AO4ELT 2

SPHERE non-common path aberrations measurement and pre-compensation with optimized phase diversity processes: experimental results

Jean-Francois SAUVAGE — 2011-05-16T20:00:15Z

Submitted by J.-F. Sauvage

Authors

J-F Sauvage (1), T Fusco (1,2), D. LeMignant (2), C. Petit (1), A. Sevin (3), K. Dohlen (2), C. Robert (1), L Mugnier (1)

Affiliations

(1) ONERA, (2) Laboratoire d'Astrophysique de Marseille, (3) LESIA

Abstract

The SPHERE instrument is the 2nd generation instrument dedicated to exoplanet direct imaging and characterization. The extremely high imaging performance required by these observation mode calls for a high performance AO system. Particularly, this system has to provide a wavefront corrected from turbulent and internal defects. We present here the experimental results for the complete focal-plane calibration procedure of the SPHERE instrument internal defects. An optimized phase diversity method is applied allowing to deal with model uncertainties in the image formation (noise, residual background, amplitude fluctuation, sampling factor, défocalisation distance, object size and SH-model for reference slope modifications) The full procedure includes both Non-Common Path Aberrations (NCPA) compensation at the level of the coronagraphic mask using the eXtreme AO system itself (by the mean of modification of the filtered SH WFS reference slopes during close loop operations), but also additional measurements of IRDIS differential optical path aberrations for post processing of dual-band images.

We validate the algorithm and the pre-compensation procedure using data obtained during the first eXtreme AO bench (SAXO) integration and tests. We also applied the Diversity tool on stand-alone IRDIS data obtained at LAM during its local integration. In both cases, we demonstrate the robustness and the ultimate performance (nanometric precision of the residual quasi-static pattern) of the phase diversity approach which will allow us to obtain nanometric accuracy on the final SPHERE system.

Phase correction of segment diffraction for high-contrast imaging

Laurent PUEYO — 2009-02-28T23:00:00Z

Submitted by Bruce Macinstosh

Authors

Laurent Puyeo, Bruce Macintosh, Remi Soummer, Mitchell Troy

Affiliations

LLNL

Abstract

The exquisite angular resolution of segmented extremely large telescopes will provide astronomers with unique science opportunities in exoplanet imaging, from the ability to characterize the birth of exoplanets in star-forming regions to the direct detection of mature exoplanets in reflected light. However segmented apertures complicate the design of coronagraphic solutions for these instruments. While fill factor is a crucial figure of merit, e.g. many small segments with small gaps greatly simplify coronagraphic designs compared to a few large segments with large gaps, the static contrast is ultimately limited by optical artifacts due to the image of the segments gaps leaking through the starlight suppression system. Recent developments have shown how to accommodate segmented geometries using tailored coronagraphic designs (such as the generalized APLC and double stage Optical Vector Vortex Coronagraph). The successful implementation of such solutions at the very high contrast level can potentially degrade throughput and render the whole starlight suppression system more sensitive to both manufacturing and segments phasing errors. In this paper we propose an alternative solution that treats segment gaps can as a special case in reflectivity errors, with favorable spatial frequency properties but very high amplitude. Such reflectivity errors will have to be controlled in even a monolithic high-contrast system. We present the results of a numerical study which includes two sequential deformable mirrors as an extra degree of freedom in the design of the coronagraphic solution.

Tomographic phase diversity for phase retrieval on wide-field AO systems

Damien GRATADOUR — 2009-02-28T23:00:00Z

Submitted by Damien GRATADOUR

Authors

Damien Gratadour(1), François Rigaut(2)

Affiliations

(1) LESIA, Observatoire Paris , (2) Gemini Observatory

Abstract

Phase diversity is a commonly used technique to retrieve the wavefront at the focal plane. The usual algorithm involves two or more images of the same target with known phase changes like defocus. It has been shown to be very efficient at measuring on-axis the non-common path aberrations of classical AO systems. In this paper, we present an evolution of this algorithm towards tomographic measurements. This novel technique is dedicated to wide-field AO systems, allowing phase retrieval on multiple layers, conjugated at various altitudes. While the general grounds are very similar to classical phase diversity, the tomographic algorithm involves two or more images with known phase changes of several targets dispatched over the entire field of view. Regularization on the phase is usualy done by factorizing it on a basis of modes, traditionally Zernike polynomials. In this paper, we discuss the choice of a proper basis in the tomographic case and show that other basis such as disk harmonics are interesting alternatives in the case of real AO systems. We additionally propose two versions for this algorithm: an image-based and a Fourier-based both leading to comparable results. We finally present the results obtained on simulated data as well as on real data obtained on the Gemini MCAO system on which this algorithm has been used to estimate and compensate for non common path aberrations.

Pyramids, layers and no laser guide stars!

Roberto RAGAZZONI — 2009-02-28T23:00:00Z

Submitted by Roberto RAGAZZONI

Authors

Roberto Ragazzoni, Marco Dima, Jacopo Farinato, Demetrio Magrin, Valentina Viotto

Affiliations

INAF - Astronomical Observatory of Padova

Abstract

A decade after the first achievement into the improved capabilities in sensing by the pyramid wavefront sensor and from the outlining of novel classes of Multi Conjugated Adaptive Optics experimental verification of such approaches has been vastly proved by results from MAD onboard VLT and from FLAO onboard LBT. Refinement and extensions of these techniques promises to achieve similar goals within references scattered in a Field of View much larger than the one being compensated, an approach only marginally exploited in the so-called Multiple Field of View approach while the adoption of virtual DMs would allow a much deeper exploitation of such possibilities. As in the meantime the diameter of the largest project shortened somehow it is time to gather all these concept, to assemble -possibly- in an efficient way in order to continue to pursue the goal of achieving diffraction limited imagery at a level concorrential with what is being promised by artificial references, by the usage of solely natural guide stars. The resulting approach is a robust one (as it is not incompatible with Laser generated beacons) and can be implemented into existing optical design of the current extremely large telescopes under development

Post-coronagraphic wave-front sensing dedicated to exoplanet detection

Jean-Francois SAUVAGE — 2009-02-28T23:00:00Z

Submitted by Jean-francois SAUVAGE

Authors

Sauvage Jean-Francois, Mugnier Laurent, Paul Baptiste

Affiliations

ONERA-DOTA / HRA

Abstract

The final performance of current instruments dedicated to exoplanet search and imaging (such as SPHERE and EPICS) is strongly limited by uncorrected optical aberrations.

After correction of the atmospheric turbulence by an extreme AO system, the main contribution comes from the quasi-static aberrations introduced upstream of the coronagraph.

In order to measure and precompensate for these, we propose a focal-plane sensor which we call coronagraphic phase diversity (CPD). It is an adaptation of conventional phase diversity to the coronagraphic case and uses an analytical model for coronagraphic imaging.

In this communication, we validate two essential aspects of CPD:

we validate by realistic simulations that our analytical imaging model, which assumes a perfect coronagraph, can indeed be used with real-life coronagraphs, and we assess the CPD performance;

we perform the very first validation of CPD on experimental data obtained on an in-house AO bench.

OCAM2: world's fastest and most sensitive camera system for advanced Adaptive Optics wavefront sensing

Jean-Luc GACH — 2009-02-28T23:00:00Z

Submitted by Gach JEAN-LUC

Authors

Jean-Luc Gach, Philippe Balard, Eric Stadler, Christian Guillaume, Philippe Feautrier

Affiliations

LAM, Laboratoire d'Astrophysique de Marseille, Technopôle de Château-Gombert - 38, rue Frédéric Joliot-Curie -13388 Marseille, France; IPAG, Domaine Universitaire, 414 rue de la Piscine, BP 53 38041 Grenoble Cedex 9, France; OHP, Observatoire de Haute Provence, 04870 St.Michel l'Observatoire, France First Light Imaging S.A.S. Technopôle de Château-Gombert - 38, rue Frédéric Joliot-Curie -13388 Marseille, France

Abstract

For the first time, sub-electron read noise has been achieved with a camera suitable for astronomical wavefront-sensing (WFS) applications. The OCam system has demonstrated this performance at 1500 Hz frame rate and with 240x240-pixel. ESO and JRA2 OPTICON have jointly funded e2v technologies to develop a custom CCD for Adaptive Optics (AO) wavefront sensing applications. The device, called CCD220, is a compact Peltier-cooled 240x240 pixel frame-transfer 8-output back-illuminated sensor using the EMCCD technology. This talk demonstrates sub-electron read noise at frame rates from 25 Hz to 1500 Hz and dark current lower than 0.01 e-/pixel/frame. It reports on the comprehensive, quantitative performance characterization of OCam and the CCD220 such as readout noise, dark current, multiplication gain, quantum efficiency, charge transfer efficiency... OCam includes a low noise preamplifier stage, a digital board to generate the clocks and a microcontroller. The data acquisition system includes a user friendly timer file editor to generate any type of clocking scheme. A second version of OCam, called OCAM2, was designed offering enhanced performances, a completely sealed camera package and an additional Peltier stage to facilitate operation on a telescope or environmentally rugged applications. OCAM2 offers two types of built-in data link to the Real Time Computer: the CameraLink industry standard interface and various fiber link options like the sFPDP interface. OCAM2 includes also a modified mechanical design to ease the integration of microlens arrays for use of this camera in all types of wavefront sensing AO system. The front cover of OCAM2 can be customized to include a microlens exchange mechanism. A picture of OCAM2, the commercial version of OCam, is shown in Figure 2. OCAM2 is commercialized by the "First Light Imaging" company

LGS WFS on ELTs II: Impact of the sodium layer fluctuations

Sandrine THOMAS — 2009-02-28T23:00:00Z

Submitted by Sandrine THOMAS

Authors

S. Thomas, N. Muller, V. Michau, T. Fusco, D. gavel

Affiliations

UCO/Lick Osbervatory Onera

Abstract

The application of laser guide stars to large aperture telescopes has spurred many new studies in wavefront sensing. For the particular case of the Shack-Hartmann wavefront sensor (WFS), wavefront sensing is prone to new sources of errors not previously considered. The primary source of error is spot elongation resulting from the resolution of the finite thickness of the sodium layer. The elongation spreads the signal over several pixels, resulting in a decrease of the signal to noise ratio and an increase of non-linearity. Also, the SHWFS performance becomes sensitive to temporal and spatial variations of the density of the sodium atoms. Among the different centroid algorithms to be used with this WFS, the most powerful methods (correlation, matched filter, WCoG) require a reference. Although straightforward for a point source, the use of a resolved laser guide star is more cumbersome because the sodium layer variations affect their performance. In this paper we look at the impact of the sodium layer fluctuations and the reference choice on the performance of the WFS. We consider real as well as analytical non-symmetric profiles and investigate errors both at the subaperture level and at the wavefront reconstruction level. We conclude on the compromise between the use of the best possible reference versus the difficulty of implementation.

Laboratory results for speckle suppression with a self-coherent camera.

Pierre BAUDOZ — 2009-02-28T23:00:00Z

Submitted by Pierre BAUDOZ

Authors

Pierre Baudoz(1), Marion Mas(1), Raphael Galicher(2), Gerard Rousset(1)

Affiliations

1 :LESIA, Observatoire de Paris-Meudon, France 2: Herzberg institute of astrophysics , Victoria, Canada

Abstract

Direct imaging is a powerful tool for exoplanet atmosphere characterization. High performance of these techniques requires extreme wavefront correction for ground-based instruments as well as space projects. Wavefront sensors are usually physically separated from the common optics by a beam splitter in classical AO system. This separation introduces differential aberrations that are not measured by the wavefront sensor, which limits the performance of a planet finder instrument. We propose to use a Self-Coherent Camera (SCC) to directly measure the differential aberrations in the final coronagraphic science focal plane. The SCC is based on the principle of light coherence and allows us to estimate the wavefront errors upstream the coronagraph by spatially encoding the speckles with fringes in the final image. After recalling the SCC principle, we will present laboratory results on speckle suppression and compare it with expected performances from numerical simulations. We will also show the solution we developed to measure and correct the tip-tilt errors directly from the coronagraphic image.

LGS WFS on ELTs I: Wave Front Sensor Design & Analysis

Nicolas MULLER — 2009-02-28T23:00:00Z

Submitted by Nicolas MULLER

Authors

N. Muller, S. Thomas, V. Michau, T. Fusco

Affiliations

N. Muller: Onera DOTA / HRA, France S. Thomas: UCO / Lick Observatory, USA V. Michau, T. Fusco: Onera DOTA / HRA, France

Abstract

Adaptive Optics (AO) relies on a Wave Front Sensor (WFS) to measure properly the perturbations induced by the turbulence. In case of a Shack-Hartmann (SH) WFS, the elongation of the spot due to the longitudinal extension of Laser Guide Stars (LGS) affects the quality of the WFS measurements. As the diameter of the telescope increases, non-linearities appear through spot truncation and sampling effects. Various centroid algorithms have been proposed to reduce the effects of these non-linearities. In addition, the spatial structure of the LGS, set by the Sodium layer density profile, has an impact on the measurements. Both effects are coupled.

In this paper we aim at optimizing the design of a SH WFS in the case of a LGS on an ELT. Considering a Gaussian Sodium profile, we show that the impact of the WFS non-linearities can be neglected with regards to the noise. Therefore, taking into account the reconstruction filtering, the impact of the longitudinal extent of the LGS on the WFS measurement accuracy is moderate. Moreover this result is established with an extensive range of analytical Sodium profiles, provided that the Sodium density profile is known. The impact of the Sodium layer fluctuations is specifically studied in the second part of this paper, namely LGS WFS on ELTs II.

Experimental validation of the linearized focal-plane technique (LIFT)

Serge MEIMON — 2009-02-28T23:00:00Z

Submitted by Serge MEIMON

Authors

Serge Meimon, Cédric Plantet, Thierry Fusco, Jean-Marc Conan

Affiliations

ONERA

Abstract

Laser-assisted adaptive optics (AO) systems should increase dramatically the system sky coverage. Unfortunately, the laser guide star (LGS) wavefront sensing (WFS) principle is insensitive to tip/tilt, and focus measurement is corrupted by the evolution of the sodium concentration in altitude. Additionally, volumic structures of the LGS may induce quasi-static WFS errors.

Hence, low-order modes have to be measured separately using faint natural guide stars (NGSs), and a so-called "truth sensor" has to be used to calibrate higher order LGS induced WFS errors. In that framework, we have proposed a new focal-plane WFS concept called the linearized focal-plane technique (LIFT), which allows us to efficiently deal with low-order mode measurement under low flux conditions. It can also be used on long exposure as a truth sensor without any hardware modification.

We show here an experimental validation of LIFT in both low-order and truth sensor configurations. We compare the experimental linearity and noise propagation of LIFT to classical sensors, such as the quad-cell wavefront sensor (WFS), pyramid WFS, and Shack–Hartmann WFS.