Download 9 - Ohio State Astronomy

Observatories and Telescopes
Mauna Kea, Hawaii (14,000 ft)
Why do telescopes need to be located at high altitude and dry climate ?
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 ?
Retina
Collecting Area
Eye could be a “refracting” telescope,
but the size is very small
Lens
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
Large Binocular Telescope
Mount Graham, Arizona
Objective and eyepiece
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
Properties of Light and Telescopes
• Reflection  Mirrors
• Refraction  Lenses
Simple Refracting Telescope
Convex lens
Focus
Objective Lens
Secondary Lens
(Eyepiece)
Spheres of light from distant source
 parallel rays at the observer
Different speeds in different media
 Bending or Refraction
Refraction of light beam
Normal
(Perpendicular)
Light bends towards the perpendicular going into denser medium, and vice-versa
Refraction by prism and lens
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
Chromatic Aberration:
Different colors at different focus
Chromatic aberration affects refracting telescopes;
therefore use reflecting telescopes in modern observatories
Law of Reflection: Angle i = Angle r
REFLECTING TELESCOPES
Prime
Focus
Secondary
Mirror
Primary
Mirror
Cassegrain
Focus
Reflecting Concave Mirror
Telescopic Configurations
Spherical and Parabolic Mirrors
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
Visible (Optical) and Radio
“Windows” in the Atmosphere
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
Radio Telescope
Aracebo Radio Telescope(Puerto Rico)
Rotational Speed and Doppler Shift
(Line profile broadens on both the blue and red side)
Doppler Radar
Hubble Space Telescope
Ground and HST images
Unresolved
Resolved
Visible and IR images of Saturn
Gamma-Ray View of the Sky
Each wavelength band presents a different and mutually complementary view
Telescope and Instruments
Intensities of Lines in Absorption
Spectra: Atoms absorb energy
Emission spectra are a set of bright lines: atoms emit energy
Observing Planets and Moons
Spectra of Titan (Moon of Saturn):
Methane (CH4)