observatories and telescopes mauna kea, hawaii (14,000 ft) why do telescopes need to be located at...
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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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Collecting Area
Lens
Retina
Eye could be a “refracting” telescope,but the size is very small
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Collecting Area = Power• A = d2 ; 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
Objective Lens
Secondary Lens (Eyepiece)
Focus
Convex 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
Light bends towards the perpendicular going into denser medium, and vice-versa
Normal(Perpendicular)
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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 = ‘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
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Chromatic Aberration:Different 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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PrimaryMirrorPrime
Focus
Secondary Mirror
Cassegrain Focus
REFLECTING TELESCOPES
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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 m = 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
Unresolved Resolved
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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)