Transcript Light and Telescope

Light and
Telescopes
Almost all astronomical
information is obtained
through the electromagnetic
radiation, i.e. light, we
receive from cosmic objects
Assignment
Chapter 6. All of it
Goals
1) To investigate the nature of light
2) To become familiar with the electromagnetic
spectrum
3) To introduce telescopes
4) To understand how we collect and study
light using telescope
5) All of this is covered in Chapter 6
What is light?
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Light is electromagnetic radiation, i.e. coupled electric and
magnetic fields that oscillate in strength and that propagate in
space while carrying energy.
Technically, light is the part of electromagnetic (e.m.) radiation
that humans (and other animals) see
Humans also sense (“see” with sense other than sight) other part
of the e.m. spectrum, like heat (through skin)
Although incorrectly, we usually call “light” all type of
electromagnetic radiation, like X-ray light or UltraViolet light
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Light really is a small portion of the spectrum of e.m. radiation
Types of e.m. radiation differ from each other by wavelengths
• Blue light: short wavelength; red: long one
• X-ray: very short wavelength; radio: very long one
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Identical situation with sound pitch
• High pitch: short wavelength; bass: long one
What is Electromagnetic
Radiation?
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Made of propagating waves of electric and
magnetic field
It carries energy with it
• Sometimes called “radiant energy”
• Think – solar power, photosynthesis,
photo-electric cells, the fireplace …
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It also carries information
• the signal received by your car radio
• the signals received by telescopes staring at
stars
• the signals received by your eyes right now!
What is the electromagnetic wave?
It is electricity and magnetism moving through space.
So, when we say the speed of light is “c” what we really mean
is that the speed of the electromagnetic wave is “c”, regardless
of its frequency
Light as a wave
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Waves you can see:
e.g., ocean waves
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Waves you cannot
see:
• sound wave
• electromagnetic waves
Light is an electromagnetic wave
Light as a Wave
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
Properties of Waves
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Wavelength – the
distance between
crests (or troughs)
of a wave.
For light in general:
Frequency – the
speed = c = s/t = λ 
number of crests
λ=c
(or troughs) that
λ = c/
pass by each
second.
frequency
Speed – the rate at wavelength
which a crest (or
speed of light = 3x105 km/s
trough) moves.
in vacuum
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 comes in quanta of energy
called photons – little bullets of
energy.
• Photons are massless, but they
have momentum and and energy.
• They also react to a gravitational
field (because they follow the
curved space-time).
Wave-particle duality
All types of electromagnetic radiation act as both
waves and particles.
The two views are connected by the relation
E = h = h c / l
h is the Planck's constant, c is the speed of light
 is the frequency, l is the wavelength
The energy of a photon does not depend
on the intensity of the light!!!
Intensity
A photon's energy depends on the wavelength (or frequency)
only, NOT the intensity.
But the energy you experience depends also on the intensity
(total number of photons).
It turns out that particles of matter,
such as electrons, also behave as both
wave and particle.
The theory that describes these puzzles
and their solution, and how light and
atoms interact is quantum mechanics.
In Summary: properties of Light
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All light travels through (vacuum) space
with a velocity = 3x105 km/s
The frequency (or wavelength) of photon
determines how much energy the photon
has:
The number of photons (how many)
determines the intensity
Light can be described in terms of either
energy, frequency, or wavelength.
Visible Light
Shorter
Wavelength
Longer
Wavelength
Remember: visible light isn’t the whole story. It’s
just a small part of the entire electromagnetic
spectrum
Short Wavelength
Long Wavelength
(high frequency)
(high energy)
(low frequency)
(low energy)
Wavelengths and size of things
Example of Electromagnetic
Radiation
Short wavelength
Long wavelength
If light is thermally generated, by a heated body,
the dominant color reflects the temperature of the
body
Compared to visible light, radio
waves have:
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higher energy and longer wavelength
higher energy and shorter
wavelength
lower energy and longer wavelength
lower energy and shorter wavelength
all light has the same energy
The Multi-wavelength Sun
Radio
infrared
optical
X-ray
Optical Sky
Near-infrared sky
Boldt et al.
Radio Sky
Soft X-ray Sky
Different wavelength carry different
type of information
• Visible light: the
glow of stars (dust
blocks light)
• Infrared: the glow of
dust
Visible light (top) and infrared
(bottom) image of the
Sombrero Galaxy
Matter interacts with light in four
different ways:
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Absorption – the energy in the photon is absorbed
by the matter and turned into thermal energy
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Reflection – no energy is transferred and the
photon “bounces” off in a new (and predictable)
direction
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E.g., Your hand feels warm in front of a fire.
E.g., Your bathroom mirror
Transmission – no energy is transferred and the
photon passes through the matter unchanged.
Emission – matter gives off light in two different
ways. We’ll come back to this next lecture.
Our eyes work via the process of:
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transmission
reflection
absorption
emission
none of the above
A red ball is red because:
it only emits frequencies
corresponding to red
it only reflects frequencies
corresponding to red
it only transmits frequencies
corresponding to red
it only absorbs frequencies
corresponding to red
Telescopes
The largest optical telescopes in the world:
The twin 10-m Keck telescopes (Hawaii)
The Hubble
Space Telescope
(HST)
An ultraviolet
(1000-3500) Ang,
Optical
(3500-8500) Ang,
and near-infrared
(8500-16000) Ang
telescope
The Five College
Radio Astronomy
Observatory
(now defunct)
UMass
LMT
The 50-m Large Millimeter
Telescope
The largest millimeterwavelength telescope in the
world
U Mass and Mexico
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
What telescopes are for?
Why do they need to be big?
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The main feature of a telescope is its capacity to collect as much
light as possible
• Like an antenna: the stronger the signal the clearest the transmission.
• Well, guess what: an antenna *is* a telescope (a radio telescope, that
is)
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The larger the light collector, I.e. the primary mirror or lens, the
more powerful the telescope (Light Gather Power= LGP)
• LGP ~ 4 p D2
• LGPA/LGPB = (DA/DB)2
• A telescope twice as large collects four times as much light
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The other primary feature is image sharpness, to faithfully
reproduce details
• Resolving power: a = 11.6/D
The last, and least important, feature is magnification
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
Seeing
Seeing
Deep Imaging of the sky:
at the edge of the Universe
To study galaxy formation both space-based sensitivity and angular resolution required!!
Note how many more details and faint objects can be observed with the Hubble Space Telescope
Ground Telescope Subaru + SUPREME
Space: HST + ACS
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!
Telescopes do not have to be
only Optical
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Different wavelengths carry different type of
information (optical: stars; X-ray: black hole; infrared:
dust; radio: gas)
To detect different wavelengths of light, eg. X-ray, UV,
optical, infrared, radio, different technologies are
required
For example, special mirrors are necessary for X-ray
telescopes or else the radiation would pass through
them.
Hence, it is necessary to specialize telescopes to the
wavelength of light one wishes to study.
We X-ray, UV, optical, infrared and radio telescopes
Different locations for telescopes
In addition, the Earth’s atmosphere affects light of different
wavelengths differently:
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As a consequence some telescopes can operate on the ground:
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optical, near-infrared, radio
Some can only work in space
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It totally absorbs X-ray and UV light: X-ray and UV telescopes
MUST be placed in space
It blurs the optical light, I.e. it destroys sharpness.
It also adds the glare of the night sky (yup! There is such thing) to
optical and infrared light, which makes faint sources hard to see.
It totally absorbs some (important) infrared light
X-ray, UV, mid- and far-infrared
For high-resolution (super-sharp) observations, or for
observations of very faint sources (i.e. to avoid the glare of the
Earth’s atmospherer) either space telescopes or very advanced
technologies (adaptive optics) are required.
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
Most wavelengths cannot penetrate the
Earth's atmosphere
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-
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.
Why different wavelengths are
required
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Regardless of the technology, different
wavelengths carries different information:
• Shorter wavelengths carry information on
very energetic phenomena (e.g. black holes,
star formation)
• Optical wavelengths carry information on the
structures of galaxies and their motions (the
assembly of the bodies of galaxies, their size)
• Longer wavelengths carry information on the
chemical composition, physical state (gas
and dust, presence, chemical elements;
temperature)
Telescope Instruments
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Cameras:
• To obtain images at desired wavelength or
wavelengths (color images)
• This yields the morphology, size of the sources
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Spectrographs:
• To study the intensity of the various
wavelengths (colors)
• This yields the physical nature (star, galaxy,
balck hole), chemical composition, physical
properties (temperature, density), dynamics
(motions, mass), distance of the sources
Variability
(change with time)
There are three basic aspects of
the light from an object that
we can study from the Earth.
Intensity
(spatial distribution of the light)
Spectra
(composition of the object
and the object’s velocity)
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 .
Spectral Lines of Some Elements
Argon
Helium
Mercury
Sodium
Neon
Spectral lines are like a cosmic barcode system for elements.
Traditional Telescopes
Secondary mirror
Traditional primary mirror: sturdy,
heavy to avoid distortions
Traditional Telescopes
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
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.
Advances in Modern Telescope Design (2)
2. Simpler, stronger mountings (“Alt-azimuth
mountings”)
to be controlled by computers
Examples of Modern Telescope
Design
Examples of Modern Telescope
Design
CCD Imaging
(no photographic films any longer
CCD = Charge-coupled device
• Much more sensitive
than photographic
plates (90% vs. 1%)
• Data can be read directly
into computer memory,
allowing easy electronic
manipulations and
analysis
Visible light (top) and infrared
(bottom) image of the
Sombrero Galaxy
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 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.
Life at the telescope. I
The telescope, before sunset
The MMT 6.5-m telescope, Univ. of Arizona
The trusty Night Assistant, who does all the work
Life at the telescope. II
The diligent Student,
who makes sure the work
is done right
The hard-working Professor, who bosses everybody around