Light and Telescopes: Extending Our Senses

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Light and Telescopes Extending Our Senses

• How Telescopes Work

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5.1.The Basis of Optics

• Reflection Law The light striking a mirror
  travels in such a way that the two angles,
  between each beam and the mirror are equal.

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Refraction Law (Snell, 1621)
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Refraction Law (continued)

• where n is called refraction index. The speed
  of light in a material with refraction index n
  is
• v c/n, (n gt 1)
  

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Fermat's Principle of Least Time (1650) (an
initial form of this principle has been suggested
by Heron of Alexandria)

• Out of all possible paths that it might take
  to get from one point to another, light takes the
  path which requires the shortest time.

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Reflection and Reflection from Fermat's Principle
lifeguard.pdf
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5.2. Types of TelescopesDesign of lenses using
Fermats principle
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Design of mirrors using Fermats principle
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Refracting telescope (Gallieo 1609)
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Reflecting telescope (Newton)
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Cassegrain reflecting telescope
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5.4. Comparison and problems

• Refractos have (a) chromatic aberation
• The white-light images are tinged with
  color,because
• different wave lenghts bent differently
• (b) Lenses are difficult to make big enough, (
  c) besides they absorbs light. The tubes should
  be very long. The largest refractor 1 m, at
  Yerks observatory of Wisconsin
• Refractors and reflectors have spherical
  abberation
• Parallel light is not brought into a good focus,
  depending on its difference from the center the
  focus is shifted.

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Comparison

• Paraboloid mirrors escape spherical aberation
• Huge reflectors are possible, the tube can be
  shorter due to additional lenses and/or mirrors
• Palomar 5m, California
• Caucasus 6m, Russia
• Gemini 8 m (Hawaii, Chile)
• Kecks 10 m (Mauna Kea in Hawaii)
• Texas 9.2 m telescope
• ESO Very Large Telescope (four 8 m)

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5.3. Characteristics of Telescopes

• Diameter of the objective D
• Focal length, f, the distance over which the
  objective brings light to focus.
• Focal ratio .
• The smaller focal ratios the brighter image at
  the focus.

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Characteristics of Telescopes

• Light-gathering power
• Example comparison between a telescope with 8
  meters mirror and unaided eye. LGP.pdf
• Resolution this is the most important
  characteristic of a telescope. By definition the
  resolution is the smallest angle discernable by
  the telescope. The resolution is limited mostly
  by the shimmering of earths atmosphere.
  Neglecting the effect of the atmosphere we define
  gt

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Theoretical Resolution
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Magnifying power (MP)

• Defined as the ratio of the focal length of the
  objective to the focal length of the eyepiece
• (least important characteristic)

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Transparency of the earths atmosphere
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Outside the Visible Spectrum

• X-ray and Gamma ray telescopes
• on satellites (HEAOs, Chandra X ray observatory
  (nested mirrors), Compton Gamma Ray
  Observatory,Integral
• Ultraviolet range telescopes
• International Ultraviolate Explorer
• 2.4 m Hubble Telescope. Far Ultraviolet
  Spectrographic Explorer

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Outside the visible

• Infrared telescopes
• Ground based At high altitude sites (Mauna Kea)
  , dry 2MASS whole sky mapped
• Airoplane SOFIA with 2.5 m telescope
• On sattelites Infrared Astronomical Satellite
  (IRAS) , COBE, WMAP
• Radio Telescopes
• 330 m in Arecibo, Puerto Rico (poor rsolution)
• Very Large Array (VLA) 27 telescopes 26 m,New
  Mexico (spanning kms) operate together
• Very Long baseline Array (spanning the whole USA)
  better resolution due to interferometry

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ConcepTest Why do x-ray telescopes have to be
put above the earth's atmosphere?

• (a) Not to depend on the weather conditions
• (b) X-rays do not penetrate earth's atmosphere.
• (c) To increase their resolution.
• (d) To increase their magnification.

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Modern Technologies

• Detectors charge-coupled devise (CCD).
• Computer image processing.
• Radio and optical interferometers.
• ESO VLT (array of 4 telescopes 8m) in Chile
• Wide filed telescopes (B. Schmidt 1930)
• in California and Australia, Sloan Digital Sky
  Survay (New Mexico) survays the sky as the Earth
  turns.
• Adaptive optics (compensate for the shimmering of
  Earths atmosphere).

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Adaptive optics
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ConcepTest Which of the following is NOT an
advantage of radio telescopes over optical
telescopes?

• They can be used in cloudy weather.
• They have greater resolution for similar size
  objectives.
• They allow to study objects optically invisible.
• They are easier to construct than large optical
  telescopes.