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Adaptive optics the telescope versus the
atmosphere

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Jon Lawrence Macquarie UniversityAnglo-Australian
Observatory
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Telescope Growth Since Galileo
Telescope size has been driven by the available
technology. 16th and 17th Century glass
quality was the limiting factor 19th Century
reached the limit of refractive (glass)
design 20 th Century large single mirror
telescopes and multi-mirror technology
WHY Sensitivity and Resolution
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Why is it difficult?

• The image you get depends on the shape of the
  mirror. It needs to be exact to within a fraction
  of the wavelength of light.
• Shape must be correct within 1/1000th of a
  millimetre over the entire telescope diameter

Mirrors usually take more than 12 months to polish
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The Anglo-Australian Telescope

• 4 m telescope run by the Anglo-Australian
  Observatory built in 1974
• located at Siding Spring near Coonabarabran, NSW
• instruments include the 2dF robotic fibre
  positioner

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8 -10 metre class telescopes Gemini and Keck
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Hubble Space Telescope

• 2.4 m diameter monolithic mirror telescope with
  series of optical/infrared instruments
• NASA launched in 1990 (so far 5 service
  missions)
• Low-Earth orbit

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James Webb Space Telescope (JWST)

• 6.5 m diameter telescope
• Comprised of multiple segments
• Due for launch to L2 in 2014
• Infrared optimised (?2-20µm)
• Cost 4-5 B USD mostly NASA

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Extremely Large Telescopes (ELTs)

• Thirty Metre Telescope (TMT)
• Caltech U.California private funding
• 30 m diameter
• Segmented design
• 500 elements x 1.4 m
• Total cost 1 B USD
• First light 2018

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Antarctic telescopes

• Antarctic plateau is high, dry, cold, and the
  atmosphere is very calm
• Several sites ideal for astronomy Dome C, Dome
  A, South Pole
• numerous projects proposed

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Sensitivity
Small telescope
Larger mirror diameter more light higher
sensitivity Can thus see fainter and more
distant objects
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Resolution
Telescope focal plane
Light waves
Main mirror
Secondary mirror
Distant star
CCD camera
2D
Image of star
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RESOLUTION What We Should Get
2 m telescope
4 m telescope
8 m telescope
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Example a footprint on the moon

• Angular diameter of the moon 0.5 degrees
• Distance to the moon 300,000 km
• At wavelength of 500 nm in the visible
• Human eye (Diameter 5 mm) ? 0.006 degrees
• Can see large craters (gt30 km)
• Amateur telescope (D10 cm) ?0.0003 degrees
• Can see smaller craters (2 km)
• Keck telescope (D10 m) ? 0.000003 degrees
• Can see large building (15 m)
• E-ELT (D42 m) ? 0.0000007 degrees
• Can see a spacecraft (4 m)
• To resolve a footprint (0.1 m)
• Requires a telescope with a diameter of
• D2000 m!!! ? 0.000000001 degrees

?0.5º
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Astronomical distances the moon (300,000 km) is
really close
The Solar System
The Nearest Stars
The Milky Way
The Local Group
The Local Group
http//heasarc.gsfc.nasa.gov/docs/cosmic/
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The atmosphere

• The Earths atmosphere
• Emits light
• Absorbs light
• Causes turbulence

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Turbulence
Main mirror
Secondary mirror
Distant star
Earths atmosphere
2D
Causes blurry image in the focal plane
Light waves are bent by turbulence
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Atmospheric Turbulence
Turbulence turns this
into this
Also causes stars to twinkle
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object
guide star
Beating the atmosphere Adaptive Optics
telescope optics
deformable mirror
image camera
wavefront control
wavefront sensor
wavefront analysis
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