ASTR 2310: Chapter 6

1
ASTR 2310 Chapter 6

• Astronomical Detection of Light
• The Telescope as a Camera
• Refraction and Reflection Telescopes
• Quality of Images
• Astronomical Instruments and Detectors
• Observations and Photon Counting
• Other Wavelengths
• Modern Telescopes

2
Refracting / Reflecting Telescopes
0
Refracting Telescope Lens focuses light onto the
focal plane
Focal length
Reflecting Telescope Concave Mirror focuses
light onto the focal plane
Focal length
Almost all modern telescopes are reflecting
telescopes.
3
Secondary Optics
0
In reflecting telescopes Secondary mirror, to
re-direct light path towards back or side of
incoming light path.
Eyepiece To view and enlarge the small image
produced in the focal plane of the primary optics.
4
Disadvantages of Refracting Telescopes
0

• 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.

5
The Best Location for a Telescope
0
Far away from civilization to avoid light
pollution
6
The Best Location for a Telescope (II)?
0
Paranal Observatory (ESO), Chile
http//en.wikipedia.org/wiki/Paranal_Observatory
On high mountain-tops to avoid atmospheric
turbulence (? seeing) and other weather effects
7
The Powers of a TelescopeSize does matter!
0
1. Light-gathering power Depends on the surface
area A of the primary lens / mirror, proportional
to diameter squared
D
T p (D/2)2
8
The Powers of a Telescope (II)?
0
2. Resolving power Wave nature of light gt The
telescope aperture produces fringe rings that set
a limit to the resolution of the telescope.
Astronomers cant eliminate these diffraction
fringes, but the larger a telescope is in
diameter, the smaller the diffraction fringes
are. Thus the larger the telescope, the better
its resolving power.
?min 1.22 (?/D) (radians)?
For optical wavelengths, this gives
?min
?min 11.6 arcsec / Dcm
The Spectrograph
A Multiwavelength Look at Cygnus A
NASAs Great Observatories in Space (III)?
Wavelengths and Colors
Chandra X-ray Observatory
Light as a Wave
Dark Side of the Moon
Atoms Electron Configuration
Secondary Optics
The Powers of a Telescope (II)?
Dark Side of the Moon
The Electromagnetic Spectrum
The Best Location for a Telescope (II)?
Radio Interferometry
Keplers Supernova with all three of NASAs Great
Observatories
Astronomical Telescopes
Adaptive Optics
Examples of Modern Telescope Design
Radio Astronomy
Infrared Astronomy
Spitzer Space Telescope
0
The Highest Tech Mirrors Ever!
Interferometry
Radio Telescopes
Seeing
CCD Imaging
NASAs Great Observatories in Space (I)?
0
0
0
Advances in Modern Telescope Design
Science of Radio Astronomy
NASAs Great Observatories in Space (IV)?
Disadvantages of Refracting Telescopes
Traditional Telescopes (II)?
The Largest Radio Telescopes
Spitzer Space Telescope Images
The Future of Space-Based Optical/Infrared
Astronomy
0
Kirchoffs laws
Hydrogen Lines
0
0
0
Refracting / Reflecting Telescopes
Ultraviolet Astronomy
Hubble Space Telescope Images
NASAs Great Observatories in Space (II)?
Light as Particles
Temperature and Heat
0
0
0
0
0
Temperature Scales
Planck Black Body Radiation
0
0
0
0
0
The Powers of a TelescopeSize does matter!
0
Infrared Telescopes
Planck and other Formulae
Example of Wiens law
0
The Best Location for a Telescope
0
0
0
0
0
0
0
0
Why is energy per photon so important?
0
0
Traditional Telescopes (I)?
0
0
Terrestrial Planet Finder
Light as a Wave
0
0
0
0
0

• Molecules Multiple atoms sharing/exchanging
  electrons (H2O, CH4)?
• Ions Single atoms where one or more
  electrons have escaped (H)?
• Binding energy Energy needed to let electron
  escape
• Permitted orbits or energy levels
• From quantum mechanics, only certain orbits are
  allowed
• Ground State Atom with electron in lowest
  energy orbit
• Excited State Atom with at least one atom in a
  higher energy orbit
• Transition As electron jumps from one energy
  level orbit to another, atom must
  release/absorb energy different, usually as
  light.
• Because only certain orbits are allowed, only
  certain energy jumps are allowed, and atoms can
  absorb or emit only certain energies
  (wavelengths) of light.
• In complicated molecules or solids many
  transitions are allowed
• Can use energy levels to fingerprint elements
  and estimate temperatures.

Using a prism (or a grating), light can be split
up into different wavelengths (colors!) to
produce a spectrum.
0

• Hot objects glow (emit light) as seen in
  PREDATOR, SSC Video, etc.
• Heat (and collisions) in material causes
  electrons to jump to high energy orbits, and as
  electrons drop back down, some of energy is
  emitted as light.
• Reason for name Black Body Radiation
• In a solid body the close packing of the atoms
  means than the electron orbits are complicated,
  and virtually all energy orbits are allowed. So
  all wavelengths of light can be emitted or
  absorbed. A black material is one which readily
  absorbs all wavelengths of light. These turn out
  to be the same materials which also readily emit
  all wavelengths when hot.
• The hotter the material the more energy it emits
  as light
• As you heat up a filament or branding iron, it
  glows brighter and brighter
• The hotter the material the more readily it emits
  high energy (blue) photons
• As you heat up a filament or branding iron, it
  first glows dull red, then bright red, then
  orange, then if you continue, yellow, and
  eventually blue

Light and Other Forms of Radiation

• Thermal energy is kinetic energy of moving
  atoms and molecules
• Hot material energy has more energy available
  which can be used for
• Chemical reactions
• Nuclear reactions (at very high temperature)?
• Escape of gasses from planetary atmospheres
• Creation of light
• Collision bumps electron up to higher energy
  orbit
• It emits extra energy as light when it drops back
  down to lower energy orbit
• (Reverse can happen in absorption of light)?

• Hot solids emit continuous spectra
• Hot gasses try to do this, but can only emit
  discrete wavelengths
• Cold gasses try to absorb these same discrete
  wavelengths

• Energy absorbed/emitted depends on upper and
  lower levels
• Higher energy levels are close together
• Above a certain energy, electron can escape
  (ionization)?
• Series of lines named for bottom level
• To get absorption, lower level must be occupied
• Depends upon temperature of atoms
• To get emission, upper level must be occupied
• Can get down-ward cascade through many levels

Just as for optical telescopes, the resolving
power of a radio telescope depends on the
diameter of the objective lens or mirror ?min
1.22 ?/D.
Most infrared radiation is absorbed in the lower
atmosphere.

• Want temperature scale with energy proportional
  to T
• Celsius scale is arbitrary (Fahrenheit even
  more so)?
• 0o C freezing point of water
• 100o C boiling point of water
• By experiment, available energy 0 at Absolute
  Zero 273oC (-459.7oF)?
• Define Kelvin scale with same step size as
  Celsius, but 0K -273oC Absolute Zero
• Use Kelvin Scale for most astronomy work
• Available energy is proportional to T, making
  equations simple (really! OK, simpler)?
• 273K freezing point of water
• 373K boiling point of water
• 300K approximately room temperature

Recall Resolving power of a telescope depends on
diameter D.
?
In reflecting telescopes Secondary mirror, to
re-direct light path towards back or side of
incoming light path.
CCD Charge-coupled device
The Spitzer Space Telescope

• Planck formula gives intensity of light at each
  wavelength
• It is complicated. Well use two simpler
  formulae which can be derived from it.
• Wiens law tells us what wavelength has maximum
  intensity
• Stefan-Boltzmann law tells us total radiated
  energy per unit area

• What is wavelength at which you glow?
• Room T 300 K so
• This wavelength is about 20 times longer than
  what your eye can see. Thermal camera operates
  at 7-14 µm.
• What is temperature of the sun which has
  maximum intensity at roughly 0.5 ?m?

The Chandra X-ray Telescope
Computer-controlled mirror support adjusts the
mirror surface (many times per second) to
compensate for distortions by atmospheric
turbulence
Recall Radio waves of ? 1 cm 1 m also
penetrate the Earths atmosphere and can be
observed from the ground.
2. Resolving power Wave nature of light gt The
telescope aperture produces fringe rings that set
a limit to the resolution of the telescope.

• Real life example Ultra-Violet light hitting
  your skin (important in Laramie!)?
• Threshold for chemical damage set by energy
  (wavelength) of photons
• Below threshold (long wavelengths) energy too
  weak to cause chemical changes
• Above threshold (short wavelength) energy
  photons can break apart DNA molecules
• Number of molecules damaged number of photons
  above threshold
• Very unlikely two photons can hit exactly
  together to cause damage

• Ultraviolet radiation with ? lt 290 nm is
  completely absorbed in the ozone layer of the
  atmosphere.

The Compton Gamma-Ray Observatory

• 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

Lighter mirrors with lighter support
structures, to be controlled dynamically by
computers
Radio astronomy reveals several features, not
visible at other wavelengths
The Hubble Space Telescope

• Launched in 1990 maintained and upgraded by
  several space shuttle service missions throughout
  the 1990s and early 2000s

Refracting Telescope Lens focuses light onto the
focal plane
Large dish focuses the energy of radio waves onto
a small receiver (antenna)?

• What is wrong with this picture?
• Front Not all primary colors (eg, pink,
  magenta), also refraction angles inconsistent
• Back Spectrum is Convergent I think done for
  arts sake

Often very large to gather large amounts of light.
The Electromagnetic Spectrum, Light, Astronomical
Tools

• Chromatic aberration Different wavelengths are
  focused at different focal lengths (prism effect).

Launched in 1999 into a highly eccentric orbit
that takes it 1/3 of the way to the moon!

• Wavelengths of light are measured in units of
  nanometers (nm) or angstrom (Å)

The Powers of a Telescope (III)?
0

• Just 400 years ago (Oct. 9, 1604)?
• Then a bright, naked eye object (no telescopes)?
• Its still blowing up now 14 light years wide
  and expanding at 4 million mph.
• Theres material there at MANY temperatures, so
  many wavelengths are needed to understand it.

Weather conditions and turbulence in the
atmosphere set further limits to the quality of
astronomical images
Spectral lines in a spectrum tell us about the
chemical composition and other properties of the
observed object
Wavelength

• More sensitive than photographic plates

However, from high mountain tops or high-flying
aircraft, some infrared radiation can still be
observed.
Operated from 1991 to 2000

• Neutral hydrogen clouds (which dont emit any
  visible light), containing 90 of all the
  atoms in the universe.

The Electromagnetic Spectrum
Floppy mirror
Launched in 2003
1. Light-gathering power Depends on the surface
area A of the primary lens / mirror, proportional
to diameter squared
Astronomers cant eliminate these diffraction
fringes, but the larger a telescope is in
diameter, the smaller the diffraction fringes
are. Thus the larger the telescope, the better
its resolving power.
3. Magnifying Power ability of the telescope to
make the image appear bigger.

• Ultraviolet astronomy has to be done from
  satellites.

X-rays trace hot (million degrees), highly
ionized gas in the universe.
? Combine the signals from several smaller
telescopes to simulate one big mirror ?
Interferometry
Focal length
Can be corrected, but not eliminated by second
lens out of different material.
In order to observe forms of radiation other than
visible light, very different telescope designs
are needed.
1 nm 10-9 m 1 Å 10-10 m 0.1 nm
Mars with its polar ice cap
E hf
Frequency
c 300,000 km/s 3108 m/s
A Comet
Front cover Back cover
Infrared light traces warm dust in the universe.
In astronomy, we cannot perform experiments with
our objects (stars, galaxies, ).
From our text Horizons, by Seeds
D

• Molecules (often located in dense clouds, where
  visible light is completely absorbed).

Reflecting Telescope Concave Mirror focuses
light onto the focal plane
Observation of high-energy gamma-ray emission,
tracing the most violent processes in the
universe.

• Avoids turbulence in Earths atmosphere

Warm dust in a young spiral galaxy
Eyepiece To view and enlarge the small image
produced in the focal plane of the primary optics.

• Data can be read directly into computer memory,
  allowing easy electronic manipulations

• Several successful ultraviolet astronomy
  satellites IUE, EUVE, FUSE

The 4-m Mayall Telescope at Kitt Peak National
Observatory (Arizona)?
For radio telescopes, this is a big problem
Radio waves are much longer than visible light
h 6.626x10-34 Js is the Planck constant.
A new VLT image of a possible planet around a
brown dwarf star.
A larger magnification does not improve the
resolving power of the telescope!

• Difficult and expensive to produce All surfaces
  must be perfectly shaped glass must be flawless
  lens can only be supported at the edges.

n3
Paranal Observatory (ESO), Chile

• Light waves are characterized by a wavelength
  ??and a frequency f.

?min 1.22 (?/D)?
Secondary mirror
The detector needs to be cooled to -273 oC (-459
oF).
n2
Two colliding galaxies, triggering a burst of
star formation
Visible light has wavelengths between 4000 Å and
7000 Å ( 400 700 nm).
Segmented mirror
The only way to investigate them is by analyzing
the light (and other radiation) which we observe
from them.
Very hot gas in a cluster of galaxies
http//en.wikipedia.org/wiki/Paranal_Observatory
Newborn stars that would be hidden from our view
in visible light

• Ultraviolet radiation traces hot (tens of
  thousands of degrees), moderately ionized gas in
  the universe.

• Radio waves penetrate gas and dust clouds, so we
  can observe regions from which visible light is
  heavily absorbed.

• Extends imaging and spectroscopy to (invisible)
  infrared and ultraviolet

Amplified signals are stored in computers and
converted into images, spectra, etc.
?min
High flying air planes or satellites

• Chandra is the first X-ray telescope to have
  image as sharp as optical telescopes.

The Very Large Telescope (VLT)
Traditional primary mirror sturdy, heavy to
avoid distortions.
Spitzer Space Telescope
A ? (D/2)2
The Very Large Array (VLA) 27 dishes are
combined to simulate a large dish of 36 km in
diameter.
The 100-m Green Bank Telescope in Green Bank,
West Virginia.
For optical wavelengths, this gives
The energy of a photon does not depend on the
intensity of the light!!!
Focal length

• f and ? are related through

Need satellites to observe
Different colors of visible light correspond to
different wavelengths.
My Bet Renamed after Carl Sagan. Will use both
interferometry and coronagraphs to image
Earth-like planets.

• Discovered by a Wyoming grad student and
  professor. The Cowboy Cluster an overlooked
  Globular Cluster.

A dust-filled galaxy
On high mountain-tops to avoid atmospheric
turbulence (? seeing) and other weather effects
The 300-m telescope in Arecibo, Puerto Rico
False-color image to visualize brightness contours
? Use interferometry to improve resolution!
Nebula around an aging star
Shuttle launched, highly eccentric orbit. Grazing
incidence mirrors nested hyperboloids and
paraboloids.
n1
From our text Horizons, by Seeds
The northern Gemini Telescope on Hawaii
?min 11.6 arcsec / Dcm
Far away from civilization to avoid light
pollution
The James Webb Space Telescope
f c/?

• There is no dark side really. Its all dark.
  -- Pink Floyd

Almost all modern telescopes are reflecting
telescopes.
WIRO 2.3m
Bad seeing
Good seeing

• A merger-product, and powerful radio galaxy.

NASA infrared telescope on Mauna Kea, Hawaii
More accurate, from Richard Berg
From our text Horizons, by Seeds
From our text Horizons, by Seeds
8.1-m mirror of the Gemini Telescopes
Saturn
9
The Powers of a Telescope (III)?
0

• 3. Magnifying Power ability of the telescope to
  make the image appear bigger.

A larger magnification does not improve the
resolving power of the telescope!
10
Traditional Telescopes (I)?
0
Secondary mirror
Traditional primary mirror sturdy, heavy to
avoid distortions.
11
Traditional Telescopes (II)?
0
The 4-m Mayall Telescope at Kitt Peak National
Observatory (Arizona)?
12
Astronomical Telescopes
0
Often very large to gather large amounts of light.
In order to observe forms of radiation other than
visible light, very different telescope designs
are needed.
The northern Gemini Telescope on Hawaii
13
Examples of Modern Telescope Design
0
The Very Large Telescope (VLT)
8.1-m mirror of the Gemini Telescopes
14
Seeing
0
Weather conditions and turbulence in the
atmosphere set further limits to the quality of
astronomical images
Bad seeing
Good seeing
15
Advances in Modern Telescope Design
Lighter mirrors with lighter support
structures, to be controlled dynamically by
computers
Floppy mirror
Segmented mirror
16
Adaptive Optics
0
Computer-controlled mirror support adjusts the
mirror surface (many times per second) to
compensate for distortions by atmospheric
turbulence
17
Interferometry
0
Recall Resolving power of a telescope depends on
diameter D.
? Combine the signals from several smaller
telescopes to simulate one big mirror ?
Interferometry
18
CCD Imaging
0
CCD Charge-coupled device

• More sensitive than photographic plates

• Data can be read directly into computer memory,
  allowing easy electronic manipulations

False-color image to visualize brightness contours
19
The Spectrograph
0
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
20
Radio Astronomy
0
Recall Radio waves of ? 1 cm 1 m also
penetrate the Earths atmosphere and can be
observed from the ground.
21
Radio Telescopes
0
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.
22
Radio Interferometry
0
Just as for optical telescopes, the resolving
power of a radio telescope depends on the
diameter of the objective lens or mirror ?min
1.22 ?/D.
For radio telescopes, this is a big problem
Radio waves are much longer than visible light
The Very Large Array (VLA) 27 dishes are
combined to simulate a large dish of 36 km in
diameter.
? Use interferometry to improve resolution!
23
The Largest Radio Telescopes
0
The 100-m Green Bank Telescope in Green Bank,
West Virginia.
The 300-m telescope in Arecibo, Puerto Rico
24
Science of Radio Astronomy
0
Radio astronomy reveals several features, not
visible at other wavelengths

• Neutral hydrogen clouds (which dont 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.

25
Infrared Astronomy
0
Most infrared radiation is absorbed in the lower
atmosphere.
However, from high mountain tops or high-flying
aircraft, some infrared radiation can still be
observed.
NASA infrared telescope on Mauna Kea, Hawaii
26
Infrared Telescopes
Spitzer Space Telescope
WIRO 2.3m
27
Ultraviolet Astronomy
0

• Ultraviolet radiation with ? lt 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.

28
NASAs Great Observatories in Space (I)?
0
The Hubble Space Telescope

• Launched in 1990 maintained and upgraded by
  several space shuttle service missions throughout
  the 1990s and early 2000s

• Avoids turbulence in Earths atmosphere

• Extends imaging and spectroscopy to (invisible)
  infrared and ultraviolet

29
Hubble Space Telescope Images
0
Mars with its polar ice cap
A dust-filled galaxy
Nebula around an aging star
30
NASAs Great Observatories in Space (II)?
0
The Compton Gamma-Ray Observatory
Operated from 1991 to 2000
Observation of high-energy gamma-ray emission,
tracing the most violent processes in the
universe.
31
NASAs Great Observatories in Space (III)?
0

• The Chandra X-ray Telescope

Launched in 1999 into a highly eccentric orbit
that takes it 1/3 of the way to the moon!
X-rays trace hot (million degrees), highly
ionized gas in the universe.
Two colliding galaxies, triggering a burst of
star formation
Very hot gas in a cluster of galaxies
Saturn
32
Chandra X-ray Observatory
Shuttle launched, highly eccentric orbit. Grazing
incidence mirrors nested hyperboloids and
paraboloids.
33
The Highest Tech Mirrors Ever!

• Chandra is the first X-ray telescope to have
  image as sharp as optical telescopes.

34
NASAs Great Observatories in Space (IV)?
0
The Spitzer Space Telescope
Launched in 2003
Infrared light traces warm dust in the universe.
The detector needs to be cooled to -273 oC (-459
oF).
35
Spitzer Space Telescope Images
0
A Comet
Warm dust in a young spiral galaxy
Newborn stars that would be hidden from our view
in visible light
36
Spitzer Space Telescope

• Discovered by a Wyoming grad student and
  professor. The Cowboy Cluster a new Globular
  Cluster.

37
Keplers Supernova with all three of NASAs Great
Observatories

• Just 400 years ago (Oct. 9, 1604)?
• Then a bright, naked eye object (no telescopes)?
• Its still blowing up now 14 light years wide
  and expanding at 4 million mph.
• Theres material there at MANY temperatures, so
  many wavelengths are needed to understand it.

38
A Multiwavelength Look at Cygnus A

• A merger-product, and powerful radio galaxy.

39
(No Transcript)
40
The Future of Space-Based Optical/Infrared
Astronomy
The James Webb Space Telescope