Telescopes

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Telescopes

• Amateur and Professional

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Galileo 1609
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The Moon as a World
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Jupiter has Moons
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Refracting telescopes
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Long focus refractors were awkward but suffered
less from chromatic aberration
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Isaac Newtons reflecting telescope
Mirrors do not have chromatic aberration
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Reflecting telescope
Objective mirrors instead of lenses
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Three Powers

• Magnifying
• Resolving
• Light Gathering

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Magnifying Power

• Ability to make objects appear larger in angular
  size
• One can change the magnifying power of a
  telescope by changing the eyepiece used with it
• Mag Power focal length of objective divided by
  the focal length of the eyepiece

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Resolving Power

• Ability to see fine detail
• Depends on the diameter of the objective lens or
  mirror

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Light Gathering Power

• The ability to make faint objects look brighter
• Depends on the area of the objective lens or
  mirror
• Thus a telescope with an objective lens 2 inches
  in diameter has 4 times the light gathering power
  of a telescope with a lens 1 inch in diameter

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Herschel Lord Rosse
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19th century epoch of the large refractors
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Refracting telescopes
Lick
Vienna
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Yerkes Observatory
Largest refracting telescope with a one meter
objective
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20th century Large Reflectors Come of Age
Mount Wilson Observatory 1.5m (1908) and 2.5m
(1918)
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Palomar 5-m(entered operation in 1948)
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4 meter Reflecting telescope
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Objective Mirror
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Dome of 4 meter Kitt Peak
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Keck Telescopes
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SOAR Telescope
4.1 meter
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SOAR Telescope -- Cerro Pachon
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SOAR Observing Room
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SOAR Image of the planetary nebula NGC 2440
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MSU Campus Observatory
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Boller Chivens reflecting telescope with a
24-inch objective mirror
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More on resolution

• Eagle-eyed Dawes
• The Dawes Limit
• R 4.56/D
• Where
• R resolution in seconds of arc
• D diameter of objective in inches
• More appropriate for visible light and small
  telescopes

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A more general expression for the theoretical
resolving power

• Imagine that star images look like Airy disks

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Minimum Angle that can be resolved

• R 1.22 x 206,265 l / d
• R resolution in seconds of arc
• l wavelength of light
• d diameter of the objective lens or mirror
• Note that the wavelength of light and the
  diameter of the objective should be in the same
  units

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Examples

• For Visible light around 500nm
• Our 24-inch telescope
• R 0.20 seconds
• This may be compared with the Dawes limit of 0.19
  seconds
• But with large ground-based telescopes it is
  difficult to achieve this

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Astronomical seeing

• Blurring effect of looking through air
• Causes stars to twinkle and planetary detail to
  blur
• At the SOAR site good seeing means stellar
  images better than about 0.7 seconds of arc
• In Michigan, good seeing means better than about
  3 seconds of arc
• Not to be confused with good transparency

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Bad seeing on this side
Good seeing on this side
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Electromagnetic Spectrum
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Radio TelescopesArecibo
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Very Large Array
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Radio telescope resolution

• 1m d 100m
• R 2500 seconds 42 minutes!
• Even though radio telescopes are much bigger,
  their resolving power is much worse than for
  optical telescopes
• Interferometric arrays get around this

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Very Large Array
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Interferometry

• Size of array 10 km for a VLA
• This becomes the effective d
• Now R becomes 25 secsec for a
• 1-m wavelength
• For VLBI (very long baseline interfeormetry) the
  d 10,000km and R 0.025 seconds

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Observing from space

• No clouds
• Perfect seeing
• Can see wavelengths of light blocked by the
  earths atmosphere

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Hubble Space Telescope
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Rooftop telescopes