Atmospheric Turbulence and Astronomical Adaptive Optics

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Atmospheric Turbulence and Astronomical Adaptive
Optics

• Matthew Britton
• Caltech Optical Observatories

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Outline

• Atmospheric turbulence profiles
• Turbulence and image quality
• Angular anisoplanatism
• PSF estimation and deconvolution
• Differential tilt jitter
• The PSF of a laser guide star AO system
• Wide field adaptive optics architectures

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Atmospheric Turbulence
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Atmospheric Turbulence
Wavefront Phase
1.25 mm PSF
1.65 mm PSF
2.2 mm PSF
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Atmospheric Turbulence Monitors

• Differential Motion Measurement (DIMM)
• Slope Detection and Ranging (SLODAR)
• Sonic Detection and Ranging (SODAR)
• Scintillation Detection and Ranging (SCIDAR)
• Multi Aperture Scintillation Sensor (MASS)
• Shadow Band Ranging (SHABAR)

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DIMM
http//www.ociw.edu/birk/CDIMM/cdimm.html
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SLODAR
From Wang, Chanan Schoeck
http//eraserhead.caltech.edu/palomar/MGSU/MGSU.ht
ml
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Scintillation
5 meters
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SCIDAR
http//www.iac.es/galeria/bgarcia/SCIDAR_webpage/S
CIDAR_IAC.html
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Multi Aperture Scintillation Sensor
http//www.ctio.noao.edu/atokovin/profiler/index.
html
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DIMM/MASS
http//odata1.palomar.caltech.edu/massdimm/
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Fried Parameter and Seeing
The Fried parameter r0 is the lateral coherence
scale of the wavefront phase
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Fried Parameter Variability
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Seeing Variability
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Seeing and r0 Statistics
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Atmospheric Turbulence and Image Quality
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The Long Exposure Point Spread Function
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Long Exposure Seeing Limited PSF Variability
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An AO Simulation
Aberrated Phase
TTM Surface
DM Surface
DL PSF
PSF
Residual Phase
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Strehl Ratio vs Time
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Guide Star Strehl Variability

• Short term Strehl variability arises from the
  random realizations of atmospheric turbulence.
• Long term Strehl variability arises from the
  evolution of the atmospheric turbulence profile
  and the resulting change in AO system
  performance. For example, fitting error for an
  AO system with subapertures of size d is

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Angular Anisoplanatism
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(No Transcript)
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Anisoplanatic Wavefront Phase Errors
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http//www.gemini.edu/sciops/instruments/adaptiveO
ptics/MCAO.html
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Isoplanatic Angle
Q0 is the angular decorrelation scale for
wavefront phase aberrations arising from
atmospheric turbulence.
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Isoplanatic Angle Variability
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Q0 Statistics
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Long Exposure PSF of a Perfect AO System
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Trapezium Images from Palomar
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Astronomical Impact of PSF Variability in Time
and Field

• Turbulence profile variability is not stationary
  on observational timescales, so PSF variability
  doesnt average away.
• Variations depend on turbulence and wind
  profiles, which impact AO system performance and
  anisoplanatic degradation.
• Variability in Strehl ratio can be factors of
  several on timescales of a minute or less.

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Quantitative Applications of Astronomical
Adaptive Optics

• Differential photometry
• Differential astrometry
• High contrast imaging
• Imaging and IFU spectroscopy of resolved objects

To understand the level of precision AO can
provide, one must understand the consequences of
turbulence profile variability on PSF stability
in a particular observational context.
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PSF Estimation and Deconvolution
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PSF Estimation

• From a PSF calibration star
• From the data
• From a parameterized model
• From a measured turbulence profile

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The Anisoplanatic Transfer Function
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PSF Estimation from Cn2 Profile

• Guide Star

Companion
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Observed and Predicted PSFs
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Observations of a Quadruple
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Modelling the Observation
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Trapezium PSF Predictions
Star 4
Star 3
Stars 1,2
Stars 1,2
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Differential Tilt Jitter
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Differential Tilt Jitter
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(No Transcript)
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Tilt Jitter vs Angular Separation
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Probability of the Measurements
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Grid Astrometry
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Grid Astrometry in Trapezium
10 stars averaged
25 Images Averaged
300 uas in 30 sec
Cameron, Britton Kulkarni, in prep.
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The PSF of a Laser Guide Star AO System
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LGS Adaptive Optics
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Focal Anisoplanatism Parameter
In the differential wavefront phase between the
LGS and the science target, d0 is the size of the
coherence patch in the pupil plane.
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d0 Variability
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d0 Statistics
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LGS Focal and Angular Anisoplanatism
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0 asec
0 asec
5 asec
10 asec
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Strehl Histories
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The LGS PSF
The PSF is stable over a field of view of size
The PSF core width is
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Long Exposure LGS PSF
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Wide Field AO Architectures
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Wavefront Sensing with Multiple Laser Beacons
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Multiconjugate Adaptive Optics
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ESO Multiconjugate AO Demonstrator

• Uses two 60 actuator bimorph deformable mirrors
  conjugated to 0km, 8.5km
• Wavefront sensing performed with three natural
  guide stars

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Multiobject Adaptive Optics
Optically separate the light from each science
target and correct wavefront phase aberrations
using a deformable mirror
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Summary

• Measurement of atmospheric turbulence profiles
• Effects of turbulence on image quality
• Variability of the AO PSF in time and field
• Estimation of the AO PSF
• Quantitative applications of AO
• The motivation for future wide field astronomical
  adaptive optics architectures

Talk available at http//eraserhead.caltech.edu/t
alks/britton_cfao_ss_2007/cfao.html
Thank you