Chapter 5 Light and Matter: Reading Messages from the Cosmos

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Chapter 5Light and Matter Reading Messages
from the Cosmos
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Chapter 5 Light

• Learning goals
• Properties of light
• Light as waves and particles -
• Energy, Frequency, Wavelength, speed
• The electromagnetic spectrum
• Interactions with matter - energy levels
• Doppler effect

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How do we experience light?

• Light is a form of energy
• Flux of energy watts (m.k.s. unit)
• 1 watt 1 joule/sec
• In c.g.s. 1 joule 107 erg
• Thus, 1 Watt 107 erg/sec

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Colors of Light

• Colors of white light

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How do light and matter interact?

• Emission
• Absorption
• Transmission
• Transparent objects transmit light
• Opaque objects block (absorb) light
• Reflection or Scattering

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Reflection and Scattering
Mirror reflects light in a particular direction
Movie screen scatters light in all directions
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Thought QuestionWhy is a rose red?

• The rose absorbs red light.
• The rose transmits red light.
• The rose emits red light.
• The rose reflects red light.

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Thought QuestionWhy is a rose red?

• The rose absorbs red light.
• The rose transmits red light.
• The rose emits red light.
• The rose reflects red light.

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Properties of Waves

• Wavelength is the distance between two wave peaks
• Frequency is the number of times per second that
  a wave vibrates up and down
• wave speed wavelength x frequency

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Light Electromagnetic Waves

• A light wave is a vibration of electric and
  magnetic fields
• Light interacts with charged particles
  (electrons)
• Electromagnetic wave wave causes charges to
• oscillate.

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Wavelength and Frequency

• wavelength x frequency speed of light c
• c is an absolute constant, independent of motion!

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Particles of Light

• Particles of light are called photons
• Each photon has a wavelength and a frequency
• The energy of a photon depends on its frequency

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Wavelength, Frequency, and Energy

• l x f c
• l wavelength , f frequency
• c 3.0 x 108 m/s 3.0 x 1010 cm/sec
• speed of light
• E h x f photon energy
• h 6.626 x 10-34 joule x s photon energy
• In c.g.s. h 6.626 x 10-27

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The electromagnetic spectrum
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The Electromagnetic Spectrum
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Thought QuestionThe higher the photon energy

• the longer its wavelength.
• the shorter its wavelength.
• energy is independent of wavelength.

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Thought QuestionThe higher the photon energy

• the longer its wavelength.
• the shorter its wavelength.
• energy is independent of wavelength.

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5.3 Properties of Matter

• Learning goals
• What is the structure of matter?
• What are the phases of matter
• How is energy stored in atoms?
• How does light interact with matter?

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Structure of matter
Electron Cloud
Nucleus
Atom
10-8 cm 10-13 cm Size of
atoms Size of protons
neutrons
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Atomic Terminology

• Atomic Number of protons in nucleus
• Atomic Mass Number of protons neutrons

• Molecules consist of two or more atoms (H2O,
  CO2)

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Atomic Terminology

• Isotope same of protons but different of
  neutrons. (4He, 3He)

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Phases of matter

• Familiar phases
• Solid (ice)
• Liquid (water)
• Gas (water vapor)
• Plasma (ionized gas)

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Phases of Water
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Phase Changes

• Ionization Stripping of electrons, changing
  atoms into plasma
• Dissociation Breaking of molecules into atoms
• Evaporation Breaking of flexible chemical bonds,
  changing liquid into solid
• Melting Breaking of rigid chemical bonds,
  changing solid into liquid

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How is energy stored in atoms?
1 electron Volt (1 eV) 1.6 x10-13 erg
Hydrogen (H)
Excited States
Ground State

• Electrons in atoms are restricted to particular
  energy levels

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Energy Level Transitions

• The only allowed changes in energy are those
  corresponding to a transition between energy
  levels

Allowed
Not Allowed
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What have we learned?

• Electrons in atoms can only have descrete energy
  levels.
• Atoms gain and lose energy only in amounts
  corresponding to changes in energy levels.
• Atoms (and molecules and ions)
• gain or lose energy by absorbing or emitting
• photons with specific energies, frequencies,
• and wavelengths.
• E h x f where E is the energy
  difference
• between energy
  levels
• E hc / ? because ? f
  c
• 
  f c/ ?

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5.4 Learning from Light

• 3 types of spectra
• Light tells us what things are made of
• Light traces temperatures
• Interpretation of spectra

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What are the three basic types of spectra?
Continuous Spectrum
Emission Line Spectrum
Absorption Line Spectrum
Spectra of astrophysical objects are usually
combinations of these three basic types
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Three Types of Spectra
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Continuous Spectrum

• The spectrum of a common (incandescent) light
  bulb spans all visible wavelengths, without
  interruption

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Emission Line Spectrum

• A thin or low-density cloud of gas emits light
  only at specific wavelengths that depend on its
  composition and temperature, producing a spectrum
  with bright emission lines

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Absorption Line Spectrum

• A cloud of gas between us and a light bulb can
  absorb light of specific wavelengths, leaving
  dark absorption lines in the spectrum

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How does light tell us what things are made of?
Spectrum of the Sun
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Chemical Fingerprints

• Each type of atom has a unique set of energy
  levels
• Each transition corresponds to a unique photon
  energy, frequency, and wavelength

Energy levels of Hydrogen
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Chemical Fingerprints

• Downward transitions produce a unique pattern of
  emission lines

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Chemical Fingerprints

• Because those atoms can absorb photons with those
  same energies, upward transitions produce a
  pattern of absorption lines at the same
  wavelengths

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Chemical Fingerprints

• Each type of atom has a unique spectral
  fingerprint

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Chemical Fingerprints

• Observing the fingerprints in a spectrum tells us
  which kinds of atoms are present

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Example Solar Spectrum
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Energy Levels of Molecules

• Molecules have additional energy levels because
  they can vibrate and rotate

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Energy Levels of Molecules

• The large numbers of vibrational and rotational
  energy levels can make the spectra of molecules
  very complicated
• Many of these molecular transitions are in the
  infrared part of the spectrum

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Thermal Radiation

• All large objects emit thermal radiation,
  including stars, planets, you
• An objects thermal radiation spectrum depends on
  only one property its temperature

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Properties of Thermal Radiation

• Hotter objects emit more light at all frequencies
  per unit area.
• Hotter objects emit photons with a higher average
  energy.

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Thought QuestionWhich is hotter?

• A blue star.
• A red star.
• A planet that emits only infrared light.

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Thought QuestionWhich is hotter?

• A blue star.
• A red star.
• A planet that emits only infrared light.

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Thought QuestionWhy dont we glow in the dark?

• People do not emit any kind of light.
• People only emit infrared light that is invisible
  to our eyes.
• People only emit radio waves.
• People do not contain enough radioactive material.

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Thought QuestionWhy dont we glow in the dark?

• People do not emit any kind of light.
• People only emit infrared light that is invisible
  to our eyes.
• People only emit radio waves.
• People do not contain enough radioactive material.

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What is this object?
Carbon Dioxide Absorption lines are the
fingerprint of CO2 in the atmosphere. Strongest
absorption is at ? 14 to 15 ?m This
absorption prevents the escape of some thermal
infrared radiation emitted by the ground to
space More CO2 gt warmer Earth!
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How does light tell us the speed of a distant
object?
The Doppler Effect
Change in f / f Change in ? /?? V / c
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Measuring the Shift
Stationary
Moving Away
Away Faster
Moving Toward
Toward Faster

• We generally measure the Doppler Effect from
  shifts in the wavelengths of spectral lines

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Doppler shift tells us ONLY about the part of an
objects motion toward or away from us
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Thought QuestionI measure a line in the lab at
500.7 nm.The same line in a star has wavelength
502.8 nm. What can I say about this star?

• It is moving away from me.
• It is moving toward me.
• It has unusually long spectral lines.

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Thought QuestionI measure a line in the lab at
500.7 nm.The same line in a star has wavelength
502.8 nm. What can I say about this star?

• It is moving away from me.
• It is moving toward me.
• It has unusually long spectral lines.

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How does light tell us the rotation rate of an
object?

• Different Doppler shifts from different sides of
  a rotating object spread out its spectral lines

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Spectrum of a Rotating Object

• Spectral lines are wider when an object rotates
  faster

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What have we learned?

• How does light tell us the speed of a distant
  object?
• The Doppler effect tells us how fast an object is
  moving toward or away from us.
• Blueshiftobjects moving toward us
• Redshift objects moving away from us
• How does light tell us the rotation rate of an
  object?
• The width of an objects spectral lines can tell
  us how fast it is rotating

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Chapter 6Telescopes Portals of Discovery
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6.1 Eyes and Cameras Everyday Light Sensors

• Our goals for learning
• How does your eye form an image?
• How do we record images?

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How does your eye form an image?
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Refraction

• Refraction is the bending of light when it passes
  from one substance into another
• Your eye uses refraction to focus light

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Example Refraction at Sunset

• Sun appears distorted at sunset because of how
  light bends in Earths atmosphere

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Focusing Light

• Refraction can cause parallel light rays to
  converge to a focus

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Image Formation

• The focal plane is where light from different
  directions comes into focus
• The image behind a single (convex) lens is
  actually upside-down!

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How do we record images?
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Focusing Light
Digital cameras detect light with charge-coupled
devices (CCDs)

• A camera focuses light like an eye and captures
  the image with a detector
• The CCD detectors in digital cameras are similar
  to those used in modern telescopes

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6.2 Telescopes Giant Eyes

• Collect as much light as possible
• Diameter of primary mirror or lens D
• Area of collecting mirror or lens ? (D /
  2)2
• Resolve small details in images
• ?????? / D (in radians)
• There are 206,265 2 x 105 arc-seconds in
  a radian
• Obtain spectra
• Telescopes can work at all wavelengths

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What are the two most important properties of a
telescope?

• Light-collecting area Telescopes with a larger
  collecting area can gather a greater amount of
  light in a shorter time.
• Angular resolution Telescopes that are larger
  are capable of taking images with greater detail.

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Light Collecting Area

• A telescopes diameter tells us its
  light-collecting area Area p(diameter/2)2
• The largest telescopes currently in use have a
  diameter of about 10 meters

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Bigger is better
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Thought QuestionHow does the collecting area of
a 10-meter telescope compare with that of a
2-meter telescope?

• Its 5 times greater.
• Its 10 times greater.
• Its 25 times greater.

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Thought QuestionHow does the collecting area of
a 10-meter telescope compare with that of a
2-meter telescope?

• Its 5 times greater.
• Its 10 times greater.
• Its 25 times greater.

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Angular Resolution

• The minimum angular separation that the telescope
  can distinguish.

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Angular Resolution

• Ultimate limit to resolution comes from
  interference of light waves within a telescope.
• Larger telescopes are capable of greater
  resolution because theres less interference

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Angular Resolution

• Ultimate limit to resolution comes from
  interference of light waves within a telescope.
• Larger telescopes are capable of greater
  resolution because theres less interference

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Angular Resolution

• The rings in this image of a star come from
  interference of light wave.
• This limit on angular resolution is known as the
  diffraction limit

Close-up of a star from the Hubble Space Telescope
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What are the two basic designs of telescopes?

• Refracting telescope Focuses light with lenses
• Reflecting telescope Focuses light with mirrors

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Refracting Telescope

• Refracting telescopes need to be very long, with
  large, heavy lenses

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Reflecting Telescope

• Reflecting telescopes can have much greater
  diameters
• Most modern telescopes are reflectors

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Designs for Reflecting Telescopes
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Mirrors in Reflecting Telescopes
Twin Keck telescopes on Mauna Kea in Hawaii
Segmented 10-meter mirror of a Keck telescope
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What do astronomers do with telescopes?

• Imaging Taking pictures of the sky
• Spectroscopy Breaking light into spectra
• Timing Measuring how light output varies with
  time

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Imaging

• Astronomical detectors generally record only one
  color of light at a time
• Several images must be combined to make
  full-color pictures

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Imaging

• Astronomical detectors can record formsof light
  oureyes cant see
• Color is sometimes used to represent different
  energies of nonvisible light

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Spectroscopy

• A spectrograph separates the different
  wavelengths of light before they hit the detector

Diffraction grating breaks light into spectrum
Light from only one star enters
Detector records spectrum
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Spectroscopy

• Graphing relative brightness of light at each
  wavelength shows the details in a spectrum

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Timing

• A light curve represents a series of brightness
  measurements made over a period of time

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Want to buy your own telescope?

• Buy binoculars first (e.g. 7x35) - you get much
  more for the same money.
• Ignore magnification (sales pitch!)
• Notice aperture size, optical quality,
  portability.
• Consumer research Astronomy, Sky Tel, Mercury.
  Astronomy clubs.

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What have we learned?

• What are the two most important properties of a
  telescope?
• Collecting area determines how much light a
  telescope can gather
• Angular resolution is the minimum angular
  separation a telescope can distinguish
• What are the two basic designs of telescopes?
• Refracting telescopes focus light with lenses
• Reflecting telescopes focus light with mirrors
• The vast majority of professional telescopes are
  reflectors

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What have we learned?

• What do astronomers do with telescopes?
• Imaging
• Spectroscopy
• Timing

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6.3 Telescopes and the Atmosphere

• Our goals for learning
• How does Earths atmosphere affect ground-based
  observations?
• Why do we put telescopes into space?

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How does Earths atmosphere affect ground-based
observations?

• The best ground-based sites for astronomical
  observing are
• Calm (not too windy)
• High (less atmosphere to see through)
• Dark (far from city lights)
• Dry (few cloudy nights)

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Light Pollution

• Scattering of human-made light in the atmosphere
  is a growing problem for astronomy

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Twinkling and Turbulence
Star viewed with ground-based telescope
Same star viewed with Hubble Space Telescope

• Turbulent air flow in Earths atmosphere
  distorts our view, causing stars to appear to
  twinkle

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Adaptive Optics
Without adaptive optics
With adaptive optics

• Rapidly changing the shape of a telescopes
  mirror compensates for some of the effects of
  turbulence

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Calm, High, Dark, Dry

• The best observing sites are atop remote mountains

Summit of Mauna Kea, Hawaii
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Why do we put telescopes into space?
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Transmission in Atmosphere

• Only radio and visible light pass easily through
  Earths atmosphere
• We need telescopes in space to observe other forms

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What have learned?

• How does Earths atmosphere affect ground-based
  observations?
• Telescope sites are chosen to minimize the
  problems of light pollution, atmospheric
  turbulence, and bad weather.
• Why do we put telescopes into space?
• Forms of light other than radio and visible do
  not pass through Earths atmosphere.
• Also, much sharper images are possible because
  there is no turbulence.

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6.4 Eyes and Cameras Everyday Light Sensors

• Our goals for learning
• How can we observe nonvisible light?
• How can multiple telescopes work together?

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How can we observe nonvisible light?

• A standard satellite dish is essentially a
  telescope for observing radio waves

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Radio Telescopes

• A radio telescope is like a giant mirror that
  reflects radio waves to a focus

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IR UV Telescopes
SOFIA
Spitzer

• Infrared and ultraviolet-light telescopes operate
  like visible-light telescopes but need to be
  above atmosphere to see all IR and UV wavelengths

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X-Ray Telescopes

• X-ray telescopes also need to be above the
  atmosphere

Chandra
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X-Ray Telescopes

• Focusing of X-rays requires special mirrors
• Mirrors are arranged to focus X-ray photons
  through grazing bounces off the surface

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Gamma Ray Telescopes

• Gamma ray telescopes also need to be in space
• Focusing gamma rays is extremely difficult

Compton Observatory
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How can multiple telescopes work together?
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Interferometry

• Interferometery is a technique for linking two or
  more telescopes so that they have the angular
  resolution of a single large one

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Interferometry

• Easiest to do with radio telescopes
• Now becoming possible with infrared and
  visible-light telescopes

Very Large Array (VLA)
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Future of Astronomy in Space?

• The Moon would be a good observing site
• - no atmosphere!
• Deep space is even better
• - no gravity
• - can see most of
• the sky at once!
• James Web Space
• Telescope JWST
• D 6.5 meter
• infrared telescope
• launch in 2013 .