Light and Telescopes

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Light and Telescopes

• Paul J. Thomas
• Department of Physics and Astronomy
• UW - Eau Claire

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The Visible Sky
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Waves
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Wave Equation

• c f ?
• where
• f frequency in Hertz (Hz)
• ? wavelength in meters (m)
• c speed of light
• 299 792 458 m/s in a vacuum (exactly)

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Wave Equation

• All forms of EM radiation move at the same speed
  in a vacuum.
• Visible light waves typically have
• ? 500 ? 10-9 m 500 nm
• f 6 ? 1014 Hz
• FM radio waves typically have
• ? 3 m
• f 108 Hz 100 MHz

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Visible Light Spectrum
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Producing a Spectrum
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Refracting Telescopes
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Refracting Telescopes
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Yerkes Observatory

• Lake Geneva, Wisconsin.
• Largest refracting telescope, 1m diameter.
• Built in 1897.

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Telescope Diameter

• Building a telescope with the largest possible
  diameter increases
• light-gathering capability (ability to see
  fainter objects)
• resolution (ability to see finer details).
• Resolution is compromised by atmospheric
  turbulence (twinkling).

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Reflecting Telescopes
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Reflecting Telescopes
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Reflecting Telescopes
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Atmospheric EM Absorption
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Radio Telescopes
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The Radio Sky (480 MHz)
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Interferometers

• Signals from many telescopes can be combined to
  provide a single signal.
• To do this, we need to know the distance between
  the telescopes (baseline) to a fraction of a
  wavelength.
• This enables us to have the resolving power of a
  big telescope, but not the light-gathering power.

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Very Long Baseline Interferometry (VLBA)
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Keck Telescope
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Keck Telescope
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NASAs Great Observatory Program

• Hubble Space Telescope
• launched 1990
• Compton Gamma Ray Observatory
• launched 1991, reentered atmosphere 2000
• Chandra X-ray Observatory
• launched 1999
• Spitzer Infrared Telescope
• launched 2003

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Compton Gamma Ray Observatory
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The Gamma Ray Sky
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Hubble Space Telescope

• 2.4 m Richley-Cretien reflecting telescope
• Orbital Altitude of 600 km
• Launched in 1990
• Every day, Hubble archives 3 to 5 gigabytes of
  data and delivers between 10 and 15 gigabytes to
  astronomers all over the world.

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HST Servicing Missions

• Space shuttle crews service the HST every few
  years.
• In the most recent servicing mission
  (STS-109/Columbia), the crew installed the
  Advanced Camera for Surveys (ACS), which can
  operate in ultraviolet light.
• HST also has an infrared camera (NICMOS)

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Hubble Deep Field Image
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Chandra X-ray Observatory
Schematic of Grazing Incidence, X-ray Mirrors
CXC
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The X-Ray Sky
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Chandra X-ray Observatory

• X-rays reveal details of hot (1 million degree )
  gases.
• Reveals presence of dark matter around galaxies.

Chandra image of Crab Nebula Pulsar
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Spitzer Infrared Telescope
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The Infrared Sky
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Blackbody Spectra

• The wavelengths of light emitted by a star depend
  on the stars surface temperature.
• The hotter the star, the smaller the wavelength
  of the peak radiation.
• The Suns surface temperature is 5,800 K and it
  mostly emits visible light.

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Spectroscopy

• The study of spectra the components of light
  from a star.
• A spectrum can be produced by a prism or a
  diffraction grating at the focus of a telescope.
• Stars have absorption spectra that are
  characteristic of their composition.

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Three Types of Spectra
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Three Types of Spectra
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Solar Spectrum
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The Parsec

• Parsec parallax second.
• A unit of interstellar distance.
• If a star was 1 parsec away, it would appear to
  move 1 second of arc (1/36000) in six months.
• 1 parsec 3.26 light years.

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Orion parallax, with a baseline of 1.5 light years
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