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Chapter 5
Light and Telescopes
Guidepost
In the early chapters of this book, you looked at the sky the way ancient
astronomers did, with the unaided eye. In this chapter, you will see how
modern astronomers use telescopes and other instruments to gather and
focus light and its related forms of radiation. That will lead you to answer
five essential questions about the work of astronomers:
• What is light?
• How do telescopes work, and how are they limited?
• How do astronomers record and analyze light?
• Why must some telescopes go into space?
Guidepost (continued)
Astronomy is almost entirely an observational science, so astronomers
must think carefully about the limitations of their instruments. That will
introduce you to an important question about scientific data:
• How do we know? What limits the detail you can
see in an image?
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. New-Generation Telescopes
D. Interferometry
III. Astronomy from Space
A. The Ends of the Visual Spectrum
B. Telescopes in Space
I. Radiation: Information from
Space
Common Misconception:
For some people, radiation means threat.
So what is radiation?
Anything that radiates from a source.
A. Light and Other Forms of Radiation
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.
What is light as a whole?
Light as a Wave (1)
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
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.
B. The Electromagnetic Spectrum
Wavelength
Frequency
Need satellites
to observe
High
flying air
planes or
satellites
Another Misconception
Radio Waves are related to sound.
No.
Radio waves are a type of light
Scientific Argument
• What would you see if your eyes were
sensitive only to X-rays?
• Would be in the dark if your eyes were
sensitive only to radio wavelengths?
Just to make sure…..
Most waves require a material medium in which to be
transmitted:
* water waves travel along the surface of water
* sound waves move through air
* earthquake waves propagate through the solid earth
* e.m waves may propagate through a pure vacuum at
the speed of light c.
Summary (1)
• Light is a form of electromagnetic wave.
• Complete electromagnetic spectrum includes: gamma rays,
X-rays, ultraviolet (UV), visible light, infrared (IR),
microwaves, and radio waves.
• Earth’s atmosphere is transparent in only two atmospheric
windows: visible light and radio.
II. Optical Telescopes
Video Trailer:
Eyes on the Sky – 400 Years of
Telescopic Discoveries
II. Optical Telescopes
Astronomers use
telescopes to gather
more light from
astronomical objects.
The larger the
telescope, the more
light it gathers.
A. Two kinds of Optical Telescopes: Refracting/Reflecting
Focal length
Refracting
Telescope:
Lens focuses
light onto the
focal plane
Reflecting
Telescope:
Concave Mirror
Focal length
focuses light
onto the focal
plane
Almost all modern telescopes are reflecting telescopes.
Secondary Optics
In reflecting
telescopes:
Secondary
mirror, to redirect the light
path towards the
back or side of
the incoming light
path.
Eyepiece:
To view and
enlarge the
small image
produced in
the focal plane
of the primary
optics.
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
Review Questions
1. ____________ has (have) wavelengths that are longer than visible
light.
a. Gamma-rays
b. Ultraviolet light
c. Infrared radiation
d. X-rays
2. Chromatic aberration occurs in a __________telescope when
a. reflecting; different colors of light do not focus at the same point.
b. refracting; different colors of light do not focus at the same point.
c. reflecting; light of different wavelengths get absorbed by the mirror.
d. refracting; light of different wavelengths get absorbed by the lens.
3. List two main reasons why a reflecting telescope is better than a
refracting telescope.
Chapter Summary (2)
• There are two types of optical telescopes: refracting
and reflecting telescopes.
• Refracting telescopes use a primary lens, and a
reflecting telescope use a primary mirror.
• Refracting telescopes suffer from chromatic
aberration and expensive to make.
• Reflecting telescopes are easier to build and less
expensive than refracting telescopes of the same
diameter.
B. The Powers of a Telescope
1. Light-gathering power
2. Resolving power
3. Magnifying power
The Powers of a Telescope (1) : Size Does Matter
Video 2: Bigger is Better
1. Light-gathering
power:
Depends on the surface
area A of the primary lens /
mirror, proportional to
radius squared:
A = p R2
D = 2R
Small Telescope
Large Telescope
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)
For optical wavelengths, this gives
amin = 11.6 arcsec / D[cm]
amin
Andromeda galaxy using telescopes of different resolutions
10'
1'
5"
1"
The smaller the resolution,
the better is the image.
Seeing
Weather
conditions
and
turbulence in
the
atmosphere
set further
limits to the
quality of
astronomical
images.
Bad seeing
Good seeing
Cause of Bad Seeing – Jupiter Comparison
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!
Another Misconception
• The purpose of a telescope is to magnify
images.
• Magnifying power is the least important of
the three powers.
The Best Location for a Telescope
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
C. New-Generation Telescopes
Traditional Telescopes (1)
Secondary mirror
Traditional primary mirror: sturdy,
heavy to avoid distortions
Traditional Telescopes (2)
The 4-m
Mayall
Telescope at
Kitt Peak
National
Observatory
(Arizona)
Advances in Modern Telescope Design (1)
Modern computer technology has made significant
advances in telescope design possible:
Active Optics: Video 3
Segmented mirror
1. Lighter mirrors with
lighter support
structures, to be
controlled dynamically
by computers
Floppy mirror
Adaptive Optics
Computer-controlled mirror support adjusts the mirror surface
(many times per second) to compensate for distortions by
atmospheric turbulence
Adaptive optics
using laser technology.
to correct for
atmospheric smearing
Advances in Modern Telescope Design (2)
2. Simpler, stronger mountings (“Alt-azimuth mountings”)
to be controlled by computers
Examples of Modern Telescope Design (1)
Design of the
Large Binocular
Telescope (LBT)
Examples of Modern Telescope Design (2)
The Very Large Telescope (VLT)
8.1-m mirror of the Gemini Telescopes
Chapter Summary (3)
Light-Gathering-Power
(LGP)refers
is proportional
the area
• Light-gathering
power
to thetoability
ofofathe
lens or the mirror,
i.e., LGP 
D2
telescope
to produce
bright
images.
• Resolving
power
to the
abilitytoofthea linear
telescope
to
Resolving-Power
(RP) refers
is inversely
proportional
size of the
lens or thefine
mirror,
i.e., RP  1/D -- Resolving power:
resolve
detail.
• Magnifying
power,
the
toratio
make
object
Magnifying-Power
(MP or
M) ability
is just the
of thean
focal
lengthlook
of
the objective
focal length telescope
of the lens or power.
the mirror, i.e.,
bigger,
is aover
lesstheimportant
M=fo/fe
• Adaptive optics techniques involve measuring seeing
distortions caused by Earth’s atmosphere and
corrected by using laser technologies.
Let us compare…..
A student named “Antar” has a reflecting telescope with a
diameter of 20 cm having a focal length of 100 cm equipped
with an eyepiece of focal length of 0.4 cm. Another student
named “Kais” has a refracting telescope with a diameter of
10 cm having a focal length of 100 cm and equipped with an
eyepiece of focal length of 0.2 cm.
1.Antar’s telescope has better light gathering power, but less
magnifying power than Kais’s telescope
2.Antar’s telescope has less light gathering power, but better
magnifying power than Kais’s telescope
3.Kais’s telescope has better light gathering power, but less
magnifying power than Antar’s telescope
4.Kais’s telescope has less light gathering power, and less
magnifying power than Antar’s telescope
D. 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
Examples….
Very Large Array
Keck Observatory
27 Radio Telescopes
NM (USA)
Hawaii (USA)
10m mirror
ESO Paranal
Observatory
8.2m mirror
CCD Imaging: Video 4
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
CCD Chip: Charge-coupled device
Scientific Argument
• Why do astronomers build optical observatories
at the tops of mountains?
• What considerations do astronomers make in
choosing the location of a new radio telescope?
Chapter Summary (4)
• Interferometry refers to the technique of connecting
two or more separate telescopes to act as a single
large telescope.
• Modern electronic systems such as charge-coupled
devices (CCDs) have replaced both photographic
plates and photometers.
III. Astronomy from Space
III. Astronomy from Space
• Radiation that cannot reach Earth’s surface:
– Infrared
– Ultraviolet
– X-rays
– Gamma ray
Need Space Telescopes
Video 5
Radio Infrared Visible Ultraviolet X-rays Gamma
Infrared Astronomy
• Most infrared radiation is absorbed in the lower atmosphere.
NASA infrared
telescope on Mauna
Kea, Hawaii
Infrared cameras need
to be cooled to very low
temperatures, usually
using liquid nitrogen.
However, from high
mountain tops or
high-flying air planes,
some infrared
radiation can still be
observed.
NASA’s Space Infrared Telescope Facility (SIRTF)
Infrared light with wavelengths much longer
than visible light (“Far Infrared”) can only be
observed from space.
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:
IUE, EUVE, FUSE
• Ultraviolet radiation traces hot (tens of thousands of
degrees), moderately ionized gas in the Universe.
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
Gamma-Ray Astronomy
Gamma-rays: most energetic electromagnetic radiation;
traces the most violent processes in the Universe
The Compton
Gamma-Ray
Observatory
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
False Color Images
Chapter Summary (1)
• Light is a form of electromagnetic wave.
• Complete electromagnetic spectrum includes: gamma rays,
X-rays, ultraviolet (UV), visible light, infrared (IR),
microwaves, and radio waves.
• Earth’s atmosphere is transparent in only two atmospheric
windows: visible light and radio.
• There are two types of optical telescopes: refracting and
reflecting telescopes.
• Refracting telescopes use a primary lens, and a reflecting
telescope use a primary mirror.
Chapter Summary (2)
• Refracting telescopes suffer from chromatic aberration and
expensive to make.
• Reflecting telescopes are easier to build and less expensive
than refracting telescopes of the same diameter.
• Light-gathering power refers to the ability of a telescope to
produce bright images.
• Resolving power refers to the ability of a telescope to resolve
fine detail.
• Magnifying power, the ability to make an object look bigger,
is a less important telescope power.
Chapter Summary (3)
• Modern electronic systems such as charge-coupled devices
(CCDs) have replaced both photographic plates and
photometers.
• Adaptive optics techniques involve measuring seeing
distortions caused by Earth’s atmosphere and corrected by
using laser technologies.
• Interferometry refers to the technique of connecting two or
more separate telescopes to act as a single large telescope.
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