Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).

1
Note that the following lectures include
animations and PowerPoint effects such as fly ins
and transitions that require you to be in
PowerPoint's Slide Show mode (presentation mode).
2
Light and Telescopes

• Chapter 6

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)
l
c 300,000 km/s 3108 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 10-10 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

• 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 hf
h 6.626x10-34 Js 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
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
(SLIDESHOW MODE ONLY)
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 TelescopeSize 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 gt 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 / Dcm
19
Resolution and Telescopes
(SLIDESHOW MODE ONLY)
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
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 Earths 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 dont 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
NASAs Space Infrared Telescope Facility (SIRTF)
42
Ultraviolet Astronomy

• Ultraviolet radiation with l lt 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.
NASAs 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 2000s

• Avoids turbulence in the Earths atmosphere

• Extends imaging and spectroscopy to (invisible)
  infrared and ultraviolet

46
New Terms
electromagnetic radiation wavelength frequency Nan
ometer (nm) Angstrom (Å) photon infrared
radiation ultraviolet radiation atmospheric
window focal length refracting telescope reflectin
g 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 compa
rison 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 Earths
atmosphere? 2. Most people like beautiful
sunsets with brightly glowing clouds, bright
moonlit nights, and twinkling stars. Most
astronomers dont. 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. b 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