Adaptive Optics in the VLT and ELT era Beyond Basic AO

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Adaptive Optics in the VLT and ELT era Beyond
Basic AO
François Wildi Observatoire de Genève
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Adaptive Optics wavefront errors reminder

• The residual wavefront error is the quality
  criterion in AO
• The wavefront error depends on
• The number of degrees do freedom (i.e. /- nb of
  actuators) of the deformable mirror.
• The lag (delay) in the control system
• The noise in the wavefront sensor which depends
  on the guide star magnitude
• The size of the field of view
• Side effects like WFS non-ideality, NCPA,
  disturbances like vibrations

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Dependence of Strehl on l and number of DM
degrees of freedom

• Assume bright natural guide star
• No meast error or iso-planatism or bandwidth
  error

Deformable mirror fitting error only
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Reminder 1 Dependence of Strehl on l and number
of DM degrees of freedom (fitting)

• Assume bright natural guide star
• No meast error or iso-planatism or bandwidth
  error

Deformable mirror fitting error only
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Basics of wavefront sensing

• Measure phase by measuring intensity variations
• Difference between various wavefront sensor
  schemes is the way in which phase differences are
  turned into intensity differences
• General box diagram

Wavefront sensor
Computer
Transforms aberrations into intensity variations
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Types of wavefront sensors

• Direct in pupil plane split pupil up into
  subapertures in some way, then use intensity in
  each subaperture to deduce phase of wavefront.
  REAL TIME
• Slope sensing Shack-Hartmann, pyramid sensing
• Curvature sensing
• Indirect in focal plane wavefront properties
  are deduced from whole-aperture intensity
  measurements made at or near the focal plane.
  Iterative methods - take a lot of time.
• Image sharpening, multi-dither
• Phase diversity

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Shack-Hartmann wavefront sensor concept - measure
subaperture tilts
CCD
CCD
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WFS implementation

• Compact
• Time-invariant

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How to reconstruct wavefront from measurements of
local tilt
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Effect of guide star magnitude (measurement error)
Because of the photons statistics, some noise is
associated with the read-out of the
Shack-Hartmann spots intensities
Assumes no fitting error or other error terms
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Effect of guide star magnitude (measurement
error)
Assumes no fitting error or other error terms
bright star
Decreaing measurement error
dim star
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Reminder 3 Strehl vs l and guide star angular
separation (anisoplanatism)
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Reminder 3 Strehl vs l and guide star angular
separation (anisoplanatism)
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Anisoplanatism side effect

• Because correction quality falls off rapidly
  looking sideways from the guide star AND because
  faint stars cannot be used as guide stars,Only
  a very small part of the sky is accessible to
  natural guide star AO systems!

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Sky coverage accounting for guide star densities
LGS coverage 80
Tip/tilt sensor magnitude limit
Hartmann sensor magnitude limit
Galactic latitude
NGS coverage 0.1
Isokinetic angle qk
Isoplanatic angle q0
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(Temporary) conclusion on isoplanatism

• With 0.1 sky coverage, classical AO is of
  limited use for general astronomy.
• This is perticularly true for extra-galactic
  astronomy, where the science object is diffuse,
  often faint and cannot be used for wavefront
  sensing.

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AOs great divide

• High precision
• ExAO

Wide field LTAO (high coverage) GLAO MCAO
MOAO
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ExAO in a nutshell

• Like classical AO but more of the same
• The wavefront error minimized on axis
• Large number of degrees do freedom (i.e. /- nb
  of actuators) of the deformable mirror.
• Minimal lag (delay) in the control system
• Low noise in the wavefront sensor Bright guide
  star
• No field of view
• WFS non-ideality fought with spatial filter, NCPA
  measured and corrected, disturbances like
  vibrations countered with advanced signal
  processing

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High contrast imaging

• Highest contrast observations require multiple
  correction stages to correct for
• Atmospheric turbulence
• Diffraction Pattern
• Quasi-static instrumental aberrations

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NCPA compensation

• Use of phase diversity for NCPA correction on
  Vis. path

• Strong improvement of bench internal SR (45 -gt 85
  in Vis)
• various optimisations still to be performed

• 11

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NCPA compensation for IR path
Ghosts
NCPA compensation 320 modes estimated, 220
corrected
No compensation
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Implementation
CPI
Focus 1
HWP2
De-rotator
ITTM
HWP1
PTTM
Polar Cal
Focus 2
DM
Focus 4
NIR ADC
VIS ADC
DTTS
VIS corono
Focus 3
ZIMPOL
WFS
NIR corono
DTTP
IFS
IRDIS
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Sky coverage and Wide field in a nutshell

• To circumvent the sky coverage problem, several
  ways have been devised and are actively pursued
• Laser Tomography Adaptive Optics (LTAO)Laser
  guide stars are used to probe the atmosphere and
  project it in the science object direction
• Ground Layer Adaptive Optics (GLAO)Laser guide
  stars are used to probe the atmosphere but only
  the ground layer is corrected
• Multi-Conjugate Adaptive Optics (MCAO)Laser
  guide stars are used to probe the atmosphere and
  turbulence is projected and corrected in several
  layers
• Multi Object Adaptive Optics (MOAO)Laser guide
  stars are used to probe the atmosphere and
  turbulence is projected in several directions.
  Each direction has one (or several DMs)

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Laser tomography AO

• In LTAO, the atmosphere is probed by multiple
  Wave Front Sensors to form a model of the
  atmosphere. This model is used to compute the
  wavefront distorsion in a perticular direction
  and therefore calculate a correction in that
  direction.
• It allows a good correction in a direction that
  lacks a good natural guide star at the expense of
  system complexity
• Field is not increased!

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Proper use of the system requires several
wavefront sensors to perform Tomography
Altitude Layer (phase aberration )
Ground Layer Pupil (phase aberration O)
Tomography Stereoscopy
WFS1
WFS2
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WFS Set-up and LTAO reconstruction
Telescope
Turb. Layers
2
1
DM corrects 1 2 in red direction
WFS
Atmosphere
UP
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Ground layer AO

• In GLAO, the atmosphere is probed by multiple
  Wave Front Sensors to form a model of the
  atmosphere. Only the ground layer is extracted
  form the model and used to feed back a correction
  mirror conjugated to the ground.
• It allows a correction of the atmospheric
  wavefront error that happens in the common path
  of all objects at the expense of system
  complexity
• Field is very large but performance is limited

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Performance expected from GLAO (Gemini)
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WFS Set-up and GLAO reconstruction
Turb. Layers
2
1
Telescope
WFS
DM corrects 1
Atmosphere
UP
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Laser guide stars vs natural guide stars

• Tomography can also be performed with natural
  guide stars BUT
• Requires planning the NGS for each observation
• Quality is not constant due to NGS geometry and
  flux distribution
• Requires movable wave front sensors
• Solution unanimously discarded today

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Multi Conjugate Adaptive Optics

• To increase the isoplanatic patch, the idea is to
  design an adaptive optical system with several
  deformable mirrors (DM), each correcting for one
  of the turbulent layerEach DM is located at an
  image of the corresponding layer in the optical
  system. (By definition, the layer and the DM are
  called conjugated  by the optical system).

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What is multiconjugate? Case without
Deformable mirror
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What is multiconjugate? Case with it
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Multiconjugate AO Set-up
Telescope
Turb. Layers
WFS
2
1
DM1
DM2
Atmosphere
UP
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Effectiveness of MCAO no correction

• Numerical simulations
• 5 Natural guide stars
• 5 Wavefront sensors
• 2 mirrors
• 8 turbulence layers
• MK turbulence profile
• Field of view 1.2
• H band

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Effectiveness of MCAO classical AO

• Numerical simulations
• 5 Natural guide stars
• 5 Wavefront sensors
• 2 mirrors
• 8 turbulence layers
• MK turbulence profile
• Field of view 1.2
• H band

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Effectiveness of MCAO MCAO proper

• Numerical simulations
• 5 Natural guide stars
• 5 Wavefront sensors
• 2 mirrors
• 8 turbulence layers
• MK turbulence profile
• Field of view 1.2
• H band

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MCAO Performance SummaryEarly NGS results, MK
Profile
2 DMs / 5 NGS
1 DM / 1 NGS
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320 stars / K band / 0.7 seeing
Stars magnified for clarity
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The reality GEMINI MCAO Module
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Example of MCAO Performance

• 13x13 actuators system
• K Band
• 5 LGSs in X of 1 arcmin on a side
• Cerro Pachon turbulence profile
• 200 PDE/sub/ms for H.Order WFS
• Four R18 TT GS 30 off axis (MCAO)
• One R18 TT GS on axis(AO)

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MCAO Performance
MCAO
Classical LGS AO
1
1
Strehl
0

• Surface plots of Strehl ratio over a 1.5 arc min
  FoV.
• 13x13 actuator system, K band, CP turbulence.

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Other nice features of MCAO

• Robustness
• Sensitivity to noise is fairly better than with
  AO
• Prop noise AO / Prop noise MCAO ? sqrt( NGS )
• Predictive algorithms possible ?

• Robustness

• Robustness
• Sensitivity to noise is fairly better than with
  AO
• Prop noise AO / Prop noise MCAO ? sqrt( NGS )

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Generalized Fitting (Finite number of DMs)
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Generalized Anisoplanatism(Finite number of
Guide Star)
Additional error terms are necessary to represent
laser guide star MCAO. Tomography error arises
from the finite number and placement of guide
stars on the sky. Generalized anisoplanatism
error results from the correction of the
continuous atmosphere at only a finite number of
conjugate layer altitudes.
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Generalized Fitting (Finite number of DMs)
Error rd2 ? (?.?h)5/3
Design Criteria e.g. Error balanced ?
?hmax(?,dact) DM Spacing 2 x ?hmax
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Generalized Anisoplanatism (Finite number of
Guide Star)

• Turbulence altitude estimation error
• OK toward GS, but error in between GS Strehl
  dips
• Maximum FoV depends upon DM pitch.
• Example for 7x7 system

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Generalized Anisoplanatism goes down with
increasing apertures
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MCAO Pros and Cons

• PROS
• Enlarged Field of View
• PSF variability problem drastically reduced
• Cone-effect solved
• Gain in SNR (less sensitive to noise, predictive
  algorithms)
• Marginally enlarged Sky Coverage (LGS systems)
• CONS
• Complexity Multiple Guide stars and DMs
• Other limitations Generalized Fitting,
  anisoplanatism, aliasing

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Multi objects adaptive optics
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• In certain case, the user does not want to (or
  need to) have a fully corrected image. He/she
  might be satisfied with having only specific
  locations (i.e.) objects corrected in the field.
• An AO system designed to provide this kind of
  correction is called a Multi Objects Adaptive
  Optics system
• MOAO are the systems of choice to feed
  spectrographs and Integral Field Units in the ELT
  era.

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• MOAO
• Up to 20 IFUs each with a DM
• 8-9 LGS
• 3-5 TTS

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MOAO for TiPi (TMT)
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Key Design Points for AO

• Key points
• 30x30 piezo DM placed at M6, providing partial
  turbulence compensation over the 5 field.
• All LGS picked off by a dichroic and directed
  back to fixed LGS WFS behind M7. Dichroic moves
  to accommodate variable LGS range.
• The OSM is used to select TT NGS and PSF
  reference targets.
• MEMS devices placed downstream of the OSM to
  provide independent compensation for each object
  16 science targets, 3 TT NGS, PSF reference
  targets.

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Laser guide stars
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Laser guide star AO needs to use a faint tip-tilt
star to stabilize laser spot on sky
from A. Tokovinin
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Effective isoplanatic angle for image motion
isokinetic angle

• Image motion is due to low order modes of
  turbulence
• Measurement is integrated over whole telescope
  aperture, so only modes with the largest
  wavelengths contribute (others are averaged out)
• Low order modes change more slowly in both time
  and in angle on the sky
• Isokinetic angle
• Analogue of isoplanatic angle, but for tip-tilt
  only
• Typical values in infrared of order 1 arc min

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Sky coverage is determined by distribution of
(faint) tip-tilt stars

• Keck gt18th magnitude

From Keck AO book
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LGS Related Problems Null modes

• Tilt Anisoplanatism Low order modes gt Tip-Tilt
  at altitude
• ? Dynamic Plate Scale changes
• Within these modes, 5 Null Modes not seen by
  LGS (Tilt indetermination problem)
• ? Need 3 well spread NGSs to control these modes
• Detailed Sky Coverage calculations (null modes
  modal control, stellar statistics) lead to
  approximately 15 at GP and 80 at b30o

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• Additional error terms are necessary to represent
  laser guide star MCAO. Tomography error arises
• from the finite number and placement of guide
  stars on the sky. Generalized anisoplanatism
  error results from the correction of the
  continuous atmosphere at only a finite number of
  conjugate layer altitudes

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LGS WFS Subsystem needs constant refocussing!

• Trombone design accomodates LGS altitudes between
  85-210 km (Zenith to 65 degrees)
• Astigmatism corrector present / Will study Coma
  corrector

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Concept Overview
TMT MIRES (proposal) LGS trombone system
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3. NGS WFS

• RadialLinear stages with encoders offer flexile
  design with min. vignetting
• 6 probe arms operating in Meatlocker just
  before focal plane
• 2x2 lenslets
• 6 FOV - 60x60 0.1 pix

EEV CCD60
Flamingos2 OIWFS
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Issues for designer of AO systems

• Performance goals
• Sky coverage fraction, observing wavelength,
  degree of compensation needed for science program
• Parameters of the observatory
• Turbulence characteristics (mean and
  variability), telescope and instrument optical
  errors, availability of laser guide stars
• AO parameters chosen in the design phase
• Number of actuators, wavefront sensor type and
  sample rate, servo bandwidth, laser
  characteristics

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Effects of laser guide star on overall AO error
budget

• The good news
• Laser is brighter than your average natural guide
  star
• Reduces measurement error
• Can point it right at your target
• Reduces anisoplanatism
• The bad news
• Still have tilt anisoplanatism stilt2
  ( ? / ?tilt )5/3
• New focus anisoplanatism sFA2 ( D /
  d0 )5/3
• Laser spot larger than NGS smeas2 (
  ?b / SNR )2