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Amateur and Professional
Galileo 1609
The Moon as a World
Jupiter has Moons
Refracting telescopes
Long focus refractors were awkward but suffered less from
                   chromatic aberration
    Isaac Newton’s reflecting telescope

Mirrors do not have
chromatic aberration
Reflecting telescope

Objective mirrors instead of lenses
            Three Powers
• Magnifying
• Resolving
• Light Gathering
          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
          Resolving Power
• Ability to see fine detail
• Depends on the diameter of the objective
  lens or mirror
      Light Gathering Power
• The ability to make faint objects look
• 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
Herschel & Lord Rosse
19th century: epoch of the large
     Refracting telescopes


Largest refracting
telescope with a
one meter objective
20th century Large Reflectors Come
               of Age

Mount Wilson Observatory 1.5m (1908) and 2.5m (1918)
       Palomar 5-m
(entered operation in 1948)
4 meter
Objective Mirror
Dome of 4 meter
Kitt Peak
Keck Telescopes
SOAR Telescope

       4.1 meter
SOAR Telescope -- Cerro Pachon
SOAR Observing Room
SOAR Image of the planetary nebula NGC 2440
MSU Campus Observatory
Boller & Chivens reflecting telescope with a 24-
             inch objective mirror
               More on resolution
• Eagle-eyed Dawes
• The Dawes Limit
  R = 4.56/D

  R = resolution in seconds of
  D = diameter of objective in
  More appropriate for visible
    light and small telescopes
 A more general expression for the
    theoretical resolving power
• Imagine that star
  images look like Airy
     Minimum Angle that can be
• 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
• 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
          Astronomical “seeing”
• Blurring effect of looking
  through air
• Causes stars to twinkle
  and planetary detail to

   – 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
Bad seeing on   Good seeing
this side       on this side
Electromagnetic Spectrum
Radio Telescopes
Very Large Array
    Radio telescope resolution
l = 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
Very Large Array
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
      Observing from space
• No clouds
• Perfect seeing
• Can see wavelengths of light blocked by
  the earth’s atmosphere
Hubble Space Telescope
Rooftop telescopes

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