Introduction to Optical and Radio Telescopes

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Introduction to Optical and Radio Telescopes

• TIP 2008

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Purpose of Telescopes

1. Collect light from faint objects
2. Resolve objects that are near each other
3. Magnify the view of an object

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Optical Telescopes

• Reflectors
• Refractors

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Optics

• Lens directs light by bending of light into
  media of differing densities
• Focus point at which light is directed
• Focal plane two-D area where image is formed
• Field of view area on the sky images cover

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Refractors

• Use the bending of light entering denser media to
  direct light to a focus
• Largest is the 100 inch Yerkes refractor

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Reflectors

• Two main types
• Newtonian telescope
• Cassegrain telescope
• Light can be redirected with other optics

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Newtonian Telescope
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Cassegrain Telescope
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Advantages of Refractors

• Superior image quality if optics are
  well-designed
• Chromatic aberration deteriorates images if
  poorly constructed

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Reflectors Advantages

• Need to figure and polish only one surface
• Mirror is at the bottom so structurally more
  stable
• No chromatic aberration
• Reflectors are shorter for the same aperture so
  construction costs are lower

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Telescope Mounts

• Alt-Az mounts (altitude azimuth)
• Equatorial (lined up with north, celestial
  equator)

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Video

• Gemini Vitrual Tour
• What kind of telescope is it?
• What sort of detectors?
• What sort of mount?

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Karl Guthe JanskyFounder of Radio Astronomy

• Hired by Bell Labs in the late 1920s, Janskys
  mission was to find sources of radio interference

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• Jansky constructed a directional 20.5 MHz antenna
  on a turntable to locate radio noise source
  positions
• Sources of noise
• Nearby storms
• Distant storms
• A faint hiss that returned every 23 hours 56
  minutes

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Tools of Radio Astronomy

• Your car radio is an example of a simple antenna
  and receiver
• Radio waves actually cause free electrons in
  metals to oscillate!
• Radio receivers amplify these oscillations, so,
  radio telescopes measure the voltage on the sky

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Receiver Feed Horn
Example signal path of a radio telescope
Amplifier
Mixer
Antenna Control
Spectrometer
Control Computer
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• Spectrometer Output
• Spectrum brightness vs. radio frequency
• Continuum total brightness over all frequencies

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• Radio waves are VERY weak!
• Radio brightness measured in units of Janskys
• 1 Jansky (Jy) 10-26 W/m2/Hz
• Typical sources
• Sun 10,000s of Jy
• Brightest Supernova Remnant 1000s of Jy
• Active Galactic Nuclei 10-100 Jy

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The Ideal Radio Telescope

• Directional antennae, such as those with
  reflectors, isolate the radio power from single
  sources to reduce confusing radiation from others
• Low temperature receivers are more sensitive
• Large collecting areas increase gain and
  resolution
• Resolution roughly 57.3 ?/D degrees (?
  observing wavelength, D diameter of aperture)

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• Optical telescopes have an advantage on radio
  telescopes in angular resolution
• A one meter optical telescope has a resolution of
  0.1 seconds of arc.
• Since radio telescopes cannot be built large
  enough to match optical resolution, they can be
  combined as an interferometer to emulate a large
  single dish

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• At 21-cm wavelengths, PARIs 26-m and Smiley (4.6
  m) have resolutions of 0.5 and 2.5 degrees
  respectively

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Greenbank (WV) 100-m telescope in has a
resolution of 7 arc-minutes
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300-m telescope in Arecibo, Puerto Rico
(resolution 2.4 arcminutes)
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Very Large Array, resolution 1.4 arc seconds
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10 Antennas of the Very Long Baseline Array
(resolution 5 milli-arcseconds)