Transcript Chap 6

Chapter 6
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 do astronomers use radio telescopes?
• 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?
Fifteen chapters remain, and every one will discuss
information gathered by telescopes.
Outline
I. Radiation (輻射): Information from Space
A. Light as a Wave and a Particle
B. The Electromagnetic Spectrum (電磁光譜)
II. Optical Telescopes
A. Two Kinds of Telescopes
B. The Powers of a Telescope
C. Buying a Telescope
D. New-Generation Telescopes
E. Interferometry (干涉儀)
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. Astronomy from Space
A. The Ends of the Visual Spectrum
B. Telescopes in Space
C. Cosmic Rays
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.
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 mm = 10-3 m
1 μm = 10-6 m
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!!!
Intensity: 在單位時間 單位面積 單位角度 單位頻率 內 所接收(放出)的能量
The Electromagnetic Spectrum
Wavelength
Frequency
Need satellites
to observe
High
flying air
planes or
satellites
The Electromagnetic Spectrum
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
折射式望遠鏡
Refracting
Telescope:
Lens focuses
light onto the
focal plane
反射式望遠鏡
Reflecting
Telescope:
Concave Mirror
focuses light
onto the focal
Focal length
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 (色差):
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
material
The Powers of a Telescope:
Size Does Matter
1. Light-gathering
power (or
collecting
power):
Depends on the
surface area A of
the primary lens /
mirror,
proportional to
diameter
squared:
A = p (D/2)2
D
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
Seeing
Weather
conditions
and
turbulence in
the
atmosphere
set further
limits to the
quality of
astronomical
images.
Bad seeing
Good seeing
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!
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
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)
4m
Advances in Modern Telescope Design (1)
Modern computer technology has made
significant advances in telescope design
possible:
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
Advances in Modern Telescope Design (2)
2. Simpler, stronger mountings (“Alt-azimuth mountings”)
to be controlled by computers
Equatorial mounting: 赤道儀
Alt-azimuth mounting: 經緯儀
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
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
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
•
1966, Charles K. Kao made a discovery
that led to a breakthrough in fiber optics.
He carefully calculated how to transmit
light over long distances via optical glass
fibers. With a fiber of purest glass it would
be possible to transmit light signals over
100 kilometers, compared to only 20
meters for the fibers available in the
1960s.
•
In 1969 Willard S. Boyle and George E.
Smith invented the first successful
imaging technology using a digital sensor,
a CCD (Charge-Coupled Device). The
CCD technology makes use of the
photoelectric effect, as theorized by Albert
Einstein and for which he was awarded
the 1921 year's Nobel Prize. By this effect,
light is transformed into electric signals.
The challenge when designing an image
sensor was to gather and read out the
signals in a large number of image points,
pixels, in a short time. The CCD is the
digital camera's electronic eye. It
revolutionized photography, as light could
now be captured electronically instead of
on film.
See Press Release (新聞稿)
http://nobelprize.org/nobel_prizes/physics/laur
eates/2009/press.html
Astronomers who won
Nobel Physics Prizes
• 1967 Bathe - 核反應理論應用在恆星如何產生能量
• 1974 Ryle - 無線電干涉儀在天文上的應用
Hewish - 發現脈衝星 (或稱”波霎” pulsar) (1967)
• 1978 Penzias & Wilson - 發現宇宙微波背景輻射 (1964)
• 1983 Chandrasekhar - 恆星的結構與演化 （理論）
Fowler - 宇宙中元素的形成與演化（理論+實驗）
• 1993 Taylor & Hulse - 發現脈衝雙星, 驗證重力波理論
• 2002 Davis & Koshiba - 太陽微中子的探測
Giacconi - 建立觀測宇宙 X 射線的望遠鏡
• 2006 Mather & Smoot - 精確測量宇宙微波背景輻射與不均向性
CCD
see Wikipedia for animation
http://en.wikipedia.org/wiki/File:CCD_charge_transfer_animation.gif
Very Large Telescopes
optical fibers are used
to observe point
sources in astronomy
-- one fiber
corresponds to one
object
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
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 Maps
Radio maps are
often color coded:
Like different colors in
a seating chart of a
baseball stadium may
indicate different seat
prices, …
colors in a radio
map can
indicate different
intensities of the
radio emission
from different
locations on the
sky.
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
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)!
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 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
(Spitzer)
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
Cosmic Rays
• Radiation from space does not only
come in the form of electromagnetic
radiation (radio, …, gamma-rays)
• Earth is also constantly bombarded by
highly energetic subatomic particles
from space: Cosmic Rays
• The source if cosmic rays is still not
well understood.