Observatories and Telescopes Mauna Kea, Hawaii (14,000 ft)

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Observatories and TelescopesMauna Kea, Hawaii
(14,000 ft)
Why do telescopes need to be located at high
altitude and dry climate ?
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Telescopes Mirrors and Lenses

• Telescopes are basically a large mirror
  (reflecting) or a lens (refracting)
• Consider the human eye as telescope
• What determines the power of a telescope ?

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Retina
Lens
Eye could be a refracting telescope, but the
size is very small
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Collecting Area Power

• A p ( d / 2)2 d diameter
• Diameter of the telescope indicates its power
• Largest optical telescope Keck 1 and 2, each
  with a mirror of 10 m diameter
• Large Binocular Telescope (LBT) Ohio State,
  Arizona, Germany, Italy ? Two 8.4 m mirrors in a
  binocular shaped mount
• HST Only 2.4 m, but with a huge advantage
• How much more powerful than HST is the Keck
  (neglecting that advantage) ?
• New 30m telescope on the drawing board

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Large Binocular TelescopeMount Graham, Arizona
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Objective and eyepiece
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Telescope ? Objective and Eyepiece

• The main function of a telescope is to collect
  as much light as possible from the source, NOT to
  magnify an image
• Need bigger and bigger telescopes!
• The main mirror or lens of a telescope is called
  the OBJECTIVE
• The Eye-Piece (small lens or mirror) is to
  magnify the image after it is formed from the
  light collected by the objective

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Properties of Light and Telescopes

• Reflection ? Mirrors
• Refraction ? Lenses

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Simple Refracting Telescope
Convex lens
Secondary Lens (Eyepiece)
Objective Lens
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Spheres of light from distant source ? parallel
rays at the observer
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Different speeds in different media? Bending or
Refraction
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Refraction of light beam
Normal (Perpendicular)
Light bends towards the perpendicular going into
denser medium, and vice-versa
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Refraction by prism and lens
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Refractive Index

• Speed of light slows down in a medium !
• The ratio of the speed of light in vacuum to the
  speed in a medium
• c / v m
• mu is called the Refractive Index
• Material R.I.
• Water 1.33
• Glass 2.6
• Bending of light (diffraction) depends on R.I of
  the medium and wavelength of light l

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Chromatic AberrationDifferent colors at
different focus
Chromatic aberration affects refracting
telescopes therefore use reflecting telescopes
in modern observatories
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Law of Reflection Angle i Angle r
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REFLECTING TELESCOPES
Primary Mirror
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Reflecting Concave Mirror
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Telescopic Configurations
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Spherical and Parabolic Mirrors
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Wavelength range of observatories and telescopes

• Ground based telescopes can measure
• - Visible (4000-7000 A),
• - Near-IR (0.7-2 microns), 1 mm 10000 A
• - Radio ( 1 mm or greater)
• All other wavelengths blocked out by the
  atmosphere
• Space based observatories for Gamma ray, X-ray,
  UV, and Far-IR astronomy

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Visible (Optical) and Radio Windows in the
Atmosphere
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Radar and Radio Astronomy

• Radio telescopes (like huge satellite dish)
  collect radio waves from astronomical objects
• Radar telescope Transmitter Radio Telescope
  
• Doppler Radar ? Transmits radio waves towards
  an object and collects reflected radio waves
  spread in signal shows distance and velocity
• Largest radio telescope is the Arecibo, 1000 ft
  diameter, in Puerto Rico

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Radio Telescope
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Aracebo Radio Telescope(Puerto Rico)
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Rotational Speed and Doppler Shift(Line profile
broadens on both the blue and red side)
Doppler Radar
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Hubble Space Telescope
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Ground and HST images
Resolved
Unresolved
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Visible and IR images of Saturn
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Gamma-Ray View of the Sky
Each wavelength band presents a different and
mutually complementary view
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Telescope and Instruments
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Intensities of Lines in Absorption Spectra Atoms
absorb energy
Emission spectra are a set of bright lines atoms
emit energy
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Observing Planets and Moons
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Spectra of Titan (Moon of Saturn)Methane (CH4)