John OByrne

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• John OByrne
• School of Physics
• University of Sydney

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What is AO?

• Adaptive Optics
• fast image correction (f ³ 1 Hz), primarily to
  correct atmospheric wavefront distortions
• Active Optics
• slow image correction (f 1 Hz), to correct
  mirror and structural deflections

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Why do we need AO?

• Scintillation - describes random amplitude
  fluctuations of wavefront (twinkling)
• Seeing - describes random phase fluctuations of
  wavefront (image motion and blurring)
• AO aims to correct seeing effects - i.e. sharpen
  images
• Science objectives - e.g. GEMINI
• http//www.gemini.anu.edu.au/sciops/instruments/ad
  aptiveOptics/Science_drivers.html

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Where does Seeing arise?
Turbulence in the atmosphere leads to refractive
index variations. Contributions are concentrated
into layers at different altitudes.
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Scidar measurements at SSO
10 minutes of data refractive index structure
constant (Cn2 ) v. altitude
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Seeing parameters - 1

• Fried parameter ro(l,z) 0.185l6/5cos3/5z(Cn2dh
  )-3/5
• Seeing disk FWHM without AO l/ro for large
  telescopes
• So at 500nm, ro 10 cm for 1 arcsec FWHM seeing
  
• At 2.5mm, this corresponds to ro 70 cm and
• 0.7 arcsec seeing

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Seeing parameters - 2

• If seeing is dominated by a layer at altitude H
• Isoplanatic angle (for wavefront distortion) qo
  0.314 ro/H - typically a few arcsec in visible
• Isokinetic angle (for image motion) qk 0.314
  Dtel/H - typically 100 arcsec in visible
• Timescale for wavefront distortion to 0.314
  ro/Vwind - typically few ms
• Timescale for image motions tk 0.314
  Dtel/Vwind - typically 100 ms

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What can we expect from AO?

• Improvement depends on
• Dtel relative to ro
• AO is easier in the infrared
• ro is larger
• qo is larger
• to is longer
• Also easier if
• H is lower
• Vwind is lower

(R/Rmax is Strehl resolution normalised
by exposure resolution of an infinte aperture)
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Essentials of an AO system

• Wavefront sensor
• Computer
• Phase modulator

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WFS - Shearing interferometer

• The Wavefront Sensor (WFS) may be
• Shearing interferometer (uncommon)
• Shears the wavefront to measure tilt in the shear
  direction

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WFS - Shack-Hartmann Sensor
Shack-Hartmann sensor (the usual choice) Uses
lenslets to sub-divide the aperture and measures
image motion in each sub-aperture.
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WFS - Curvature Sensor

• Wavefront
• Curvature
• Sensor
• Uses lenslets to sub
• divide the aperture and
• measures curvature of
• the wavefront in each
• sub-aperture.

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Phase Modulator

• The phase modulators are always a deformable
  mirror
• - usually tip-tilt and higher order separately.
• Actuators used
• piezoelectric (PZT)
• electrostrictive
• voice-coil
• electrostatic
• But other technologies are possible
• Liquid Crystal phase screen devices
• More actuators gt better correction.

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Tit-tilt correction

• Tip-tilt mirror mounted on
• 4 piezoelectric stacks.
• Segmented surface deformable
• mirrors use tip-tilt on
• individual segments

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Stacked-array Mirrors

• Continuous faceplates
• attached to
• piezoelectric stacks
• Visible on the edges of
• each mirror are the PZT
• actuators.

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Bimorph mirrors

• Bimorph mirror made
• from piezoelectric wafers
• (sometimes one piezo and
• one glass) with an
• electrode pattern to control
• deformation

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Membrane Mirrors

• Continuous faceplates
• deformed electrostatically by
• an underlying electrode
• pattern.

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Sample of an AO result - 1
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Sample of an AO result - 2
Core diameter is recovered with low order
correction, but a surrounding halo remains
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AO limitations

• AO systems have limitations (e.g. light loss, IR
  emissivity
• driven by the large number of optical surfaces)
  but more
• fundamental are limits imposed by the guiding
  star, which is
• monitored by the wavefront sensor, and is likely
  to be
• different from the science target

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Natural Guide Stars (NGS)

• temporal anisoplanatism - delays introduced by
  the servo loop
• angular anisoplanatism - NGS is usually offset
  from science target, but can't be too far away or
  it lies outside isoplanatic patch angle (qo) -
  can be improved by making the WFS conjugate to
  the primary turbulence layer (or multiple layers
  in multi-conjugate AO MCAO)
• WFS sensitivity limit gt limited sky coverage

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Laser Guide Stars (LGS) - 1

• Use a laser to generate a star in
• the atmosphere, very close to the
• science targets light path through
• the atmosphere. This may be a
• Rayleigh guide star at 7-20 km
• or a Sodium guide star at 90 km.
• Overcomes NGS sky coverage limitation

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Laser Guide Stars (LGS) - 2

• Provides no tip-tilt information
• Cost!
• Problem to other telescopes on the site caused by
  back-scattered light

Sodium guide star and Rayleigh back-scatter
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Laser Guide Stars (LGS) - 3

• Focus anisoplanatism
• the laser does not fully sample the stars light
  path through the atmosphere
• worse for a Rayleigh guide star
• provide multiple LGS?

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AO Projects - 1

• Australian projects
• RSAA 2.3m tip-tilt system
• Anglo-Australian Telescope
• International projects
• (e.g. see University of Durham list of links to
  other projects http//aig-www.dur.ac.uk/fix/adapt
  ive-optics/area_main_ao.html)
• GEMINI http//www.gemini.anu.edu.au/sciops/instrum
  ents/adaptiveOptics/AOIndex.html
• AO at ESO / VLT http//www.eso.org/projects/aot/

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AO Projects - 2

• Keck II and now Keck I http//www2.keck.hawaii.edu
  3636/realpublic/inst/ao/ao.html
• University of Durham (UK) http//aig-www.dur.a
  c.uk/fix/adaptive-optics/area_main_ao.html
• University of Hawaii
• most recently Hokupaa on GEMINI
  http//www.ifa.hawaii.edu/ao/
• Earlier PUEO on CFHT http//www.cfht.hawaii.edu/I
  nstruments/Imaging/AOB/

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Hokupaa images - 1
CFHT
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Hohupaa Images - 2

• QSO PG1700518 and its
• companion starbust galaxy.
• These deep (2hr.) images
• were made by guiding on the
• 16th mag QSO itself.

Raw AO PSF subtr. Deconlv.
J H
CFHT
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Hohupaa Images - 3
GEMINI
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Keck
Keck I AO image in H band taken during the first
Keck I AO night (Dec.12,2000). Io angular
size 1.23 arcsecond Spatial resolution 120 km
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Starfire Optical Range (SOR)
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References

• Information on AO projects can be obtained from
  their web sites or from the
• Proceedings of the (all too frequent) AO
  conferences (e.g. SPIE, OSA or ESO).
• A few other useful references
• Popular level
• Sharper Eyes on the Sky - Sky Space, 9, 30
  (1996)
• Untwinkling the Stars - Sky Telescope, 87, May
  24 Jun 20, (1994)
• Adaptive Optics - Scientific American, Jun (1994)
• Reviews
• Young, A.T. (1974), ApJ, 189, 587
• Roddier, F. (1981), Progress in Optics, 19, 281
• Coulman ARAA (1985), 23, 19
• Beckers, J.M. (1993), ARAA 31, 13
• Wilson, R.W.,Jenkins C.R. (1996), MNRAS, 268, 39