Big telescopes for a small world

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Big Telescopes
for a Small World

• A new view of the Universe II
• Fred Watson, AAO
• April 2005

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The secret obsessions of astronomers
Big Telescopes for a Small World
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Characteristics of astronomy today

• Highly comprehensive range of instrumentation
• Infinite computing power
• Access to every part of the electromagnetic
  spectrum?-rays, X-rays, UV, visible (optical),
  IR, mm-wave, radio
• Not to mention particles, gravitational waves
• (So we wont.)

Big Telescopes for a Small World
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The Universe through different eyes...
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Whats so good about optical astronomy?

• Visible light is emitted by ordinary matter in
  the Universei.e. stars
• The visible spectrum is rich in the bar-code of
  atomic and molecular features
• Optical observations bridge long and short
  wavebands
• You can do it with your feet on the ground

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The Schematic Ground-Based Optical Telescope

• Something large to collect and focus the
  radiation
• A complicated bit in the middle for analysis
• An optical detector
• A ground-based mounting

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Detectors
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Astronomical cameras are not small(This is
IRIS2, a multi-purpose infrared camera on the
AAT)
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Subtracting the sky
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Other complicated bits
Spectrographs conventionally use a grating, prism
or grism
Sends light of different wavelengths in different
directions Hence (via the spectrograph camera)
to different positions on the detector (which is
a CCD or an infrared array). (This slide and the
next three courtesy Gordon Robertson)
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Reflection grating spectrograph (schematic)
collimator
slit
grating
?
b
?cc
?
?d
?i
detector
camera
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Volume phase holographic (VPH) gratings

• 3-d modulations of refractive index in gelatin
  layer
• Peak efficiency up to 90
• Wavelength of peak efficiency can be tuned
• Transmission gratings
• DCG layer (hologram) is protected on both sides
• Each grating is an original, made to order
• Large sizes possible

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Test of a prototype VPH grating
Big Telescopes for a Small World
Note no antireflection coatings
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Why make telescopes ever bigger?

• To gather more light from faint sources because
  there are no further gains to be made in detector
  sensitivity
• To improve resolution

R 1.22 ? / D
As the mirror diameter D gets bigger, the
resolution R gets finer.
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Large Telescope Mirror, 1969
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A 3.9-metre mirror can resolve 0.03 arcsec

• BUT

r0 is Frieds parameter for wavefront
distortion Cn2 is the refractive index structure
constant Cn2 is integrated over the full height
of the atmosphere
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The end-product is
This is very depressing indeed ?
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Can you do anything useful in such conditions?
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Detection of extra-solar planets
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Multi-object spectroscopy with fibre optics
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Galaxies

• Basic building-blocks of the Universe

• Around 100,000,000,000 stars
• Lots of gas and dust (in spirals)
• Around 100,000 light years across (or
  1,000,000,000,000,000,000 km)

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Antidotes to atmospheric turbulence
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The Eagle Nebulastellar birthplace
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But the Hubble projects total cost isUS 6
billion. That would buy 60 of todays
ground-based 8-metre telescopes
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The worlds largest telescopes, 2005
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The crowded summit of Mauna Kea
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Antu, Kueyen, Melipal and Yepun
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Its all to do with atmosphere
1 arcsecond
But at the VLT, on the same scale
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What do we do next?
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The thinking goes like this
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VLT Very Large Telescope 48 m (16 m
equiv.) ELT Extremely Large Telescope 25 m CELT
California Extremely Large Telescope 30 m GSMT
Giant Segmented-Mirror Telescope 30m TMT
Thirty-metre Telescope (US Canada ?) Euro50
formerly SELT
Future plans for large telescopes...
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OWLSharp-eyed and OverWhelmingly Large
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And what can we do with such monsters?
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What might we study with OWL?
Earth-like planets out to about 75 l.y. by direct
imaging
Individual stars in moderately distant galaxies
galactic archaeology
Galaxies forming at look-back times up to 10
billion years
Exploding stars at look-back times up to 12.5
billion years
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Not to mention the completely unexpected
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Big Telescopes for a Small World
The End