Powers of a Telescope
C. Buying a Telescope
D. New-Generation Telescopes
E. Interferometry
III. Special Instruments
A. Imaging Systems
B. The Spectrograph

5. Outline (continued) IV. Radio Telescopes
A. Operation of a Radio Telescope
B. Limitations of the Radio Telescope
C. Advantages of Radio Telescopes
V. Space Astronomy
A. Infrared Astronomy
B. Ultraviolet Astronomy
C. X-Ray Astronomy
D. Gamma-Ray Telescopes
E. Cosmic Rays
F. The Hubble Space Telescope

6. Light and Other Forms of Radiation
The Electromagnetic Spectrum
In astronomy, we cannot perform experiments with our objects (stars, galaxies, …).
The only way to investigate them, is by analyzing the light (and other radiation) which we observe from them.

7. Light as a Wave (1) f = c/l l
c = 300,000 km/s = 3*108 m/s
Light waves are characterized by a wavelength l and a frequency f.
f and l are related through
f = c/l

8. Light as a Wave (2)
Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å):
1 nm = 10-9 m
1 Å = m = 0.1 nm
Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).

9. Wavelengths and Colors
Different colors of visible light correspond to different wavelengths.

10. Light as Particles E = h*f
Light can also appear as particles, called photons (explains, e.g., photoelectric effect).
A photon has a specific energy E, proportional to the frequency f:
E = h*f
h = 6.626x10-34 J*s is the Planck constant.
The energy of a photon does not depend on the intensity of the light!!!

11. The Electromagnetic Spectrum
Wavelength
Frequency
High flying air planes or satellites
Need satellites to observe

12. The larger the telescope, the more light it gathers.
Optical Telescopes
Astronomers use telescopes to gather more light from astronomical objects.
The larger the telescope, the more light it gathers.

13. Refracting/Reflecting Telescopes
Refracting Telescope: Lens focuses light onto the focal plane
Focal length
Reflecting Telescope: Concave Mirror focuses light onto the focal plane
Focal length
Almost all modern telescopes are reflecting telescopes.

14. Secondary Optics
In reflecting telescopes: Secondary mirror, to re-direct light path towards back or side of incoming light path.
Eyepiece: To view and enlarge the small image produced in the focal plane of the primary optics.

15. Refractors and Reflectors

16. Disadvantages of Refracting Telescopes
Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect).
Can be corrected, but not eliminated by second lens out of different material.
Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges

17. The Powers of a Telescope: Size Does Matter
1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared: D
A = p (D/2)2

18. The Powers of a Telescope (2)
2. Resolving power: Wave nature of light => The telescope aperture produces fringe rings that set a limit to the resolution of the telescope.
Resolving power = minimum angular distance amin between two objects that can be separated.
amin = 1.22 (l/D)
amin
For optical wavelengths, this gives
amin = 11.6 arcsec / D[cm]

19. Resolution and Telescopes

20. Seeing
Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images.
Bad seeing
Good seeing

21. The Powers of a Telescope (3)
3. Magnifying Power = ability of the telescope to make the image appear bigger.
The magnification depends on the ratio of focal lengths of the primary mirror/lens (Fo) and the eyepiece (Fe):
M = Fo/Fe
A larger magnification does not improve the resolving power of the telescope!

22. The Best Location for a Telescope
Far away from civilization – to avoid light pollution

23. The Best Location for a Telescope (2)
Paranal Observatory (ESO), Chile
On high mountain-tops – to avoid atmospheric turbulence ( seeing) and other weather effects

24. Traditional Telescopes (1)
Secondary mirror
Traditional primary mirror: sturdy, heavy to avoid distortions.

25. Traditional Telescopes (2)
The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona)

26. Advances in Modern Telescope Design
Modern computer technology has made possible significant advances in telescope design:
1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers
Floppy mirror
Segmented mirror
2. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers

27. Adaptive Optics
Computer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence

28. Examples of Modern Telescope Design (1)
Design of the Large Binocular Telescope (LBT)
The Keck I telescope mirror

29. Examples of Modern Telescope Design (2)
The Very Large Telescope (VLT)
8.1-m mirror of the Gemini Telescopes

30. Interferometry Interferometry
Recall: Resolving power of a telescope depends on diameter D:
amin = 1.22 l/D.
This holds true even if not the entire surface is filled out.
Combine the signals from several smaller telescopes to simulate one big mirror 
Interferometry

31. CCD Imaging CCD = Charge-coupled device
More sensitive than photographic plates
Data can be read directly into computer memory, allowing easy electronic manipulations
Negative image to enhance contrasts
False-color image to visualize brightness contours

32. The Spectrograph
Using a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum.
Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object

33. Radio Astronomy
Recall: Radio waves of l ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be observed from the ground.

34. Radio Telescopes
Large dish focuses the energy of radio waves onto a small receiver (antenna)
Amplified signals are stored in computers and converted into images, spectra, etc.

35. Radio Interferometry
Just as for optical telescopes, the resolving power of a radio telescope is amin = 1.22 l/D.
For radio telescopes, this is a big problem: Radio waves are much longer than visible light
 Use interferometry to improve resolution!

36. Radio Interferometry (2)
The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter.
Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)!

37. The Largest Radio Telescopes
The 300-m telescope in Arecibo, Puerto Rico
The 100-m Green Bank Telescope in Green Bank, WVa.

38. Science of Radio Astronomy
Radio astronomy reveals several features, not visible at other wavelengths:
Neutral hydrogen clouds (which don’t emit any visible light), containing ~ 90 % of all the atoms in the Universe.
Molecules (often located in dense clouds, where visible light is completely absorbed).
Radio waves penetrate gas and dust clouds, so we can observe regions from which visible light is heavily absorbed.

39. Infrared Astronomy
Most infrared radiation is absorbed in the lower atmosphere.
However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed.
NASA infrared telescope on Mauna Kea, Hawaii

40. Space Astronomy

41. NASA’s Space Infrared Telescope Facility (SIRTF)

42. Ultraviolet Astronomy
Ultraviolet radiation with l < 290 nm is completely absorbed in the ozone layer of the atmosphere.
Ultraviolet astronomy has to be done from satellites.
Several successful ultraviolet astronomy satellites: IRAS, IUE, EUVE, FUSE
Ultraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the Universe.

43. X-Ray Astronomy X-rays are completely absorbed in the atmosphere.
X-ray astronomy has to be done from satellites.
X-rays trace hot (million degrees), highly ionized gas in the Universe.
NASA’s Chandra X-ray Observatory

44. Gamma-Ray Astronomy
Gamma-rays: most energetic electromagnetic radiation;
traces the most violent processes in the Universe
The Compton Gamma-Ray Observatory

45. The Hubble Space Telescope
Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s
Avoids turbulence in the Earth’s atmosphere
Extends imaging and spectroscopy to (invisible) infrared and ultraviolet

46. New Terms electromagnetic radiation wavelength frequency
Nanometer (nm)
Angstrom (Å)
photon
infrared radiation
ultraviolet radiation
atmospheric window
focal length
refracting telescope
reflecting telescope
primary lens, mirror
objective lens, mirror
eyepiece
chromatic aberration
achromatic lens
light-gathering power
resolving power
diffraction fringe
seeing
magnifying power
light pollution
prime focus
secondary mirror
Cassegrain focus
Newtonian focus
Schmidt-Cassegrain focus
sidereal drive
equatorial mounting
polar axis
alt-azimuth mounting
active optics
adaptive optics

47. New Terms (continued) interferometry charge-coupled device (CCD)
false-color image
spectrograph
grating
comparison spectrum
radio interferometer
cosmic ray

48. 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. Most astronomers don’t. Why?

49. Quiz Questions
1. The visible part of the electromagnetic spectrum can be divided into seven color bands of Red, Orange, Yellow, Green, Blue, Indigo, and Violet (from long to short wavelength). A single photon of which of these colors has the greatest amount of energy?
a. Red
b. Orange
c. Green
d. Blue
*e. Violet

50. Quiz Questions
2. The entire electromagnetic spectrum can be divided into the seven bands of Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, and Gamma-ray (from longest to shortest wavelength). To which of these two bands is Earth's atmosphere the most transparent?
a. X-ray & Gamma-ray
b. Ultraviolet & Infrared
c. Visible & Ultraviolet
d. Microwave & Radio
*e. Visible & Radio

51. Quiz Questions
3. Why do the pupils of a cat's eyes open wider at night?
a. To reduce the buildup of cat eye wax.
b. Cats are the only animals besides humans to observe the stars.
c. The cat sleeps all day and is wide awake at night.
*d. To increase light gathering power.
e. To attract a mate.

52. Quiz Questions
4. Astronomers are both hindered and assisted by chromatic aberration. In which device is chromatic aberration a big problem for astronomers?
a. The primary mirrors of reflecting telescopes.
b. The primary lenses of refracting telescopes.
c. The prism.
d. Both a and b above.
*e. All of the above.

53. Quiz Questions
5. Why have no large refracting telescopes been built in the years since 1900?
a. Refracting telescopes suffer from chromatic aberration.
b. Making large glass lenses without interior defects is difficult.
c. Refracting telescopes have several surfaces to shape and polish.
d. Large glass lenses are more difficult to support than large mirrors.
*e. All of the above.

54. Quiz Questions
6. What do large-diameter gently curved convex (thicker in the middle) lenses and large-diameter gently curved concave (thinner in the middle) mirrors have in common?
a. They both have short focal lengths.
*b. They both have long focal lengths.
c. They can be used as primary light collectors for a telescope.
d. Both a and c above.
e. Both b and c above.

55. Quiz Questions
7. Which power of a telescope might be expressed as "0.5 seconds of arc"?
a. Light gathering power.
*b. Resolving power.
c. Magnifying power.
d. Both a and b above.
e. Both a and c above.

56. Quiz Questions 8. Which power of a telescope is the least important?
a. Light gathering power.
b. Resolving power.
*c. Magnifying power.
d. Both a and b above.
e. Both a and c above.

57. Quiz Questions
9. Which power of an optical telescope is determined by the diameter of the primary mirror or lens?
a. Light gathering power.
b. Resolving power.
c. Magnifying power.
*d. Both a and b above.
e. Both a and c above.

58. Quiz Questions
10. What advantage do the builders of large telescopes today have over the previous generation of telescope builders?
a. Large mirrors can now be made thinner and lighter than before.
b. Tracking celestial objects today is computer controlled and can take advantage of simpler, stronger mounts.
c. High-speed computing today can be used to reduce the effect of Earth's atmosphere.
d. Both b and c above.
*e. All of the above.

59. Quiz Questions
11. In which device do astronomers take advantage of chromatic aberration?
a. The primary mirrors of reflecting telescopes.
b. The primary lenses of refracting telescopes.
*c. The prism.
d. Both a and b above.
e. All of the above.

60. Quiz Questions
12. Which power of a large ground-based optical telescope is severely limited by Earth's atmosphere on a cloudless night?
a. Light gathering power.
*b. Resolving power.
c. Magnifying power.
d. Both a and b above.
e. Both a and c above.

61. Quiz Questions
13. The primary mirror of telescope A has a diameter of 20 cm, and the one in telescope B has a diameter of 100 cm. How do the light gathering powers of these two telescopes compare?
a. Telescope A has 5 times the light gathering power of telescope B.
b. Telescope B has 5 times the light gathering power of telescope A.
c. Telescope A has 25 times the light gathering power of telescope B.
*d. Telescope B has 25 times the light gathering power of telescope A.
e. The light gathering power depends on the focal length of the eyepiece also.

62. Quiz Questions
14. What do the newer light-sensitive electronic CCD chips do better than the older photographic plates coated with light-sensitive chemicals?
a. They have a greater sensitivity to light.
b. They can detect both bright and dim objects in a single exposure.
c. Photometry can be done with the CCD images.
d. The CCD images are easier to manipulate.
*e. All of the above.

63. Quiz Questions
15. What can radio telescopes do that optical telescopes cannot?
a. Find the location of cool hydrogen gas.
b. See through dust clouds.
c. Detect high temperature objects.
*d. Both a and b above.
e. All of the above.

64. Quiz Questions
16. What is a disadvantage of radio telescopes compared to optical telescopes?
a. Radio photons have lower energy, thus radio waves have low intensity.
b. Interference from nearby sources of radio waves.
c. Poor resolving power.
d. Both a and b above.
*e. All of the above.

65. Quiz Questions
17. Radio telescopes are often connected together to do interferometry. What is the primary problem overcome by radio interferometry?
a. Poor light gathering power.
*b. Poor resolving power.
c. Poor magnifying power.
d. Interference from nearby sources of radio waves.
e. The low energy of radio photons.

66. Quiz Questions
18. Why are near-infrared telescopes located on mountaintops and ultraviolet telescopes in Earth orbit?
a. The primary infrared blocker, water vapor, is mostly in the lower atmosphere.
b. The primary ultraviolet blocker, ozone, is located high in the atmosphere, far above mountaintops.
c. Ultraviolet telescopes require the low temperature of space to operate.
*d. Both a and b above.
e. Both a and c above.

67. Quiz Questions
19. Why must far-infrared telescopes be cooled to a low temperature?
*a. To reduce interfering heat radiation emitted by the telescope.
b. To protect the sensitive electronic amplifiers from overheating by sunlight.
c. To improve their poor resolving power.
d. To improve their poor magnifying power.
e. To make use of the vast supplies of helium stockpiled by the United States.

68. Quiz Questions
20. Why are the sources of cosmic rays difficult to locate?
a. Cosmic rays are high-energy photons that penetrate the surfaces of telescope mirrors rather than reflecting to a focal point.
*b. Cosmic rays are charged particles, thus their paths are curved by magnetic fields, which masks the location of their source.
c. Cosmic rays are neutral particles that weakly interact with matter and are difficult to detect.
d. Cosmic rays are positively and negatively charged particles, which masks the location of their source.
e. Cosmic rays are theoretical and have never been detected.

69. Answers
1. e
2. e
3. d
4. d
5. e
6. e
7. b
8. c
9. d
10. e
11. c
12. b
13. d
14. e
15. d
16. e
17. b
18. d
19. a
20. b