Transcript Adaptive Optics

John O’Byrne
School of Physics
University of Sydney
Adaptive Optics
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What is AO?

Adaptive Optics:
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
fast image correction (f  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/adaptiveOptics/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
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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:
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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
(R/Rmax is Strehl resolution normalised by
exposure resolution of an infinte aperture)
AO is easier in the infrared
 ro is larger
 qo is larger
 to is longer
Also easier if
 H is lower
 Vwind is lower
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Essentials of an
AO system
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Wavefront sensor
Computer
Phase modulator
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WFS - Shearing interferometer
The Wavefront Sensor (WFS) may be
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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 => 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)
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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 => 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 target’s 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
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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

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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/adaptive-optics/area_main_ao.html)
 GEMINI
http://www.gemini.anu.edu.au/sciops/instruments/adaptiveOptics/AOIndex.html
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AO at ESO / VLT
http://www.eso.org/projects/aot/
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AO Projects - 2
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Keck II and now Keck I
http://www2.keck.hawaii.edu:3636/realpublic/inst/ao/ao.html
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University of Durham (UK)
http://aig-www.dur.ac.uk/fix/adaptive-optics/area_main_ao.html
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University of Hawaii
 most recently Hokupa’a on GEMINI
http://www.ifa.hawaii.edu/ao/
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Earlier PUEO on CFHT
http://www.cfht.hawaii.edu/Instruments/Imaging/AOB/
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Hokupa’a
images - 1
CFHT
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Hohupa’a Images - 2
QSO PG1700+518 and its
companion starbust galaxy.
These deep (2hr.) images
were made by guiding on the
16th mag QSO itself.
Raw AO
PSF
subtr.
Deconlv.
CFHT
J
H
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Hohupa’a 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
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