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8.4-m Mirror Blank for Large Binocular Telescope - PowerPoint PPT Presentation

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8.4-m Mirror Blank for Large Binocular Telescope. Polishing one LBT 8.4-m mirror. TELESCOPES. Palomar 200-in. The Electromagnetic Spectrum. Functions of Telescopes. Collect more light --- depends on (diam) 2 Resolve sources better (see more detail) Magnify images.

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functions of telescopes
Functions of Telescopes
  • Collect more light --- depends on (diam)2
  • Resolve sources better (see more detail)
  • Magnify images
telescope objectives specially shaped main optical element
Telescope “Objectives”: Specially Shaped Main Optical Element

Purpose: form an accurate representation of original scene at a “focus”

  • Lens ---> “refracting” telescope
  • Mirror ---> “reflecting” telescope
refracting telescope
Refracting Telescope

Minimum 2 lenses needed for visual use

Place detector here

telescope performance characteristics
Telescope Performance Characteristics
  • Focal Ratio (f/ number)
  • Magnification ("power")
  • Field of view
  • Light Gathering Power
  • Resolution
focal ratio
Focal Ratio
  • f/ number = Obj FL / Obj Diam
  • Smaller numbers give more concentrated light in focal plane (better for faint extended objects); allow shorter exposures with film/electronic detectors
  • Higher numbers have better resolution; better for high magnification (e.g. for planets)
  • Defined to be ratio of apparent angular size of image to original angular size (without telescope)
  • Mag = FL (telescope) / FL (eyepiece)
  • For Celestrons, Mag = 2034 mm/FLE (mm)
  • Moderate magnifications (<150) best
field of view
Field of View
  • True angular diameter -- i.e. as viewed without telescope -- of field visible in eyepiece.
  • Usually quoted in degrees or minutes of arc
  • Depends on eyepiece used
  • Is smaller for higher magnification with given telescope
light gathering power
Light Gathering Power
  • Most important attribute of telescope
  • Light collected is proportional to the area of the objective, or to Dobj2
  • If the pupil diameter of your eye is 5mm, an 8" telescope collects (203/5)2 = 1600x more light

Table by

D. Haworth

image quality resolution
Image Quality (Resolution)
  • Design optics to reduce "aberrations" -- e.g. chromatic, spherical, etc.
  • Optical figuring to intended shape: must be better than “1/4 wavelength”
  • Larger telescopes better because of “diffraction” of light waves
  • Turbulence in air strongly affects image blur. “Seeing” = size of blur.
“Schmidt-Cassegrain” design uses a thin refractive corrector to eliminate spherical aberration from a spherically-shaped primary
Video of enlarged image of bright star

in a large telescope. Image size/motion caused by

Earth’s atmosphere.

telescope designs a multitude
Telescope Designs: A Multitude
  • Optical design
  • Mounting design
    • Equatorial
    • Altitude-Azimuth

Dedication (1948)

(Largest equatorial mount for

optical telescope; "horseshoe")

McCormick 26-in


Equatorial mount