Advances in Modern Telescope Design (1)

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Advances in Modern Telescope Design (1)
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Modern computer technology has made significant
advances in telescope design possible
Segmented mirror
1. Lighter mirrors with lighter support
structures, to be controlled dynamically by
computers
Floppy mirror
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Advances in Modern Telescope Design (2)
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2. Simpler, stronger mountings (Alt-azimuth
mountings) to be controlled by computers
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Adaptive Optics
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Computer-controlled mirror support adjusts the
mirror surface (many times per second) to
compensate for distortions by atmospheric
turbulence
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CCD Imaging
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CCD Charge-coupled device

• More sensitive than photographic plates

• Data can be read directly into computer memory,
  allowing easy electronic manipulations

Negative image to enhance contrasts
False-color image to visualize brightness contours
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Examples of Modern Telescope Design (1)
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Design of the Large Binocular Telescope (LBT)
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Examples of Modern Telescope Design (2)
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The Very Large Telescope (VLT)
8.1-m mirror of the Gemini Telescopes
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Interferometry
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Recall Resolving power of a telescope depends on
diameter
This holds true even if not the entire surface is
filled out.

• Combine the signals from several smaller
  telescopes to simulate one big mirror ?
• Interferometry

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Radio Telescopes
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Large dish focuses the energy of radio waves onto
a small receiver (antenna)
Amplified signals are stored in computers and
converted into images, spectra, etc.
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Radio Maps
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Radio maps are often color coded
Like different colors in a seating chart of a
baseball stadium may indicate different seat
prices,
colors in a radio map can indicate different
intensities of the radio emission from different
locations on the sky.
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Radio Telescopes Advantages
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1. Can reveal clouds of cool hydrogen because the
emit a radio signal.
2.They can penetrate dust clouds that obscure
visible light.
3.They can detect the most distant objects in the
universe.
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Radio Interferometry
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For radio telescopes, resolving power is a big
problem since Radio waves are much longer than
visible light.
? Use interferometry to improve resolution!
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Radio Interferometry (2)
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The Very Large Array (VLA) 27 dishes are
combined to simulate a large dish of 36 km in
diameter.
Even larger arrays consist of dishes spread out
over the entire U.S. (VLBA Very Long Baseline
Array) or even the whole Earth (VLBI Very Long
Baseline Interferometry)!
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The Largest Radio Telescopes
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The 300-m telescope in Arecibo, Puerto Rico
The 100-m Green Bank Telescope in Green Bank, WVa.
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NASAs Space Infrared Telescope Facility (SIRTF)
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Infrared light with wavelengths much longer than
visible light (Far Infrared) can only be
observed from space.
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The Hubble Space Telescope
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• Launched in 1990 maintained and upgraded by
  several space shuttle service missions throughout
  the 1990s and early 2000s

• Avoids turbulence in the Earths atmosphere

• Extends imaging and spectroscopy to (invisible)
  infrared and ultraviolet

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Optical Telescopes
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Astronomers use telescopes to gather more light
from astronomical objects.
The larger the telescope, the more light it
gathers.
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The Powers of a TelescopeSize Does Matter
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• Light-gathering power Depends on the surface
  area A of the primary lens / mirror.
• Area depends on diameter.
• More light collected means you can see fainter
  objects.

D
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The Powers of a Telescope (2)
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2. Resolving power the ability to make out fine
detail. The larger the telescope the more
resolving power.

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Seeing
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Weather conditions and turbulence in the
atmosphere set further limits to the quality of
astronomical images.
Bad seeing
Good seeing
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The Best Location for a Telescope
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Far away from civilization to avoid light
pollution
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The Best Location for a Telescope (2)
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Paranal Observatory (ESO), Chile
On high mountain-tops to avoid atmospheric
turbulence (? seeing) and other weather effects
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The Powers of a Telescope (3)
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3. Magnifying Power ability of the telescope to
make the image appear bigger.
The magnification depends on the ratio of focal
lengths of the primary mirror/lens (Fo) and the
eyepiece (Fe)
A larger magnification does not improve the
resolving power of the telescope!
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Refracting/Reflecting Telescopes
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Refracting Telescope Lens focuses light onto the
focal plane
Focal length
Reflecting Telescope Concave Mirror focuses
light onto the focal plane
Focal length
Almost all modern telescopes are reflecting
telescopes.
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Secondary Optics
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In reflecting telescopes Secondary mirror, to
re-direct the light path towards the back or side
of the incoming light path.
Eyepiece To view and enlarge the small image
produced in the focal plane of the primary optics.
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• Limitations of Refractors
• Lens must be made of high-quality glass with no
  imperfections
• Larger lenses weigh a lot lenses can be
  supported only around their rims
• The lens will sag under its own weight
• So, the largest refractor telescope anyone has
  built is about 1.1 m.

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• Usefulness of Reflectors
• Objective (mirror) can be made of many things,
  even plastic or metal
• Only one side of the mirror is polished
• Mirror can be supported from its entire back,
  not the rim only
• So, all large modern telescopes are reflectors
  because they are easier to make and maintain.

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A Prime Focus Reflector (some light blocked)
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A Newtonian Reflector (secondary mirror)
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A Schmidt Cassegrain Reflector (hole in primary
mirror)
Example Hubble Space Telescope
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Traditional Telescopes (2)
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The 4-m Mayall Telescope at Kitt Peak National
Observatory (Arizona)