Astronomical Instruments

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Chapter 5

• Astronomical Instruments

How do we learn about objects too far away for
spacecraft? How do telescopes work? Do all
telescopes use light? Why are telescopes located
on mountains? Why are some telescopes in orbit
around the Earth?
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5.1 Telescopes

• Telescopes are devices for seeing distant
  objects.
• To see faint objects, collect more light by using
  a bigger aperture.
• Magnification is much, much less important.
• Two types of optical telescopes
• Refracting
• Reflecting

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Refracting Telescopes

• Use a large lens as the primary light collecting
  element.
• Common in older telescopes.
• Chromatic aberrations and ultraviolet cutoff.

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Reflecting Telescopes

• Use a large mirror as the primary light
  collecting element.
• Easier to make a large mirror than a large lens.
• Modern telescopes are reflectors.
• Reduced chromaticity problems.

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Refracting versus Reflecting
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Prime, Newton, and Cassegrain
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5.2 Optical Detectors and Instruments

• Second greatest advance in astronomy was the use
  of photographic film
• Longer integration time than the eye.
• Images available for later (re)inspection.
• The 3 uses of telescopes
• Imaging
• Measuring brightness and color
• Spectroscopy
• All benefit from longer integration time.

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Charge-Coupled Device

• A modern advance is the use of charge-coupled
  devices or CCDs.
• These are the digital film of digital cameras.

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5.3 Optical and Infrared Observatories

• Ground based telescopes are housed in domes or
  other structures to protect them when not in use.
• They are often located on remote mountains, away
  from lights, and smog, and above humid and
  turbulent air.

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NOAO on Kitt Peak, AZ
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South African Large Telescope
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European Southern Observatory
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Choice of Telescope Sites

• Weather clouds, wind, and rain limit observing
  time.
• Water vapor water vapor in the atmosphere
  filters out infrared wavelengths.
• Darkness city light can overwhelm faint stars.
• Turbulence turbulent air blurs images.

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Observatory Sites

• Newest telescopes are sited in the Chilean Andes,
  desert peaks of Arizona, Canary Islands, and
  Mauna Kea in Hawaii.
• Modern telescopes can use segmented mirrors,
  adaptive optics, and interferometry.

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Observing Other Radiation

• To learn the most about planets, stars, and
  galaxies, astronomers use as many types of
  radiations as possible.
• Radio
• Infrared and ultraviolet
• X-rays
• Gamma rays

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5.4 Radio Telescopes

• Many objects emit radio waves, from long wave to
  microwaves.
• Radio telescopes generally look like large
  satellite dishes.
• They can operate during the day and night.
• Large facilities in Puerto Rico, West Virginia,
  and New Mexico.

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The Very Large Array (VLA)
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The VLBA Spans A Continent
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Radio Image of Quasar Jets
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5.5 Observatories Outside the Earths Atmosphere

• Some wavelengths of radiation are absorbed by
  Earths atmosphere (gamma rays, x-rays, and
  ultraviolet).
• Telescopes for these wavelengths must be put in
  space.
• The Hubble telescope demonstrates the advantages
  for an optical telescope to be above the
  atmosphere.

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The Hubble Space Telescope
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The Swift Satellite (GRBs)