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Chapter 6
Light and Telescopes
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
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.
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
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
Light and Other Forms of
• 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.
Light as a Wave (1)
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
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).
Wavelengths and Colors
Different colors of visible light
correspond to different wavelengths.
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 = 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!!!
The Electromagnetic Spectrum
Need satellites
to observe
flying air
planes or
Optical Telescopes
Astronomers use
telescopes to gather
more light from
astronomical objects.
The larger the
telescope, the more
light it gathers.
Refracting/Reflecting Telescopes
Focal length
Focal length
Lens focuses
light onto the
focal plane
Concave Mirror
focuses light
onto the focal
Almost all modern telescopes are reflecting telescopes.
Secondary Optics
In reflecting
mirror, to redirect light path
towards back or
side of
incoming light
Eyepiece: To
view and
enlarge the
small image
produced in
the focal
plane of the
Refractors and Reflectors
Disadvantages of Refracting
• Chromatic aberration: Different
wavelengths are focused at different
focal lengths (prism effect).
• Difficult and expensive to
produce: All surfaces must be
perfectly shaped; glass must
be flawless; lens can only be
supported at the edges
Can be
corrected, but
not eliminated
by second lens
out of different
The Powers of a Telescope:
Size Does Matter
1. Light-gathering
power: Depends
on the surface
area A of the
primary lens /
proportional to
A = p (D/2)2
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
Resolving power = minimum
angular distance amin between
two objects that can be
amin = 1.22 (l/D)
For optical wavelengths, this gives
amin = 11.6 arcsec / D[cm]
Resolution and Telescopes
turbulence in
set further
limits to the
quality of
Bad seeing
Good seeing
The Powers of a Telescope (3)
3. Magnifying Power = ability of the
telescope to make the image appear
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!
The Best Location for a
Far away from civilization – to avoid light pollution
The Best Location for a
Telescope (2)
Paranal Observatory (ESO), Chile
On high mountain-tops – to avoid atmospheric
turbulence ( seeing) and other weather effects
Traditional Telescopes (1)
Secondary mirror
Traditional primary mirror: sturdy,
heavy to avoid distortions.
Traditional Telescopes (2)
The 4-m
Telescope at
Kitt Peak
Advances in Modern Telescope Design
Modern computer technology has made
possible significant advances in telescope
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
Adaptive Optics
Computer-controlled mirror support adjusts the mirror
surface (many times per second) to compensate for
distortions by atmospheric turbulence
Examples of Modern Telescope
Design (1)
Design of the Large
Binocular Telescope
The Keck I telescope mirror
Examples of Modern Telescope
Design (2)
The Very Large Telescope (VLT)
8.1-m mirror of the Gemini Telescopes
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 
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
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
Radio Astronomy
Recall: Radio waves of l ~ 1 cm – 1 m also
penetrate the Earth’s atmosphere and can be
observed from the ground.
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.
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!
Radio Interferometry (2)
The Very
Large Array
(VLA): 27
dishes are
combined to
simulate a
large dish of
36 km in
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)!
The Largest Radio Telescopes
The 300-m telescope in
Arecibo, Puerto Rico
The 100-m Green Bank Telescope in
Green Bank, WVa.
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.
Infrared Astronomy
Most infrared radiation is absorbed in the lower
from high
tops or highflying air
can still be
NASA infrared telescope on Mauna Kea, Hawaii
Space Astronomy
NASA’s Space Infrared Telescope
Facility (SIRTF)
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
• Several successful ultraviolet astronomy
satellites: IRAS, IUE, EUVE, FUSE
• Ultraviolet radiation traces hot (tens of
thousands of degrees), moderately ionized
gas in the Universe.
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.
Chandra X-ray
Gamma-Ray Astronomy
Gamma-rays: most energetic electromagnetic radiation;
traces the most violent processes in the Universe
The Compton
The Hubble Space Telescope
• Launched in 1990;
maintained and
upgraded by several
space shuttle service
missions throughout
the 1990s and early
• Avoids
turbulence in
the Earth’s
• Extends
imaging and
to (invisible)
infrared and
New Terms
electromagnetic radiation
Nanometer (nm)
Angstrom (Å)
infrared radiation
ultraviolet radiation
atmospheric window
focal length
refracting telescope
reflecting telescope
primary lens, mirror
objective lens, mirror
chromatic aberration
achromatic lens
light-gathering power
resolving power
diffraction fringe
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
New Terms (continued)
charge-coupled device (CCD)
false-color image
comparison spectrum
radio interferometer
cosmic ray
Discussion Questions
1. Why does the wavelength response of the human
eye match so well the visual window of Earth’s
2. Most people like beautiful sunsets with brightly
glowing clouds, bright moonlit nights, and twinkling
stars. Most astronomers don’t. Why?
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
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
a. X-ray & Gamma-ray
b. Ultraviolet & Infrared
c. Visible & Ultraviolet
d. Microwave & Radio
*e. Visible & Radio
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
c. The cat sleeps all day and is wide awake at night.
*d. To increase light gathering power.
e. To attract a mate.
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.
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
d. Large glass lenses are more difficult to support than large
*e. All of the above.
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.
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.
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.
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.
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.
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.
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.
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
e. The light gathering power depends on the focal length of the
eyepiece also.
Quiz Questions
14. What do the newer light-sensitive electronic CCD chips do
better than the older photographic plates coated with lightsensitive chemicals?
a. They have a greater sensitivity to light.
b. They can detect both bright and dim objects in a single
c. Photometry can be done with the CCD images.
d. The CCD images are easier to manipulate.
*e. All of the above.
Quiz Questions
15. What can radio telescopes do that optical telescopes
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.
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.
Quiz Questions
17. Radio telescopes are often connected together to do
interferometry. What is the primary problem overcome by radio
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.
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
*d. Both a and b above.
e. Both a and c above.
Quiz Questions
19. Why must far-infrared telescopes be cooled to a low
*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.
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.