Transcript The Milky Way - Computer Science Technology

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Chapter 6
Light and Telescopes
Guidepost
In earlier chapters of this book, you looked at the sky
the way ancient astronomers did, with the unaided
eye. In the previous chapter, you got a glimpse through
Galileo’s telescope that revealed amazing things
about the moon, Jupiter, and Venus. Now you can
consider the telescopes, instruments, and techniques
of modern astronomers.
Telescopes gather and focus light, so you need to
study what light is, and how it behaves, on your way to
understanding how telescopes work. You will learn
about telescopes that capture invisible types of light
such as radio waves and X-rays. These enable
astronomers to put together a more complete view of
celestial objects.
Guidepost (continued)
This chapter will help you answer these five important
questions:
• What is light?
• How do telescopes work?
• What are the powers and limitations of telescopes?
• What kind of instruments do astronomers use to record
and analyze light gathered by telescopes?
•Why must some telescopes be located in space?
The science of astronomy is based on observations.
Astronomers cannot visit distant galaxies and far-off
worlds, so they have to study them using telescopes.
Twenty chapters remain in your exploration, and every one
will present information gained by astronomers using
telescopes.
Outline
I. Radiation: Information from Space
A. Light as Waves and as Particles
B. The Electromagnetic Spectrum
II. Telescopes
A. Two Ways to Do It: Refracting and Reflecting
Telescopes
B. The Powers and Limitations
III. Observatories on Earth: Optical and Radio
A. Modern Optical Telescopes
B. Modern Radio Telescopes
Outline (continued)
IV. Airborne and Space Observatories
A. Airborne Telescopes
B. Space Telescopes
C. High Energy Astronomy
V. Astronomical Instruments and Techniques
A. Imaging Systems and Photometers
B. Spectrographs
C. Adaptive Optics
D. Interferometry
VI. Nonelectromagnetic Astronomy
A. Particle Astronomy
B. Gravity Wave Astronomy
Radiation: Information from Space
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 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
Wavelength
Frequency
Need satellites
to observe
High
flying air
planes or
satellites
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
Refracting
Telescope:
Lens focuses
light onto the
focal plane
Reflecting
Telescope:
Concave Mirror
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).
• 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: 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 (Fp) and the
eyepiece (Fe):
M = Fp/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)
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
Advances in Modern Telescope Design (2)
2. Simpler, stronger mountings (“Alt-azimuth
mountings”)
to be controlled by computers
Examples of Modern
Telescope Design (1)
Examples of Modern Telescope
Design (2)
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.
The Largest Radio Telescopes
The 300-m telescope in
Arecibo, Puerto Rico
The 100-m Green Bank Telescope in
Green Bank, WVa
Observing Beyond the Ends of the
Visible Spectrum
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.
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
Infrared Astronomy from Orbit:
NASA’s Spitzer Space Telescope
Infrared light with wavelengths much longer
than visible light (“Far Infrared”) can only be
observed from space.
CCD Imaging
CCD = Charge-coupled device
• More sensitive
than photographic
plates
• Data can be read
directly into
computer memory,
allowing easy
electronic
manipulations
Visible light (top) and infrared
(bottom) image of the
Sombrero Galaxy
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 .
Adaptive Optics
Computer-controlled mirror support adjusts the mirror
surface (many times per second) to compensate for
distortions by atmospheric turbulence
A laser beam produces an artificial
star, which is used for the
computer-based seeing correction.
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
Radio Maps
Radio maps are often color coded:
Like different colors in
a weather map may
indicate different
weather patterns, …
… 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)!
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.