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Adaptive Optics in the VLT and ELT eraAdaptive Optics in the VLT and ELT era

Beyond Basic AO Beyond Basic AO

François WildiObservatoire de Genève

Adaptive Optics wavefront errors Adaptive Optics wavefront errors reminderreminder

• 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

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

• Assume bright natural guide star

• No meas’t error or iso-planatism or bandwidth error

S exp 2 exp 0.28 d / r0 5 /3

r0 r0 0.5m / 0.5m 6 /5

S exp 0.28d

r0 0.5m

5 /30.5m

2

Deformable mirror fitting error only

Decreasing fitting error

• Assume bright natural guide star

• No meas’t error or iso-planatism or bandwidth error

Deformable mirror fitting error only

Reminder #1: Dependence of Strehl on Reminder #1: Dependence of Strehl on and number of DM degrees of and number of DM degrees of freedom (fitting)freedom (fitting)

Basics of wavefront sensingBasics 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:

Guidestar

Turbulence

Telescope Optics Detector Recon-structor

Wavefront sensor

Transforms aberrations into intensity variations

Computer

Types of wavefront sensorsTypes 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

Shack-Hartmann wavefront sensor Shack-Hartmann wavefront sensor concept - measure subaperture concept - measure subaperture tiltstilts

CCD CCD

f

Pupil plane Image plane

WFS implementationWFS implementation

• Compact

• Time-invariant

How to reconstruct wavefront How to reconstruct wavefront from measurements of local “tilt”from measurements of local “tilt”

Effect of guide star magnitude Effect of guide star magnitude (measurement error)(measurement error)

Assumes no fitting error or other error terms

S H2

6.3

SNR

2

S exp S H2 exp

6.3

SNR

2

SNR 1

N photons

Because of the photons statistics, some noise is associated with the read-out of the Shack-Hartmann spots intensities

Effect of guide star Effect of guide star magnitude (measurement magnitude (measurement error)error)

Decreaing measurement error

Assumes no fitting error or other error terms

bright star

dim star

Reminder #3: Strehl vs Reminder #3: Strehl vs and guide and guide star angular separation star angular separation (anisoplanatism)(anisoplanatism)

S exp iso2 exp

0

5 /3

0 r0h

6 /5

S exp

0 (0.5m)

5 /30.5m

2

Reminder #3: Strehl vs Reminder #3: Strehl vs and guide and guide star angular separation star angular separation (anisoplanatism)(anisoplanatism)

Anisoplanatism side effect: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!

Sky coverage accounting for Sky coverage accounting for guide star densitiesguide star densities

Tip/tilt sensor magnitude limit

Isokinetic angle k

LGS coverage ~80 %

Galactic latitude

Hartmann sensor magnitude limit

Isoplanatic angle 0

NGS coverage 0.1 %

(Temporary) conclusion on (Temporary) conclusion on isoplanatism: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.

AO’s great divideAO’s great divide

High precision

ExAO

Wide field

LTAO (high coverage)

GLAO

MCAOMOAO

ExAO in a nutshellExAO 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

High contrast imaging

Highest contrast observations require multiple correction stages to correct for

1. Atmospheric turbulence

2. Diffraction Pattern

3. Quasi-static instrumental aberrations

XAOXAO

visiblevisiblecoronagraphcoronagraph

infraredinfraredcoronagraphcoronagraph

Diff. Pol.Diff. Pol.

IFSIFS

SDISDI

XAO, S~90%XAO, S~90% CoronagraphCoronagraphDifferential MethodsDifferential Methods

NCPA compensationNCPA compensation Use of phase diversity for NCPA correction on

Vis. path

11

-Strong improvement of bench internal SR (45 -> 85 in Vis)

- various optimisations still to be performed

NCPA compensation for IR pathNCPA compensation for IR path

No compensation

NCPA compensation

320 modes estimated, 220 corrected

Ghosts

ImplementationCPICPI

IRDISIRDISIFSIFS

ZIMPOLZIMPOL

ITTMITTM

PTTMPTTM

DMDM

DTTPDTTP

DTTSDTTS

WFSWFS

De-rotatorDe-rotator

VIS ADCVIS ADCNIR ADCNIR ADC

Focus 1Focus 1

Focus 2Focus 2

Focus 3Focus 3

Focus 4Focus 4

NIR coronoNIR corono

VIS coronoVIS corono

HWP2HWP2

HWP1HWP1

Polar CalPolar Cal

Sky coverage and Wide field in a Sky coverage and Wide field in a nutshellnutshell

• To circumvent the sky coverage problem, several ways have been devised and are actively pursued:

1. Laser Tomography Adaptive Optics (LTAO)Laser guide stars are used to probe the atmosphere and project it in the science object direction

2. Ground Layer Adaptive Optics (GLAO)Laser guide stars are used to probe the atmosphere but only the ground layer is corrected

3. Multi-Conjugate Adaptive Optics (MCAO)Laser guide stars are used to probe the atmosphere and turbulence is projected and corrected in several layers

4. 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 DM’s)

LASER TOMOGRAPHY AOLASER 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!

Proper use of the Proper use of the system requires system requires several wavefront several wavefront sensors to perform sensors to perform TomographyTomography Altitude Layer (phase

aberration = ++)

Ground Layer = Pupil (phase aberration = OO)

WFS#1 WFS#2

Tomography = StereoscopyTomography = Stereoscopy

WFS Set-up and LTAO WFS Set-up and LTAO reconstructionreconstruction

TelescopeTurb. Layers#2 #1

Atmosphere

WFS

UP

DM corrects #1 + #2 in red direction

GROUND LAYER AOGROUND 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

Performance expected from GLAO Performance expected from GLAO (Gemini)(Gemini)

WFS Set-up and GLAO WFS Set-up and GLAO reconstructionreconstruction

Telescope

Turb. Layers#2 #1

Atmosphere

WFS

UP

DM corrects #1

LASER GUIDE STARS VS NATURAL LASER GUIDE STARS VS NATURAL GUIDE STARSGUIDE 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

Multi Conjugate Adaptive Multi Conjugate Adaptive OpticsOptics

• 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).

What is multiconjugate? What is multiconjugate? Case Case withoutwithout

Deformable mirror

Turbulence Layers

What is multiconjugate? What is multiconjugate? Case Case with itwith it

Deformable mirrors Turbulence Layers

Multiconjugate AO Set-upMulticonjugate AO Set-up

Telescope

DM#2 DM#1

Turb. Layers#2 #1

Atmosphere

WFS

UP

Effectiveness of MCAO: no correctionEffectiveness 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

Effectiveness of MCAO: classical AOEffectiveness 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

Effectiveness of MCAO: MCAO proper 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

Classical AO MCAONo AO

165’’

MCAO Performance SummaryMCAO Performance SummaryEarly NGS results, MK ProfileEarly NGS results, MK Profile

2 DMs / 5 NGS

320 stars / K band / 0.7’’ seeing

1 DM / 1 NGS

Stars magnified for clarity

The reality…: GEMINI MCAO The reality…: GEMINI MCAO ModuleModule

DMs

TTM

LGS WFS

NGS source simulator

LGS source simulatorScience ADC

BeamsplitterDiagnostic WFSNGS

WFS

LGS zoom correctorNGSADC

shutters

Example of MCAO PerformanceExample 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 R=18 TT GS 30” off axis (MCAO)

• One R=18 TT GS on axis(AO)

MCAO PerformanceMCAO Performance

Surface plots of Strehl ratio over a 1.5 arc min FoV.

13x13 actuator system, K band, CP turbulence.

11

0

Str

ehl

Classical LGS AOClassical LGS AO MCAOMCAO

• Robustness

• Sensitivity to noise is fairly better than with AO

Prop noise AO / Prop noise MCAO sqrt( NGS )

• Predictive algorithms possible ?

Other nice features of MCAOOther nice features of MCAO

• Robustness

Ave

rage

Str

ehl

(tri

angl

es)

1

.5

+ ++

++

+ Str

ehl S

t. de

v ac

ross

FoV

% (

+)

2

4

Profile number

0

• Robustness

• Sensitivity to noise is fairly better than with AO

Prop noise AO / Prop noise MCAO sqrt( NGS )

Generalized Fitting Generalized Fitting (Finite number of DMs)

dact

Geometry of the problem

Generalized AnisoplanatismGeneralized 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 layeraltitudes.

Generalized Fitting Generalized Fitting (Finite number of DMs)

Error [rd2] (.h)5/3

Design Criteria e.g. Error balanced hmax(,dact)

DM Spacing = 2 x hmax

dact FoV [arcmin] hmax [m] NDM/GS

0.5 1 3000 3

0.2 1 1200 5- 6

0.2 10 120 50

FoV = 70”FoV = 100”

Generalized Anisoplanatism 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

Generalized Anisoplanatism goes Generalized Anisoplanatism goes down with increasing aperturesdown with increasing apertures

2D info only3D info2D info only3D info

Aperture

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MCAO Pros and ConsMCAO Pros and ConsPROS:PROS:

• Enlarged Field of ViewEnlarged Field of View– PSF variability problem drastically reduced

• Cone-effect solvedCone-effect solved

• Gain in SNR (less sensitive to noise, predictive algorithms)

• Marginally enlarged Sky Coverage (LGS systems)

CONSCONS

• Complexity: Complexity: Multiple Guide stars and DMs

• Other limitationsOther limitations: Generalized Fitting, anisoplanatism, aliasing

MULTI OBJECTS ADAPTIVE MULTI OBJECTS ADAPTIVE OPTICSOPTICS

• 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.

–MOAO

• Up to 20 IFUs each with a DM

• 8-9 LGS

• 3-5 TTS

MOAO for TiPi (TMT)MOAO for TiPi (TMT)

TiledMOAOfocal-plane

4 of 16 d-IFUspectrograph

units

Flat 3-axissteering mirrors

OAPs

MEMS-DMs

Key Design Points for AOKey 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.

LASER GUIDE STARSLASER GUIDE STARS

Laser guide star AO needs to use Laser guide star AO needs to use a faint tip-tilt star to stabilize a faint tip-tilt star to stabilize laser spot on skylaser spot on sky

from A. Tokovinin

Effective isoplanatic angle for Effective isoplanatic angle for image motion: “isokinetic angle”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

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

• Keck: >18th magnitude

1

0

271 degrees of freedom5 W cw laser

Galactic latitude = 90°Galactic latitude = 30°

From Keck AO book

LGS Related Problems: Null modesLGS Related Problems: Null modes

• Tilt Anisoplanatism : Low order modes > 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 b=30o

• 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

TMT.IAO.PRE.06.030.REL02

62

LGS WFS Subsystem needs constant LGS WFS Subsystem needs constant refocussing!refocussing!

• Trombone design accomodates LGS altitudes between 85-210 km (Zenith to 65 degrees)

• Astigmatism corrector present / Will study Coma corrector

TMT.INS.PRE.06.029.DRF01

63

Concept OverviewConcept Overview

TMT MIRES TMT MIRES (proposal)(proposal)LGS trombone LGS trombone systemsystem

TMT.IAO.PRE.06.030.REL02

64

3. NGS WFS 3. NGS WFS

• Radial+Linear 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

Issues for designer of AO systemsIssues 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

Effects of laser guide star on Effects of laser guide star on overall AO error budgetoverall 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 tilt

2 = ( / tilt )5/3

– New: focus anisoplanatism FA2 = ( D / d0 )5/3

– Laser spot larger than NGS meas2 ~ ( b /

SNR )2