Page 1 Adaptive Optics in the VLT and ELT era Beyond Basic AO François Wildi Observatoire de Genève

AO’s great divide High precision ExAO Wide field LTAO (high coverage) GLAO MCAO MOAO

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

High contrast imaging Highest contrast observations require multiple correction stages to correct for 1. Atmospheric turbulence 2. Diffraction Pattern 3. Quasi-static instrumental aberrations XAO visible coronagraph infrared coronagraph Diff. Pol. IFS SDI XAO, S~90% Coronagraph Differential Methods

NCPA compensation Use of phase diversity for NCPA correction on Vis. path 1111 -Strong improvement of bench internal SR (45 -> 85 in Vis) - various optimisations still to be performed

NCPA compensation for IR path No compensation NCPA compensation 320 modes estimated, 220 corrected Ghosts

Implementation CPI IRDIS IFS ZIMPOL ITTM PTTM DM DTTP DTTS WFS De-rotator VIS ADC NIR ADC Focus 1 Focus 2 Focus 3 Focus 4 NIR corono VIS corono HWP2 HWP1 Polar Cal

WIDE FIELD

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

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 guide 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 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.

Sky coverage and Wide field in a nutshell 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)

TOMOGRAPHIC AO LTAO, GLAO, MCAO, MOAO all required that the atmosphere is probed by multiple Wave Front Sensors to build a model of the atmosphere. What differentiates all those «AO» varieties is the way this tomographic model is exploited.

Proper use of the system requires several wavefront sensors to perform Tomography + Altitude Layer (phase aberration = + ) O Ground Layer = Pupil (phase aberration = O) WFS#1 WFS#2 Tomography = Stereoscopy

LASER GUIDE STARS VS NATURAL GUIDE STARS Tomography can also be performed with laser guide stars or natural guide stars BUT: NGS requires planning the GS for each observation With NGS Quality is not constant due to NGS geometry and flux distribution NGS requires movable wave front sensors

LASER TOMOGRAPHY AO In LTAO, the tomographic model is used to compute the wavefront distorsion in a perticular direction and calculate a correction in that direction. In principle the tomographic model is built with LGS 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!

WFS Set-up and LTAO reconstruction TelescopeTurb. Layers #2 #1 Atmosphere WFS UP DM corrects #1 + #2 in red direction

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

WFS Set-up and GLAO reconstruction Telescope Turb. Layers #2 #1 Atmosphere WFS UP DM corrects #1

Only modest Performance expected from GLAO From Gemini

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 layer Each 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? Case without Deformable mirror Turbulence Layers

What is multiconjugate? Case with it Deformable mirrors Turbulence Layers

Multiconjugate AO Set-up Telescope DM#2 DM#1 Turb. Layers #2 #1 Atmosphere WFS UP

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

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

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

Example of MCAO Performance 13x13 actuators system K Band (2.3 micron) 5 LGSs in X of 1 arcmin on a side Cerro Pachon turbulence profile 200 e-/sub/ms for WFS Four R=18 TT GS 30” off axis (MCAO) One R=18 TT GS on axis(AO)

Classical AOMCAO No AO 14’’ MAD MCAO Performance NGS results, 2DM (0km and 8.5km), K band 2 DMs / 5 NGS 1 DM / 1 NGS

The reality…: GEMINI MCAO Module DMs TTM LGS WFS NGS source simulator LGS source simulator Science ADC Beamsplitter Diagnostic WFS NGS WFS LGS zoom corrector NGS ADC shutters

Generalized Fitting Generalized Fitting (Finite number of DMs) d act Geometry of the problem

Generalized Anisoplanatism Generalized Anisoplanatism (effect of a finite number of Guide Stars) 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.

FoV = 70” FoV = 100” Generalized Anisoplanatism Generalized Anisoplanatism (effect of a finite number of Guide Stars) 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 down with increasing apertures 2D info only 3D info 2D info only 3D info Aperture

38 MCAO Pros and Cons 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)CONS Complexity:Complexity: Multiple Guide stars and DMs Other limitationsOther limitations: Generalized Fitting, anisoplanatism, aliasing

Optomechanical complexity 3 Shack-Hatmann WFS lay-out of MAD

MULTI OBJECTS ADAPTIVE OPTICS

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) Tiled MOAO focal- plane 4 of 16 d-IFU spectrograph units Flat 3-axis steering mirrors OAPs MEMS- DMs

Key Design Points for TiPi 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 STARS

LGS 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=30 o

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 REL02 50 LGS WFS Subsystem needs constant refocussing! Trombone design accomodates LGS altitudes between km (Zenith to 65 degrees)

TMT.INS.PRE DRF01 51 Concept Overview TMT MIRES (proposal) LGS trombone system

TMT.IAO.PRE REL 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