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Astronomy & Astrophysics manuscript no. main ©ESO 2015June 9, 2015

SAXO, the eXtreme AO system of SPHERE (I)System overview and global laboratory performance

Jean-Francois Sauvage1, 2, Thierry Fusco1, 2, Cyril Petit1, Anne Costille2, 3, David Mouillet3, Jean-LucBeuzit3, Kjetil Dohlen2, Markus Kasper4, Marcos Suarez4, Christian Soenke4, Andrea Baruffolo5, Sylvain

Rochat3, Enrico Fedrigo4, Pierre Baudoz6, Emmanuel Hugot2, Arnaud Sevin6, Denis Perret6, Francois Wildi7,Mark Downing4, Philippe Feautrier3, Pascal Puget3, Arthur Vigan2, 4, Jared O’Neal4, and Dimitri Mawet4

1 ONERA - Optics Department, 29 avenue de la Division Leclerc, F-92322 Chatillon Cedex, Francee-mail: [email protected]

2 Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388, Marseille,France

3 UJF-Grenoble 1 / CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Greno-ble, F-38041, France

4 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany5 Istituto Nazionale di Astrofisica, Osservatorio di Padova, Vicolo Osservatorio 5, 35122 Padova, Italia6 Laboratoire d’Etudes Spatiales et d’Instrumentation Astrophysiques - Observatoire de Paris, Section de Meudon 5,place Jules Janssen 92195 MEUDON

7 Observatoire de Genève 51 ch. des Maillettes CH-1290 Sauverny

Received ...; Accepted...

ABSTRACT

Context. The direct imaging of exoplanet is a leading field of today’s astronomy. Capturing the few photons comingfrom a planet witnesses for the chemical composition of its atmosphere. The second-generation instrument SPHERE(Spectro-Polarimetric High contrast Exoplanet REsearch), dedicated to exoplanet imaging (both detection, photometricand spectral characterisation) of Jovian-like planets, is now in operation on the European Very Large Telescope [VLT].Such an instrument relies on an extreme adaptive optics (AO) system to compensate for atmospheric turbulence aswell as internal system defects with an unprecedented accuracy.Aims. We demonstrate in this paper the high level of performance reached by SAXO (the SPHERE eXtreme AO system)during the Assembly Integration and Test [AIT] period.Methods. In order to fully characterize the instrument quality, two AIT periods have been mandatory. In the first phase atObservatoire de Paris, the performance of SAXO itself were assessed. In the second phase at IPAG Grenoble Observatory,the operation of SAXO in interaction with the overall instrument (Grenoble observatory) has been optimised. In additionto the first two phases, a final check has been performed after the re-integration of the overall instrument at Paranalobservatory, in the New Integration Hall [NIH] before integration at the telescope focus.Results. The final performance reached by the SPHERE instrument are the best Strehl Ratio ever pretended for anoperational instrument (90% in H band, 50% in V band in a realistic turbulence r0 and wind speed), a limit Rmagnitude for loop closure at 15, and a robustness with high wind speeds. The Full Width Half Maximum reached bythe instrument are 40 mas for IR in H band, and unprecedented 17 mas in V band.Conclusions. conclusions (optional: leave void)

Key words. Instrumentation: Adaptive Optics, Coronagraph, High Contrast Imaging - Methods: Wavefront Sensing,Control, Phase Diversity, Shack-Hartman

1. Introduction

The direct imaging of extra-solar planets is one of the mostexciting challenges of today’s astronomy. From a scientificpoint of view, the light coming from the extra-solar planetorbiting its parent’s star witnesses for the chemical elementsof its atmosphere. Direct imaging of exoplanets thereforeenables in a short term the search for life or for habitableareas. In a long and philosophical term it brings an answerto the question: are we alone? The whole exoplanet commu-

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nity expects the scientific reach of this thematic. From aninstrumental point of view, harvesting the few photons com-ing from the self-luminous planet and disentangling themfrom the huge flux coming from the star is one of the mostdifficult tasks ever attempted in astronomy. The last decadehas seen the first try of this challenge (Chauvin et al. 2004),2014 has seen a real revolution with the delivery of theinstrument SPHERE (Spectro Polarimetric High contrastExoplanet REsearch (Fusco et al. 2014)) and GPI (GeminiPlanet Imager (Macintosh et al. 2014)) to the community.

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The direct imaging of exoplanets is the only techniqueable to detect and measure photons from planets outside ofour solar system. The other techniques are qualified as “in-direct” as they use the star signal (and not the planet itself)to detect and characterize the planet. The direct imaging ofexoplanet, consisting in grabbing directly the photon fromthe planet, will bring rich amounts of information allowingto fully characterizing the properties of exoplanets, includ-ing indications of habitability and physics and chemistryof their atmosphere. The direct methods are therefore aunique mean to retrieve intimate information for planetaryscience.

2. SAXO Description

2.1. SPHERE overview

The SPHERE system (Beuzit et al. 2008) aims at detect-ing extremely faint sources (giant extrasolar planets) inthe vicinity of bright stars. Such a challenging goal re-quires the use of a very-high-order performance AO system(Fusco et al. 2006), a coronagraphic device to cancel outthe flux coming from the star itself, and smart focal planetechniques to calibrate any coronagraph imperfections andresidual uncorrected turbulent or static wavefronts. The de-tection limit for the SPHERE instrument is 10−6 at 0.5arcsec(i.e, 15 magnitudes between the star and the planet)with a goal around 10−8. There is no direct link betweenthe AO system performance and the final detectivity of theinstrument; nevertheless, the impact of AO on the final per-formance is related to the performance of the coronagraph.A better AO correction leads to a better coronagraph ex-tinction and therefore leads to the following improvementsin system performance (Guyon 2005):

– a reduction of the photon and flat-field noises (i.e., again in Signal-to-Noise Ratio for a given integrationtime)

– a reduction of the static speckle (through the reductionof airy pattern intensity due to the coronagraph opti-mization).

These reductions are important from the global system per-formance point of view, and the optimization of the coro-nagraph rejection is a main goal of the SPHERE system.It of course requires the use and the optimization of aneXtreme Adaptive Optics [XAO] system, as presented inthe following. Nevertheless, the ultimate detection limit willbe achieved through an extreme control of system internaldefects (non-common path aberrations [NCPA] (Sauvageet al. 2007), optical axis decentering, vibrations (Petit et al.2008a), coronagraph and imaging system imperfections,and so on). This ultimate control will also be partially en-sured by the AO system through the use of additional de-vices. To meet the requirements (and hopefully the goals) interms of detection (Mouillet et al. 2009) the proposed designof SPHERE (Beuzit et al. 2008) is divided into four sub-systems, namely, the common path optics and three sciencechannels. The common path includes pupil-stabilizing fore-optics (tip-tilt and derotator) where insertable polarimetrichalf-wave plates are also provided, the SAXO XAO systemwith a visible wavefront sensor, and near infrared [NIR]coronagraphic devices in order to feed the infrared dual-imaging spectrograph [IRDIS] (Dohlen et al. 2008) and theintegral field spectrograph [IFS] (Claudi et al. 2008) with

a highly stable coronagraphic image in the NIR. The threescientific channels gather complementary instrumentationto maximize the probability of exoplanet detection and togive us access to a large range of wavelengths and informa-tion (e.g., imaging, spectra, and polarization).

The concept behind this very challenging instrumentis illustrated in Figure 1 left, where the common NIR-Visbeam is indicated in orange, the exclusively NIR beam isindicated in red, and the exclusively visible beam is indi-cated in blue. In order to fully characterize the instrumentquality, two AIT periods have been mandatory. In the firstphase at Observatoire de Paris, the performance of SAXOitself were assessed. In the second phase at IPAG Greno-ble Observatory, the operation of SAXO in interaction withthe overall instrument (Grenoble observatory) has been op-timised. In addition to the first two phases, a final checkhas been performed after the re-integration of the overallinstrument at Paranal observatory, in the New IntegrationHall [NIH] before integration at the telescope focus. Pic-tures of Figure 2 show the status of SAXO at each of thethree different steps.

The extreme AO system [SAXO] is the core of theSPHERE instrument, and is essential for reaching the ex-tremely high contrast requirements. In this framework,SAXO must fulfill the following three high-level require-ments:

• Ensure the measurement and correction of the turbulentphase perturbations of the telescope and system com-mon optics aberrations and of the NCPAs (main AOloop);• Ensure an extremely high stability (at low temporal fre-quency) of the optical axis at the level of the corona-graphic mask (using an IR differential sensor as close aspossible to the coronagraphic mask, the Differential TipTilt Sensor [DTTS])• Ensure the measurement and the correction of any pupilmotion (using a pupil motion sensor [PMS]).

The first and critical point to be addressed for any AOsystem optimization is the performance estimation param-eter. Unlike classical AO systems, residual variance andStrehl ratio are not sufficient anymore for optimizing thesystem and deriving the pertinent trade-offs. They have tobe replaced by a more accurate parameter that can provideinformation on the coronagraphic image shape in the focalplane. During the past few years, a large number of coro-nagraphic devices have been proposed, ranging from modi-fied Lyot concepts (with apodization for instance Soummer(2005)) to interferometric devices such as the four quad-rants coronagraph (Boccaletti et al. 2004). Each approachhas its own advantages and drawbacks, and several devicehave been implemented in the SPHERE instrument. In anycase, the purpose of the coronagraph is to remove the coher-ent light coming from the on-axis guide star (GS). Thereforeone can analytically define a ”perfect coronagraph” usingthe following equations:

Cres (ρ) =

⟨∣∣∣FT [P (r)A (r) eiΦres(r) −√EcP (r)

]∣∣∣2⟩Cres (ρ) corresponds to the image intensity in the focal

plane after the coronagraphic process. ρ stands for the focalplane position, r for the pupil plane coordinates, and <


Jean-Francois Sauvage et al.: SAXO, the eXtreme AO system of SPHERE (I)

Fig. 1. Global concept of the SPHERE instrument, indicating the four subsystems and the main functionalities within the commonpath subsystem. Optical beams are indicated in red for NIR, blue for Vis, and orange for common path

Fig. 2. [Left] SAXO being tested in Meudon Observatory [Center] SPHERE in its final shape at IPAG (8 tons, 6m length, 4mlarge, 2.5m height) [Right] SPHERE at the UT3 telescope Nasmyth focus

. > for a statistical average; and, with A(r) the wavefrontamplitude, Φres(r) the residual phase after AO correction,P (r) the pupil function and Ec the short exposure coherentenergy defined as follows:

withEc = e−[−σ2

Φ−σ2log(A)]

with σ2Φ and −σ2

log(A) being respectively the variance ofphase and amplitude effect accross the pupil.

According to Fusco et al. (2006) and for faint phaseregime, it quickly comes that as a first approximation(first order expansion) the coronagraphic image intensityis proportional to the residual phase power spectral density[PSD]:

Cres (ρ) ∝⟨|FT [Φres(r)]|2

⟩This approximation allows to refine our analysis of the

image formation, and to express more clearly the require-ments. The contribution of PSD to the final coronagraphicimage can be separated according to the localisation in thefocal plane. Let us separate the PSD contribution in threeterms. First the tip-tilt residual which presents a strong im-pact close to optical axis, then the PSD contribution to thecorrected halo up to the cut-off frequency of AO, then thePSD contribution outside of the AO corrected halo.

The final Strehl Ratio can be re-written as follows:

SRIm = e−σ2TipTilt−σ

2Halo−σ

2Uncorrected

The contribution of uncorrected PSD is mainly due toatmospherical perturbation at high spatial frequency, outof sight of the XAO system and therefore uncompressible.

Finaly the Strehl Ratio in the image is a good criterionfor the behavior of SAXO, and will be used in the follow-ing as the quality criterion of the subsystem. However asthe contrast is the final criterion of the whole instrument,we will present at the end of the paper some raw contrastand differential contrast results in order to fully ensure theperformance;

2.2. SAXO general requirements

SAXO high level requirements have been extracted fromthe SPHERE Technical Specification document and theSPHERE sub-system functional requirements. They arelisted hereafter:

• Maximum residual tip-tilt in normal conditions (0.85arcsec seeing): 3 mas rms• Maximum turbulent residual wavefront variance on cor-rected modes in normal conditions: 60 nm rms• radius of AO- corrected field up to λ/2d with d = 0.2m• SR(1.6 µm) > 15% in poor or faint conditions



Fig. 3. Schematic representation of SAXO loops

• Ability to stabilize pupil in translation: < 0.2% (goal =0.1%) of pupil diameter• Ability to reproduce an image position and to stabi-lize the image in translation, (hence compensate imagemovements due to thermo-mechanical effects and dif-ferential atmospheric dispersion between Vis and NIRbands) with accuracy better than 0.5 mas (goal 0.2 mas)• The residual non common path aberrations after phasediversity measurement and AO pre-compensation shallbe lower than 0.8 (goal 0.4) nm per mode.• AO system shall pre-compensate for 50 nm rms of non-common path defocus and 40 nm rms of the 55 firstZernike modes.

These values have driven all the SAXO design.

2.3. SAXO design and implementation

The SAXO system has to compensate for any wave frontaberrations and optical misalignment. The driving goal forthe scientific instrument is very high contrast imaging inthe near IR. A general overview of SAXO is given in Figure3.

The SAXO system is composed of 3 loops and an off-linecalibration illustrated on Figure 3.

• The main AO loop corrects for atmospheric, telescopeand common path defects. The main impact is the in-crease of detection signal to noise ratio through the re-duction of the smooth PSF halo due to turbulence ef-fects• The DTTS loop ensures a fine centering on coronagraphmask (correction of differential tip-tilt between VIS andIR channel). It will ensure an optimal performance ofthe coronagraph device• The PMS loop compensates for pupil shift (due to bothtelescope and instrument) (Montagnier et al. 2007). Itwill ensure that the uncorrected instrumental aberra-tions effects (in the focal plane) will always be locatedat the same position and thus will be canceled out by aclever post-processing procedure.• Lastly, the NCPAs pre-compensation which will lead tothe reduction of persistent speckle

In addition to these loops, the XAO system will alsoprovide different features allowing a close following of theenvironment evolution:

• Anti-wind up and Garbage Collector loop (Petit et al.2010) allowing to deal with saturated actuators

• A Tip Tilt and focus off-load process on telescope M2• A turbulence parameter and system performance on-linemonitor

SAXO gathers the following components and features:

• A high spatial and temporal frequencies deformable mir-ror to correct for all phase perturbations but the tip-tilt.The 41x41 actuators DM has been provided by CILAScompany. This component does not fulfill its specifica-tion in terms of dead actuators (17 are declared dead inthe SAXO system) and its shape at rest (which evolveswith temperature and can represent up to 100% of theDM stroke itself). The DM is now identified as themain risk for the SPHERE system. Section 3.3 givesan overview of the DM status.• a fast image tip-tilt mirror (ITTM), provided by LESIA(Observatoire de Paris) and located in a pupil plane forimage motion correction. This component does not ful-fill its specification (phase lag of 13 deg and 11 deg foreach axis instead of 5 deg). Nevertheless, this discrep-ancy has been considered as acceptable by the SAXOteam and the component has been formally accepted.• A 40 × 40 visible Spatially filtered Shack-Hartmann

(VIS-WFS). It includes an EMCCD provided by E2Vwith the following characteristics 240×240 pixels (pixelsize is 24×24µm ). The final detector has been providedby ESO in july 2013. This component fulfills (or evenexceeds) its specifications in terms of noise (< 0.1e−),read-out speed (> 1200Hz), quantum efficiency andpixel transfert function. In particular, these values con-tribute to the exceptional performance of the system interms of limit magnitude (see section 8).• A weighted Centre of Gravity [WCOG] algorithm(Nicolle et al. 2004) is considered for slope estimation.This algorithm has been successfully implemented andtested in SPARTA.• A spatial filter device (Poyneer &Macintosh 2003; Fuscoet al. 2005) is added in front of the WFS to reduce thealiasing effects. Its shape is square with a variable size(from 0.7 to 2.1 arcsec). This component fulfills the spec-ification in terms of speed and accuracy of size variationsas well as centering w.r.t. optical axis (30 mas on sky)• Real Time Computer [RTC] SPARTA (Fedrigo et al.2010) provided by ESO completely fulfills all the spec-ification. In particular, it exceeds its goal in terms ofRTC latency since a 80 µs has been measured (specwas 150µs with a 100µs goal). CCD read out + CCDcontroller delay + RTC latency + DM high voltage am-plifier leads to a 2.14 frame overall delay at 1200Hz forthe main SAXO loop which exceeds the specifications.The RTC includes a mixed control law: an OMGI forthe DM control and a Kalman filter based control lawfor the tip-tilt mirror(Linear Quadratic Gaussian (LQG)control). This Kalman filter control law corrects for upto 10 vibrations per axis located between 20 and 200Hztypically (Petit et al. 2008b).• A phase diversity algorithm (Robert et al. 2008) is usedto measure and optimize the NCPAs. The algorithmhas been successfully implemented in the instrumentsoftware [INS] and the residual NCPAs after the iter-ative compensation process completely fulfill the spec-ification (measured SR is larger than 99 % in H bandwhich means less than 20 nm rms of residuals and lessthan 5 nm on the 50 first modes).



• A slow pupil mirror PMS close to the entrance focalplane to correct for pupil shifts. The PMS (which di-rectly uses the WFS intensity) has been successfullyimplemented in SAXO and it has been shown that theloop is able to stabilize the SPHERE pupil with accu-racy largely better than the 0.5 % of the full pupil.• A slow infra-red tip-tilt sensor on the scientific channel(Baudoz et al. 2010) measures the differential tip-tilt be-tween the common and imaging paths has been success-fully implemented. The DTTS loop characteristics fulfilsthe specification (in term of measurement accuracy andloop bandwidth) and the final accuracy in terms of opti-cal axis stabilisation is lower than 0.5 mas which is wellwithin the specifications.

3. AO system internal performance

3.1. Performance criteria, computation processes andaccuracies

The performance of SAXO are expressed in Strehl Ratio[SR]. This quantity characterises the optical quality of animaging system, it corresponds to the ratio between theon-axis value of the PSF iab on the on-axis value of theAiry pattern iAiry. It is well-known that phase aberrationsdegrade the SR and hence the optical quality.

Several parameters have to be taken into account in or-der to obtain accurate and unbiased estimation of SR: thepupil shape and potential apodisation, the residual back-ground, the noise in the image, the CCD pixel scale, thesize of the calibration source. The SR value can be com-puted using the following equations:

SRim =iab(0)

iAiry(0)=

∫iab(f)∫iAiry(f)

where∫iab and

∫iAiry are the optical transfer function of

the aberrant system and of the aberration-free system, andf is a position variable in Fourier space. The 3.1 showsthe Optical Transfert Function [OTF] profiles in the case ofan experimental data processing. The Airy profile (dashed-dotted line) is different than the usual Airy-like OTF be-cause the data processed is acquired with apodisation andlyot stop in place. In particular, the typical feature of cen-tral occultation at mid-frequency is visible, but also the theexact apodisation map and lyot stop pupil shape have beenaccounted for in this computation.

3.2. Ultimate performance (internal source)

Let us first consider the ultimate performance of theSPHERE system itself. The calibration source is put at theentrance focal plane. This source is an unresolved broad-band halogen source standing for a bright M star. Thesource flux can be tuned to mimic different magnitude from12 to -3. SAXO loops are closed, NCPAs have been pre-compensated for.

The final result shows a residual RMS value for the NC-PAs (over all the phase spatial frequencies) around 25 nmrms. The low orders of NCPA aberrations are compensatedfor. Even if the algorithm measures the aberrations as aphase map (described on 64x64 pixels), an equivalent of 20to 50 Zernike polynomials have been compensated for.

Fig. 4. SR computation in Fourier space. The OTF profiles areplotted here, in the case of an experimental image of SPHERE,with apodized pupil and lyot stop.

Ref slopes WFS path WFS pathonly NCPAs

SR@H [%] 97.5 ±1 99.0 ±1RMS [nm] 41 ±10 25±10

Fig. 5. Ultimate performance of SPHERE, obtained on the in-ternal calibration source of the instrument.

3.3. DM defects and mitigations

This section is dedicated to the DM problems that havebeen identified as the highest risk item in the SPHEREsystem. It briefly describes the various DM issues as wellas the mitigation procedures proposed by the consortium.The HODM has been provided by the CILAS Company. Ithad to fulfill the following high specifications

– 41x41 actuator with a rectangular pitch (4.5 and 4.51mm) to match SPHERE optical design

– 1.5 µm of mechanical interactuator stroke– 10 µm mechanical stroke with a shape at rest lower than

1 µm– No dead actuator (and in particular no stuck actuator)– High temporal response : BW > 1kHz (no resonance

below 5 kHz) for ALL the actuators– Operation conditions : up to 99% of humidity and be-

tween 5° and 15°C (usual Paranal T° range)

Most of the specifications are not fulfill with the currentDM (in italic). Although the system performance is barelyaffected by the current BW limitation, the shape at rest(and especially its evolution with temperature) and the DMdead actuators (from 2 when the DM has been deliveredup to 18 at the moment this paper is written) represent themost severe limitations and medium / long term risks ofthe SPHERE system.



Fig. 6. Raw coronagraphic image of pupil plane (top) and fo-cal plane (bottom), without masking the actuators (left), withmasking the dead actuators (right)

3.3.1. Dead actuator mitigation

The HODM dead actuators term gathers three classes ofactuators: the ones with short-circuit, the ones with con-tact issues (between wires and piezo material) and the oneswith extremely slow response (i.e. a very high resistance).All combined, they number has grown from 2 (at the verybeginning of the AIT in 2010) to 18 (at the time we arewritten this paper). This increase is mainly due to DM op-eration in humidity condition larger than 50% and high T◦.Therefore, an active dry system (Nitrogen laminar flow intothe DM itself) allowing to reduce the humidity down to lessthan 50% has been added. Since the installation of HODMat Paranal (12 months), there was no new dead actuators.Considering the HODM design, any operation on these ac-tuators is almost impossible and thus the system has tolive with them. It has been demonstrated (both in simula-tion and in labs) that the impact on performance of such anumber of dead (stuck to a position corresponding to zerovoltage) actuator is the main contributor to the contrastloss. Therefore, adopting a strategy proposed by the Gem-ini Planet Imager (Macintosh et al. 2008), modified Lyotstops (occulting the pupil area containing the dead actua-tors) have been manufactured for each coronagraph (bothin IR and VIS) of SPHERE.The Figure 6-top shows thepupil image downstream of the coronagraph. On left side,the impact of dead actuators is clearly visible, each deadactuator resulting in a residual bright peak located at theposition of the actuator. The gain in performance is demon-strated in Figure 6-bottom. The effect of the dead actuatorsin the coronagraphic PSF (left) results in bright structurescorresponding to interference pattern due to residual brightpeaks. By masking the peaks, these pattern are efficientlyblocked. With the new Lyot stop, the ultimate performanceis similar to a perfect DM (without any dead actuator), evenif one additionnal dead actuator appeared between the de-sign of the mask and the acquisition of these images. All

Fig. 7. (left) DM typical shape at rest. (right) Evolution of theshape at rest with temperature.

HODM dead actuators are devalidated in the RTC system,so as to minimize their impact in the closed-loop process.

3.3.2. Shape at rest mitigation

The shape at rest is the initial shape of the DM before anydeformation. It can be compensate by the DM itself but itreduce the DM stroke dedicated to the correction of turbu-lence itself. The specification was that less than 10% of thefull DM stroke has to be dedicated to the DM flattening.Sadly, this specification is far from being reached. Not onlythis value is too high (50% in the best possible case) butit evolves with temperature (typically 1µm per degree Cel-sius) and can reach more than 100% of the full stroke whentemperature exceeds 18 degree Celcius (see Figure 7). Nev-ertheless since the shape is mainly cylindrical, it is possibleto compensate partially for it by introducing the oppositeshape somewhere in the SPHERE common optical path. Ithas been done by modifying one of the Toric Mirror pol-ished by LAM (Hugot et al. 2012) (the 3rd one, so-calledTM3), and installing a warping harness on TM3 for cylin-der compensation. A Shape optimisation has been madewith finite element analysis, demonstrating that a defor-mation system with one single actuator is able to producethe required cylinder shape. The latest is therefore adjustedonce per night, prior to the observation (see Figure 8). This“woofer-tweeter” configuration allows us to work with ourcurrent HODM by ensuring that the final DM shape at restis always lower than 2 µm mechanical peak-to-valley (in theextreme case) and a nominal case of less than 1 µmmechan-ical peak-to-valley which is less than 10% of the total DMstroke.

Although the DM is not perfect yet, the current ver-sion allows a full SPHERE operation and does not signifi-cantly degrades the performance, its potential degradationremains a real concern for the project and one of the mostimportant risk for the whole SPHERE mission. CILAS, to-gather with ESO and TMT are currently working on a re-covery plan to improve their manufacturing processes inorder to have, for the future, a viable solution for the nextDM generation.

3.4. AO system temporal behavior

Let us now analyze the temporal behavior of the AO sys-tem. Rejection Transfer Function [RTF] is a simple andefficient criterion to quantify the overall system temporalperformance. It translates the attenuation of the turbulentsignal (in frequency domain) by the AO system, and is here



Fig. 8. (left) 3D model of constraint mecanism, (center) 3D model including the mirror in its mount and motorisation, (right)picture of implemented mecanism on the real system at IPAG

defined as the ratio between the residual phase spectrumPSDresidual and the incident turbulent phase spectrum as-suming high Signal to Noise ratio (pseudo-open loop dataPSDturbulent), each depending on the temporal frequencyf . The phases are described as their decomposition on theDM or Tip-Tilt basis (voltage basis), and each actuator(1377 for the DM, 2 for the TT) are considered indepen-dant. The PSD are therefore obtained by averaging the volt-age information on all the actuators. It is to be noted thatthe result has been validated also for each actuator inde-pendently so as to detect potential bad behaviour actuator.

RTF =PSDresidual(f)

PSDTurbulent(f)

In Figure 9 and 10 the experimental RTF for HODMand ITTM are compared to theoretical expressions assum-ing a standard integrator controller with various gains. Thesampling frequency is 1200 Hz, although the WFS cameraand RTC can drive the system up to 1380Hz, and an ex-cellent match is found on all RTF at all gain values for a2.14 frames delay. This 2.14 frames delay includes the inte-gration time, the read out time and the RTC computationdelay. This value is completely within the specification andvery close to the goals. Note that the RTC pure delay hasbeen measured seperatly and is equal to 80 µs (which isbetter than the goal of 100 µs, and demonstrates the ex-ceptional quality of the SPARTA system). All these num-bers lead to a final system bandwidth around 70 Hz (foran AO loop gain = 0.5). In addition to classical integrator,a Linear Quadratic Gaussian [LQG] control law has beenimplemented for Tip Tilt mirror control in order to be ableto deal with system and telescope vibrations and to ensurethe smallest possible residual jitter. The LQG-Kalman andits associated on-line identification processes are detailledin Section 4.2.

4. AO system performance with turbulence

The performance of AO system have been assessed on tur-bulence during extensive test period at Meudon observatory(2011-2012), at Grenoble observatory (2012-2013), and fi-nally at Paranal observatory.

Fig. 9. HODM loop Rejection Transfer Function, for gain valuesfrom 0.1 to 0.4, and theoretical fit.

4.1. Turbulence simulator

In order to quantify the performance correctly, a dedicatedmodule of turbulence generation has been used. This mod-ule called TSIM (see Figure 11) for Turbulence Simulatoruses reflective phase screen plates produced by SILIOS com-pany. After one reflection on the plate, the optical beam re-produces the atmospheric turbulence. Two rotating platesare foreseen in the TSIM to receive phase screens. Two tur-bulent phase screens can be used either one at a time (theother one being a flat screen), or simultaneously (the tur-bulence effect therefore accumulates) in the TSIM. Thanksto the two phase screens of Fried parameters at 0.5 µmr0 = 12 cm (faint turbulence) and r0 = 16 cm (nominalturbulence), the turbulence strength that can be producedis θ0 = 0.64 arcsec, θ0 = 0.84 arcsec, and θ0 = 1.12 arcsecby combining the two phase screens.

The phase screens are calibrated by different means.

– An external calibration is performed on a dedicatedbench and using a 128x128 HASOTM. This calibrationallows to estimate the strength of the phase screen it-self. The variance averaged on each Zernike radial ordershows the expected shape for the high orders (see 12).



Fig. 10. ITTM loop Rejection Transfer Function, with a gain0.3 and 2 frames delay simulated FTR.

r0 [mm] PS1 PS2Specification 160 120Calibration 168 126

Long exposure 167 124SPARTA 154 118

Table 1. Cross-check of Fried parameter estimators.

The strength of the two phase screens are estimated atθ0 = 0.62 arcsec, and θ0 = 0.81 arcsec by this mean.

– Uncorrected long exposure images are acquired with theinstrument itself. The light distribution in such images(see Figure 13) gives a precise estimation of the strengthof the turbulence.

– The real-time computer SPARTA provides regular sta-tistical estimation of the turbulence parameters, in-cluding Fried parameter and wind speed. This esti-mation relies on the computation of pseudo-open loopturbulent modes from closed loop residual slopes andclosed loop correction voltages. As detailed in Fuscoet al. (2004), the turbulence parameters are estimatedfrom the pseudo-openloop data thanks to analytical be-haviour. The precise estimation of the noise variance isthe key point to access to turbulence parameter. Thisspurious parameter is estimated in the temporal auto-correlation of pseudo-open loop data.Unlike in Fusco et al. (2004), in SAXO the pseudo-loopare not reconstructed on the Zernike basis but on theKarhunen-Loeve basis. In this case, there is no analyti-cal expression of the dependency of Fried parameter andwind speed with respect to the turbulence variance. Anumerical relation has been derived on simulated data,then used as a model to perform a least-square estima-tion.

varoln = 7.3 ∗ n−1.89

4.2. Tip-tilt correction and vibration filtering

The global performance of Tip-Tilt of the SAXO subsys-tem is decomposed into two main contributors: the turbu-lent tip-tilt correction, and the absolute centering of star

Fig. 11. Simulation of turbulence for SPHERE. The heart ofTSIM: two rotating reflective phase screens.

Fig. 12. Calibration of the phase screen r_0=168mm by HASO:the Fried parameter fits to the expectation.

Fig. 13. Long turbulent exposure (open-loop) obtained withinfrared imager. Left with phase screen 0.85 arcsec. Right withphase screen 1.12 arcsec.

on coronagraph. Both these two contributors are known tobe the limitation to the raw image contraste, and need par-ticular care. The first contribution is adressed by the dedi-cated LQG control law in section 4.2.1, while the second isadressed by the differential Tip-Tilt loop in 4.2.2.

4.2.1. Ultimate performance of the Tip-Tilt loop: the LQGcontrol law with identification of vibration peak

The Linear Quadratic Gaussian control law is an optimalcontrol approach first introduced in AO by Paschall & An-derson (1993). This optimal law, coupled with an automaticprocess of identification of turbulence vibration and mea-



Fig. 14. Temporal PSD of tip-tilt mode, with classical integra-tor (black curve) and LQG control law (red curve).

surement noise, controls the Tip-Tilt mirror of SAXO incharge of the correction of the atmospheric turbulent Tip-Tilt aberrations. The vibration identification and optimalcompensation is mandatory in a telescope environment, es-pecially when dealing with such level of performance. Thefiltering of vibrations proposed by Petit et al. (2008a) andthe identification process proposed by Meimon et al. (2010)has been implemented on SAXO control laws. The illustra-tion of Figure 16 shows that vibration peaks are modelisedbased on pseudo-open loop data by AR2 filters. Up to 10peaks can be identified on each axis between 10 and 500 Hz,and limited for operationnal reasons between 30 and 300Hz,and the corresponding control law is modified to optimallyextinct them as far as the system framerate can. This is thefirst auto-tuned LQG based control on an operational sys-tem. The LQG models and control law are thus regularlyupdated on a 2 minute time basis.

Let us first consider the ultimate LQG performance. InFigure 14 and 15 we compare (in terms of PSD and cumu-lative PSD) the temporal spectrum of the residual Tip-Tiltobtained with a classical integrator (gain = 0.54) and aLQG process (with on-line identification of vibration peak).The LQG law clearly shows a residual on the Tilt of an or-der of magnitude lower (1/16th) than with the integratorlaw (visible on the last point at 600Hz on the cumulatedPSD). The residual Tilt RMS with LQG is therefore 1/4thof the residual Tilt RMS with integrator law. The maincontribution of this is clearly visible on the two vibrationpeaks extinction at 45Hz and 70Hz.

4.2.2. Absolute centering

The absolute centering precision requested is 3.0 mas. TheAO should be able to reproduce the PSF position in thefocal plane with a reproductibility better than 0.5 mas. TheFigure 17 shows the reproductibility of PSF position, whilerepeating the following sequence.

– Open all loops (Vis + IR)– Rotate Infra-red ADC by 10 degree– Close all loops (Vis + IR)– Measure non-corono PSF position on IRDIS, with Cen-

ter of Gravity algorithm

Fig. 15. Cumulated curves of PSD of previous figure 14.

Fig. 16. Example of identification run: the temporal PSD arefitted with 10 different AR2 filters for vibration peaks, and onefor turbulence.

The rotation of infra-red ADC is used to reproduce a re-alistic behavior of the instrument during observation, butwith an internal source scheme. The maximum deviation tothe reference position is equal to 0.35 mas.

4.3. SAXO performance in nominal conditions

The SAXO nominal performance is define for the followingconditions:

– V magnitude is around 9, i.e. 100 photo-electron persub-aperture and per frame on the WFS (@1.2 kHz)

– Seeing = 0.85” and wind speed = 12.5 m/s

In these conditions, with a fully operational and optimizedSAXO system (all loops closed, all optimization processesincluding Kalman vibration filtering on Tip Tilt) we haverecorded classical PSFs and coronagraphic images on IRDISin Dual Band Imaging mode (Vigan et al. 2010), see Figure18. On classical images, a SR value is estimated and willallow us quantify AO loop residuals. The coronagraphic im-age provides the final performance in terms of detectivity.

From the classical PSF, a SR has been computed: SR =90.3±2@1589nm. The error bars are estimated empirically



Fig. 17. DTTS absolute precision : position of PSF on IRDISwhile rotating the infrared ADC.

Fig. 18. SPHERE PSF as during AIT period, obtained withnominal conditions of guide star flux and atmospheric turbu-lence.

from experience and knowledge of the system parameteraccuracy. This SR can be translated into a global residualerror: σtotal = 81±10nm rms. Knowing that the fitting errorfor a 0.85arcsec seeing is estimated to σfit = 60±10nm rms.It leads to a residual rms on corrected modes (by the AOsystem) σcorr = 54 ± 14nm rms which is well within thespecification and fully coherent with the simulation resultsSRsim = 89.8%, leading to σcorr,sim = 83nm rms.

4.4. SAXO performance for faint guide stars

The evolution of SPHERE performance with respect to fluxis a vital behavior.

Figure 19 shows the evolution of SAXO performance(in terms of SR) for various SNR conditions. R magni-tudes have been computed from flux measured on VIS-WFSsub-apertures and system measured transmissions. It showsthat ultimate performance is achieved (in H band) for mag-nitude fainter than 9 and that the performance smoothlydrops down to 15 % for magnitude down to 15.5. This limit

a b

Fig. 20. SPHERE PSF in poor turbulence conditions.a b

Wind speed [m/s] 12.5 30.0Seeing [arcsec] 1.12 1.12Strehl Ratio@H 85.5% 73.3%

magnitude is an exceptional result, especially due to thevery good system transmission and the exceptional perfor-mance of the WFS in terms of RON (which is well below1e-) and quantum efficiency. The fact that the WFS detec-tor is a deep depletion device the main contributor to thisexceptional result.

4.5. SAXO performance in poor turbulence conditions

In this section, the performance (as well as the robustness)of the system is studied in the poor condition regime, i.e. ata seeing of 1.12 arcsec and two wind speed values of 12.5 and30 m/s. These measurements have been performed in goodtemperature conditions (< 17 degree Celsius) so that theDM had a relatively good shape at rest. First of all, despitesome actuators in saturation (well handled by the anti-windup and Garbage Collector processes), the loop was stableand robust during all the acquisition process (a few tens ofminutes). The PSF corresponding to these conditions areshown on Figure 20. The corresponding SR (H band) areestimated at 85.5% and 73.3%. The main effect of the highwind speed level is a smooth pattern of light residuals allover the corrected halo along the wind direction (vertical inthe images). This smooth pattern can be partially removedby differential imaging techniques.

5. Coronagraphic imaging and performance

Let us first analyze the raw coronagraphic images profilesof Figure 21 and compare them with a full end2end simu-lation. The profiles are obtained by the azimuthal averageof residual intensity of the coronagraphic image. The twoprofiles are remarkably similar. In the corrected area upto 800 mas, the profile follows a f−2 law signature of thenoise and wind speed residual errors. After the AO cut-offfrequency a f−11/3 law is found with the correct level (for0.85arcsec seeing conditions). Only a very small over ex-citation is visible in the experimental data around 700mas(less than a factor 1.5). It can be due to a WFS spatial filtera little bit larger in experimental data than in simulationand to the dead actuators. In any case, the coronagraphicshape is very well mastered and understood. It follows thesimulation data demonstrating that the AO loop is doing



Fig. 19. Performance of SPHERE with respect to guide star magnitude as measured in laboratory. Only the H performances havebeen measured (purple line), the performance at other wavelengths have been extrapolated from the H band measurement.

exactly what it is supposed to do. Note that these datawere acquired when only 8 actuators were dead. The rawcontrast degrades with the additionnal dead actuators (18at the date of publication), but this has been solved by theuse of dedicated masks presented in Section 6.

After the analysis of the raw coronagraphic profile (withcircular average), let us focus on detectivity curves seen onFigure 22. The detectivity curves are the azimuthal RMSvalue of the residual intensity of the coronagraphic image.Once again we compare experimental results and simula-tion. For detectivity aspect, the results depend on the ex-posure time (which will allow smoothing the residual tur-bulence speckle). In Figure 22 we compare an experimen-tal data with 10s and 3.4s exposure time with simulateddata. As expected the detectivity curve is proportional tothe square root of the exposure time in the non-correctedarea (where the dominant noise is the pure speckle one). Inthe corrected area things are more complex and the depen-dency with exposure time is less clear. Nevertheless, againa remarkable (for such kind of attenuation level and such acomplex system) match can be noticed between simulationand experimental data. Finally, a SDI process is applied onboth simulated and experimental data leading to solid lineprofiles in Figure 22 and a SDI performance around 10−6

for the two exposure times considered here.

6. Conclusion

SPHERE instrument has been delivered to VLT obser-vatory in mid 2014. SAXO, the eXtreme AO System ofSPHERE, has followed a long integration period from late2011 to end 2013 when the subsystem has been integratedto the full instrument. The performance reached by SAXOdurint AIT are fully compatible with the extreme imagingquality required for exoplanet direct imaging. The Strehlratio measured in laboratory with realistic star flux and tur-bulence reach 90% in H band, with around 80nm residualaberrations. This performance ensures a very high qual-ity for high contrast imaging, and SDI performances aredemonstrated in laboratory down to 10−6 (Mesa et al. 2015;Zurlo et al. 2014).

Fig. 21. SPHERE coronagraphic residual intensity profiles innominal turbulence conditions, and comparison with simulation.The grey area on small angular separation is covered by the ALCmask. The profile in this area is therefore not fully relevant.

Fig. 22. SPHERE coronagraphic detectivity profiles in nom-inal turbulence conditions, resulting from Spectral DifferentialImaging. The grey area on small angular separation is coveredby the ALC mask. The profile in this area is therefore not fullyrelevant.

ReferencesBaudoz, P., Dorn, R. J., Lizon, J.-L., et al. 2010, in SPIE Astronomical

Telescopes+ Instrumentation, International Society for Optics andPhotonics, 77355B–77355B

Beuzit, J.-L., Feldt, M., Dohlen, K., et al. 2008, in SPIE AstronomicalTelescopes+ Instrumentation, International Society for Optics and



Photonics, 701418–701418Boccaletti, A., Riaud, P., Baudoz, P., et al. 2004, PASP, 116, 1061Chauvin, G., Lagrange, A.-M., Dumas, C., et al. 2004, A&A, 425, L29Claudi, R. U., Turatto, M., Gratton, R. G., et al. 2008, in Society of

Photo-Optical Instrumentation Engineers (SPIE) Conference Se-ries, Vol. 7014, Society of Photo-Optical Instrumentation Engineers(SPIE) Conference Series, 3

Dohlen, K., Langlois, M., Saisse, M., et al. 2008, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol.7014, Society of Photo-Optical Instrumentation Engineers (SPIE)Conference Series, 3

Fedrigo, E., Bourtembourg, R., Donaldson, R., et al. 2010, in Societyof Photo-Optical Instrumentation Engineers (SPIE) Conference Se-ries, Vol. 7736, Society of Photo-Optical Instrumentation Engineers(SPIE) Conference Series, 2

Fusco, T., Petit, C., Rousset, G., Conan, J.-M., & Beuzit, J.-L. 2005,Optics letters, 30, 1255

Fusco, T., Petit, C., Rousset, G., et al. 2006, in Astronomical Tele-scopes and Instrumentation, International Society for Optics andPhotonics, 62720K–62720K

Fusco, T., Rousset, G., Rabaud, D., et al. 2004, Journal of Optics A:Pure and Applied Optics, 6, 585

Fusco, T., Sauvage, J.-F., Petit, C., et al. 2014, in SPIE AstronomicalTelescopes+ Instrumentation, International Society for Optics andPhotonics, 91481U–91481U

Guyon, O. 2005, ApJ, 629, 592Hugot, E., Ferrari, M., El Hadi, K., et al. 2012, A&A, 538, A139Macintosh, B., Graham, J. R., Ingraham, P., et al. 2014, Proceedings

of the National Academy of Science, 111, 12661Macintosh, B. A., Graham, J. R., Palmer, D. W., et al. 2008, in Soci-

ety of Photo-Optical Instrumentation Engineers (SPIE) ConferenceSeries, Vol. 7015, Society of Photo-Optical Instrumentation Engi-neers (SPIE) Conference Series, 18

Meimon, S., Petit, C., Fusco, T., & Kulcsar, C. 2010, JOSA A, 27,A122

Mesa, D., Gratton, R., Zurlo, A., et al. 2015, A&A, 576, A121Montagnier, G., Fusco, T., Beuzit, J.-L., et al. 2007, Optics express,

15, 15293Mouillet, D., Beuzit, J.-L., Feldt, M., et al. 2009, in Science with the

VLT in the ELT Era (Springer Netherlands), 337–341Nicolle, M., Fusco, T., Rousset, G., & Michau, V. 2004, Optics letters,

29, 2743Paschall, R. N. & Anderson, D. J. 1993, Appl. Opt., 32, 6347Petit, C., Conan, J.-M., Kulcsár, C., Raynaud, H.-F., & Fusco, T.

2008a, Optics Express, 16, 87Petit, C., Fusco, T., Fedrigo, E., et al. 2008b, in SPIE Astronomical

Telescopes+ Instrumentation, International Society for Optics andPhotonics, 70151D–70151D

Petit, C., Meimon, S., Fusco, T., Kulcsar, C., & Raynaud, H.-F. 2010,in CCA, 878–883

Poyneer, L. A. & Macintosh, B. A. 2003, in Society of Photo-OpticalInstrumentation Engineers (SPIE) Conference Series, Vol. 5169,Astronomical Adaptive Optics Systems and Applications, ed. R. K.Tyson & M. Lloyd-Hart, 190–200

Robert, C., Fusco, T., Sauvage, J.-F., & Mugnier, L. 2008, in SPIEAstronomical Telescopes+ Instrumentation, International Societyfor Optics and Photonics, 70156A–70156A

Sauvage, J.-F., Fusco, T., Rousset, G., & Petit, C. 2007, JOSA A, 24,2334

Soummer, R. 2005, ApJ, 618, L161Vigan, A., Moutou, C., Langlois, M., et al. 2010, MNRAS, 407, 71Zurlo, A., Vigan, A., Mesa, D., et al. 2014, Astronomy & Astrophysics,

572, A85


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