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

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

Dependence of Strehl on l and number of DM degrees of freedom Assume bright natural guide star No meas’t error or iso-planatism or bandwidth error Deformable mirror fitting error only

Decreasing fitting error Reminder #1: Dependence of Strehl on l and number of DM degrees of freedom (fitting) Decreasing fitting error Assume bright natural guide star No meas’t error or iso-planatism or bandwidth error Deformable mirror fitting error only

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 Guide star Turbulence Telescope Optics Detector Recon-structor Computer Transforms aberrations into intensity variations

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

Shack-Hartmann wavefront sensor concept - measure subaperture tilts Pupil plane Image plane CCD CCD

WFS implementation Compact Time-invariant

How to reconstruct wavefront from measurements of local “tilt”

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

Effect of guide star magnitude (measurement error) Assumes no fitting error or other error terms bright star Decreaing measurement error dim star

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

Reminder #3: Strehl vs l 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 LGS coverage ~80 % Tip/tilt sensor magnitude limit Hartmann sensor magnitude limit Galactic latitude NGS coverage 0.1 % Isokinetic angle qk Isoplanatic angle q0

(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.

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

Like classical AO but more of the same 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 XAO, S~90% Coronagraph Differential Methods visible coronagraph infrared Diff. Pol. IFS SDI Highest contrast observations require multiple correction stages to correct for Atmospheric turbulence Diffraction Pattern Quasi-static instrumental aberrations XAO, S~90% Coronagraph Differential Methods

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

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

Implementation CPI ZIMPOL IFS IRDIS ITTM PTTM DM DTTS WFS DTTP Focus 1 De-rotator HWP2 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

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 DM’s)

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!

Proper use of the system requires several wavefront sensors to perform Tomography Altitude Layer (phase aberration = +) Ground Layer = Pupil (phase aberration = O) Inversion process describe figure Altitude of aberration proportional to shear at WFS -> retrieve altitude from 2D wfs info. Tomography = stereoscopy In MCAO case: Restricted problem -> limited number of DM. Treated as a whole. Similarly to AO were one does not explicitely reconstruct the phase, in MCAO the 3D phase distribution is not reconstructed, and then projected to the DMs. The system computes directly the DM commands that will minimize the error as measured by the WFS. More stable. Tomography = Stereoscopy WFS#1 WFS#2

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

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

Performance expected from GLAO (Gemini)

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

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

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 Turbulence Layers Deformable mirror

What is multiconjugate? Case with it Deformable mirrors Turbulence Layers

Multiconjugate AO Set-up Turb. Layers Telescope WFS #2 #1 DM#2 DM#1 Usual stuff Atmosphere 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 Numerical simulation tools written at Gemini here I present results obtained with a Monte Carlo based simulation, with the parameter listed here. What you see is a seeing limited image of a stellar field. Note speckle boiling (uncorrelated from one corner to the next) and global image motion. Close the loop with single centered guide star -> good compensation in small centered fov. Anisoplanatism of high order modes -> speckle boiling. Aniso of tilt -> image motion off axis. Image motion mostly radial because of the better correlation of tangential tilt component Add 4 stars in 4 corners, and one DM -> MCAO system

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 Numerical simulation tools written at Gemini here I present results obtained with a Monte Carlo based simulation, with the parameter listed here. What you see is a seeing limited image of a stellar field. Note speckle boiling (uncorrelated from one corner to the next) and global image motion. Close the loop with single centered guide star -> good compensation in small centered fov. Anisoplanatism of high order modes -> speckle boiling. Aniso of tilt -> image motion off axis. Image motion mostly radial because of the better correlation of tangential tilt component Add 4 stars in 4 corners, and one DM -> MCAO system

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 Numerical simulation tools written at Gemini here I present results obtained with a Monte Carlo based simulation, with the parameter listed here. What you see is a seeing limited image of a stellar field. Note speckle boiling (uncorrelated from one corner to the next) and global image motion. Close the loop with single centered guide star -> good compensation in small centered fov. Anisoplanatism of high order modes -> speckle boiling. Aniso of tilt -> image motion off axis. Image motion mostly radial because of the better correlation of tangential tilt component Add 4 stars in 4 corners, and one DM -> MCAO system

MCAO Performance Summary Early NGS results, MK Profile Classical AO MCAO No AO 1 DM / 1 NGS 2 DMs / 5 NGS Long exposure images. Pros of MCAO Enlargement of the field PSF uniformity across the field -> data reduction impacts 165’’ 320 stars / K band / 0.7’’ seeing Stars magnified for clarity

The reality…: GEMINI MCAO Module NGS source simulator LGS source Science ADC Beamsplitter Diagnostic WFS NGS WFS LGS zoom corrector ADC shutters DMs TTM LGS WFS This is a schematic showing the components of the AOM and the light path through them. The light from the telescope enters the system at the top RHS and is directed onto the 3 deformable mirrors. These are the mirrors which are conjugated to different heights in the atmosphere. The beam then reflects off a tip-tilt mirror and onto a beamsplitter. At this point, part of the beam is directed down to another beamsplitter where the laser light is split from the natural star light, probably by use of a filter at the sodium wavelength. The laser light is directed onto the LGS WFS’s while the natural star light is directed to an array of Avalanche Photo Diodes. The APD’s are used to obtain Tip/Tilt information while the LGS WFS’s are used to obtain higher order corrections for abberations such as coma, astigmatism, trefoil, etc. The remaining star light, not picked off by the first beamsplitter carries on into the telescope instrument being used for the observation.

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 R=18 TT GS 30” off axis (MCAO) One R=18 TT GS on axis(AO) Get more quantitative info. Plot of Strehl, FWHM, 50%EE and Slit coupling versus distance to the central GS. Medium order system. Computations done for the Gemini south system. 5 guide stars, 3 DM. Noise included both on LGS and NGSs. 4 NGS because of symmetry reasons to speed up results evaluation. Demonstrate later that only 3 needed, and adding a fourth only marginally improve the results. Classical LGS AO: one R=18 GS on axis Comment results

MCAO Performance Classical LGS AO MCAO 1 Strehl Surface plots of Strehl ratio over a 1.5 arc min FoV. 13x13 actuator system, K band, CP turbulence.

Other nice features of MCAO Average Strehl (triangles) 1 .5 + Strehl St. dev across FoV % (+) 2 4 Profile number Robustness Robustness Sensitivity to noise is fairly better than with AO Prop noise AO / Prop noise MCAO  sqrt( NGS ) Predictive algorithms possible ? Robustness Sensitivity to noise is fairly better than with AO Prop noise AO / Prop noise MCAO  sqrt( NGS ) Other aspects of MCAO: Robustness (smooth and forgiving dependence upon DM altitude) Noise sensitivity (need sqrt(N_GS) times less photons per GS (Gemini case studies) Slicing up the atmosphere -> Taylor ok ? predictive algorithms

Generalized Fitting (Finite number of DMs) dact Geometry of the problem Gen Fitting. Finite # of DMs. Sketch of the geometry Image sharp bump in layer -> impossible to correct in *all direction* -> filtered out of the command Command act as a spatial low pass filter of width theta.dh Simple model check: Using simulation code written at Gemini, we have simulated 2 DMs and 3 DMs system (0,4,13), (0,13), (4,13) - extrema found where expected - addition of 0 km DM does not modify perf between 4 and 13 - steep degradation of S 4-13 -> 0-13 - simple model seems to fit good. Compute error vs regular fitting error -> behavior in 5/3 power of theta.dh

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.

Generalized Fitting (Finite number of DMs) Error [rd2]  (.h)5/3 Design Criteria e.g. Error balanced  hmax(,dact) DM Spacing = 2 x hmax

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 FoV = 70” FoV = 100”

Generalized Anisoplanatism goes down with increasing apertures 2D info only 3D info 2D info only 3D info Aperture

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

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

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.

Laser guide stars

Laser guide star AO needs to use a faint tip-tilt star to stabilize laser spot on sky from A. Tokovinin

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

Sky coverage is determined by distribution of (faint) tip-tilt stars Keck: >18th magnitude 1 271 degrees of freedom 5 W cw laser Galactic latitude = 90° Galactic latitude = 30° From Keck AO book

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=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

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 TMT.IAO.PRE.06.030.REL02

TMT MIRES (proposal) Concept Overview LGS trombone system TMT.INS.PRE.06.029.DRF01

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 Flexible design - minimize vignetting Radial + linear stages Similar to OIWFS for F2 and GMOS TMT.IAO.PRE.06.030.REL02 Flamingos2 OIWFS

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

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