executive summary of the proposal - lam1. executive summary of the proposal over the 20 past-years,...

PROGRAM RETOUR POST-DOCTORANTS

EDITION 2013

Projet WASABI

DOCUMENT SCIENTIFIQUE

ANR-GUI-AAP-06 – Doc Scientifique 2013 4/28

1. EXECUTIVE SUMMARY OF THE PROPOSAL

Over the 20 past-years, Adaptive Optics [AO] for astronomy went from a demonstration phase, to a well-proven and operational technique. Since the first astronomical AO systems were opened to the community in the early 1990s, numerous technical achievements have been accomplished, a multitude of novel techniques have been established, and it is now inconceivable to consider building a large telescope without AO. AO observations have brought some of the major discoveries in astronomy with, among others, detailed study of the massive black hole at the center of our Galaxy (e.g. Ghez et al. 2008, Genzel et al. 2010), detailed images of the surface of solar systems planets (e.g. Hartung et al. 2004, De Pater et al. 2010), or precise morphology and dynamics of very distant galaxies (e.g. Huertas-Company et al. 2008, Cresci et al. 2009, Law et al. 2009, Wright et al. 2009). AO have revolutionized the ground-based telescopes by providing the highest achievable image quality of the world. We are today at the beginning of a new step forward, with the birth of a revolutionary generation of AO systems called Wide Field AO [WFAO]. By using multiple Laser Guide Stars [LGS], WFAO significantly increases the field of the AO corrected images, and the fraction of the sky that can benefit from such correction. Therefore, where the first AO systems were well suited for observations of bright and relatively small objects, the new generation of WFAO is opening the path for a multitude of new science cases. Within a decade, the world will see a new generation of telescopes with diameter up to 39m, called the Extremely Large Telescopes [ELTs]. These giants will address fundamental astrophysical science cases as for instance the direct imaging and characterization of exo-worlds or the study of bulk and evolution of the first galaxies. The scientific potential of these giants relies on challenging new AO concepts, integrated inside the telescope itself, and providing high-resolution images to all the instrumentation downstream.

With the deployment of the WFAO systems on current telescope generation, and with the venue of the ELTs, Adaptive Optics is becoming a new must for astronomical observations.

Our program aims at being an active part in preparing this transition. For this, we have defined 3 main axes around which the project will be developed: • Astronomical exploitations of new WFAO systems. This part of the project will be dedicated to execute

AO-assisted observations for a set of three science cases, identified as key in modern astronomy and for the future ELTs. Beyond the unprecedented astronomical results envisioned for these observations, we will use the data to do a fair assessment of current and near-future WFAO systems performance.

• Development of new data reduction and analysis tools. WFAO observations will be used to test, develop and validate new data reduction and analysis tools, adapted and dedicated to the optimization of the science return. These tools will be made available to the community.

• Study and validation of new AO concepts. New and innovative technological development for the optimization of the future AO-instruments will be developed in this part of the project. Our effort will be focused towards a significant improvement of the number of targets that would benefit from AO corrections

The project will gather worldwide experts in astronomy and Adaptive Optics, in order to greatly improve the scientific returns of current AO systems, and open brand new areas in the way to design, operate and optimize AO for the next generation of AO-assisted instrumentation. The project will be lead by the Laboratoire d’Astrophysique de Marseille [LAM]. LAM is internationally recognized as a major actor in the large instrumentation programs for ground and space instrumentation.

Our program aims at providing answers to key astronomical questions, optimize the use of the current AO-assisted instrumentation, and prepare the future generation of instruments.


EDITION 2013

Projet WASABI



2. CONTEXT, POSITION AND OBJECTIVES OF THE PROPOSAL 2.1. CONTEXT Today's Astronomy observations rely on state-of-the-art instrumentation and facilities. The gain in higher sensitivity, better image quality, and wider field of view allows scientists to make breakthrough in their research field. Astronomy is a science where the observations of extremely distant objects are the single source of information. Therefore, larger aperture telescopes and higher angular resolution play a crucial role.

Toward these objectives, one of the key technological breakthrough done in the past decades was the introduction of Adaptive Optics [AO] for astronomical observations.

At the cross of optics, electronics, atmospheric science, control theory, computer science and mathematics, AO is a technique that aims at compensating quickly-varying optical aberrations to restore the ultimate angular resolution limit of an optical system. It uses a combination of wave-front sensors, to analyze the light wave aberrations, and deformable mirrors to compensate them.

For astronomical telescopes, AO allows to overcome the natural ''seeing'' frontier: the blurring of images imposed by atmospheric turbulence and limiting the angular resolution of ground-based telescope to that achievable by a 10 to 50cm telescope, an order of magnitude below the diffraction limit of large 8-m class telescopes which are the current standard.

Although initially successful, two main limitations reduced the usefulness of AO and its wide adoption by the astronomical community: the need for a bright guide star close enough to measure the wave-front aberrations and the small field of view compensated around this guide star -typically a few tens of arcseconds.

The first limitation was solved by creating artificial guide stars, using lasers tuned at 589nm on the atomic sodium D2 line, which excite sodium atoms located in the mesosphere around 100km altitude (Foy & Labeyrie 1985)1. These ''Laser Guide Stars'' [LGS] could be created at arbitrary locations in the sky, thus solving partially the problem of scarcity of suitable guide stars. Nowadays, all of the major ground based telescopes are equipped with such lasers.

The second limitation arises from the fact that the atmospheric turbulence is not concentrated within a single layer but spread in a volume -typically the first 10km above sea level. New AO concepts, referred as Wide-Field AO [WFAO], were then proposed to solve this problem (Ellerbroek et al. 1994, Fusco et al. 2001). WFAO have been in study for many years, and it's only very recently that the first of these WFAO became available for the astronomical community at the Gemini observatory (Neichel et al. 2012).

This first system, called GeMS for Gemini Multi-Conjugate AO System, uses several deformable mirrors, optically conjugated to various altitudes, and several wave-front sensors, combined with tomographic techniques, in order to provide a uniform and almost diffraction limited correction over a field 10 to 20 times larger than any other current AO systems (see Fig. 2). The images delivered by GeMS at Near-Infra-Red [NIR] wavelengths are at the same high angular resolution as what the Hubble Space Telescope [HST] achieves at optical wavelengths, and open the way for new synergies between ground and space based science.

1 References are given in Sect.7

Figure 1: Gemini LGS constellation


EDITION 2013

Projet WASABI



Figure 2 - The central part of the globular star cluster NGC288. The image, taken at 1.65 microns (H band) on December 16, 2011, has a field-of-view of 87 x 87 arcseconds. The average full-width at half-maximum is slightly below 0.080 arcsecond, with a variation of 0.002 arcsecond across the entire field of the image. Exposure time was 13 minutes. Insets on the right show a detail of the image (top), a comparison of the same region with classical AO (middle; this assumes using the star at the upper right corner as the guide star), and seeing-limited observations (bottom). The pixel size in the latter was chosen to optimize the signal-to-noise ratio while not degrading the intrinsic angular resolution of the image.

GeMS is a precursor for this new generation of WFAO systems, and in a couple of years, the European Southern Observatory [ESO] will be equipped with similar technology, as part of the AO Facility [AOF]. Few years from this, the Large Binocular Telescopes [LBT], the Keck telescopes and the Subaru telescope will also be equipped with WFAO systems. This will bring the current generation of 8-m at their limits in term of angular resolution, and open a large variety of new science cases for AO astronomical observations.

The next step forward will come from the so-called Extremely Large Telescope (39 m diameter European ELT [E-ELT], 30 m north American TMT, 24 m American and Australian GMT) that should have their first light in a decade or so. These giants are all relying on performant WFAO systems, starting operations since day one. The colossal size of these telescopes (up to 39 m) and the complexity of the scientific instruments compel us on a complete rethinking, in order to improve the overall performance, but more specifically the sensitivity and the robustness of the AO systems, and thus to maximize the astrophysical returns of AO assisted instruments.

So far, the science impact of AO has benefitted greatly from case studies of relatively few intensively studied objects. In the future, the use of AO on large and diverse samples will become increasingly important. In the next decade, Adaptive Optics will become a new standard for astronomical observations.


EDITION 2013

Projet WASABI



2.2. STATE-OF-THE-ART, POSITION AND OBJECTIVES OF THE PROPOSAL

We are on the verge of a new era for ground-based astronomy, where most instruments, for existing facilities as well as for future ELTs, rely on very challenging AO concepts and highly innovative components.

The main scientific objective of our project is to assure this transition, and move AO towards more efficient, more reliable and more robust systems. For this, we have designed the WASABI project in order to cover both the scientific exploitation of current and up-coming instrumentation and the development of new and innovative instrumentation. The ultimate goal being the optimization of the scientific returns of the WFAO assisted instrumentations.

Drawing the science potential of WFAO observations:

The first part of our project will be dedicated to do a fair assessment of current and near-future WFAO systems performance for a set of key science cases. We have selected three main science cases, identified as ones of the main prominent open questions in modern astronomy, and that cover a wide range of AO-needs. These science cases are respectively (i) astrometry and exo-planet detection, (ii) star forming regions, (iii) extragalactic studies. For each of this science case, we will define and execute AO-assisted observations. We envision that the proposed observations will bring significant and unprecedented astronomical discoveries.

Optimizing the science return of WFAO observations:

In a second phase of the project, these observations will be used to test, develop and validate new data reduction and analysis tools, adapted and dedicated to the optimization of the science return. In particular, all the science cases identified above would largely benefit from a precise knowledge of the Point Spread Function [PSF]. While the PSF can sometimes be estimated, it remains a major problem when analyzing data from current AO systems. An important objective of this project will therefore be to enable the PSF to be derived from the AO-observations. The science cases have been chosen to cover different constraints on PSF knowledge, and will allow us to test the newly developed method on a wide range of astronomical cases. All the software and tools that will be developed in the frame of this project will be made available to the community on a timely manner.

Preparing the future with innovative technological developments:

Finally, the third phase of this project is related to new and innovative technological development for the optimization of the future AO-instruments. “AO ought to be accessible to targets for which the primary selection criteria are astrophysical”. To fulfill that requirement, the information gathered through the science WFAO observations will be used as a valuable feedback for system developments and optimizations. In particular, we will focus our effort around the development of new and highly sensitive Wave-Front Sensor [WFS] devices, aiming at boosting the sky coverage, and the number of targets that would benefit from AO corrections. This part of the project will largely benefit from the unique expertise developed on this domain by the coordinator partner (Laboratoire d’Astrophysique de Marseille).

In summary, the WASABI project will greatly improve the scientific returns of current WFAO systems and open brand new areas in the way to design, operate and optimize AO for the next generation of AO-assisted instrumentation. This project will bring together astronomers, AO experts and engineers at the forefront of innovative technology development. The role of the coordinator will be to act as an instrument scientist, at the frontier between the astrophysical results and the associated instrumentation.


EDITION 2013

Projet WASABI



2.2.1 OPEN A NEW WORLD OF ASTRONOMICAL OBSERVATIONS

The newly born WFAO generation opens a new, wide and unique range of astronomical studies that were not possible before. From solar system objects, to star clusters and distant galaxies, WFAO will bring a new view over our comprehension of the Universe. We have chosen three major science cases for which we will carry WFAO observations. Our program aims at providing answers to fundamental questions such as: how galaxies form and evolve? How stars are formed in our Galaxy? How planets form, and can we discover new worlds?

Precise astrometry and exo-planets detection

State-of-the-art: Wide field Adaptive Optics systems has the potential to be a premier facility for precision astrometry due to the powerful combination of high spatial resolution, large field of view, and infrared imaging.

Potential astrometric science cases cover a broad range of topics including exo-planets, star formation, stellar evolution, star clusters, black holes and neutron stars, and the Galactic center. Many of these areas cannot be addressed with HST or GAIA astrometry due to insufficient resolution or lack of sensitivity at infrared wavelengths. As an example, the image of Fig. 3 is a close view of the center of the Galaxy (SgrA*), around the location of a super massive black hole (red cross). A high-resolution, multi-year observation of this region reveals the motion of the stars around the black hole, and allows a precise study of its physical characteristics.

Astrometric precisions of < 1 mas over large field of view enable many new experiments in these areas that have not been efficient or even possible with existing ground-based AO systems due to their limited fields of view. One of the most exciting promise of precise astrometry is probably the detection of exo-planets. The search for exo-planets is a rapidly growing field. Discovering new planetary systems is crucial to understand the distribution of

planets around stars of various types, to study the range of their physical and orbital characteristics, and to estimate their number in the Galaxy (e.g. Lagrange et al. 2010, Marois et al. 2010). Determining those statistics for a wide range of planet mass, orbit radius, system age and star mass is key to understand the planet formation process and how planetary systems are evolving. The astrometry of the nearest stars is particularly interesting as they produce the largest signature for an orbiting planet and are the most likely candidates to find and image the first habitable rocky planet. However, progress in astrometric detection of exoplanets has been frustrated by the small photocentric motion induced on parent star and instrumental challenges achieving multi-epoch astrometric stability (Ammons et al. 2012). For example, an Earth-like planet at 1 AU around a Sun-like star at 10 pc only produces 0.3 microarcsec motion.

New WFAO systems are overcoming some of the main astrometric limitations, and are opening new opportunities for planet detection. In the long term, WFAO systems on ELTs should deliver the best available relative astrometric precision from the ground, which may be sufficient to detect exo-earths orbiting nearby brown and red dwarfs (< 30 µas).

Position and objectives: To move toward these exciting results, we propose to carry a complete and deep analysis of the astrometric precision provided by current WFAO system. In particular, we want to quantify the systematic errors that may dominate long-term astrometric stability with WFAO, such as dynamic optical distortion and differential atmospheric refraction, and derive solutions to overcome those. We have already started a preliminary study (Rigaut et al. 2012). We will now carry a full analysis based on a large set of WFAO images obtained on a dense globular cluster (NGC1851) over different epochs. This analysis will draw the limit for the current astrometric science cases, and set the basis for the new generation of instruments. In parallel, we will be observing a set of exo-planets candidates, and precisely derive the potential of astrometry for such studies.

Figure 3: Galactic Center observed by GeMS.


EDITION 2013

Projet WASABI



Star formation

State-of-the-art: Young star clusters are key laboratories for understanding the star formation process. Various theories of star formation predict different outcomes for the Initial Mass Function [IMF], multiplicity, dynamics, and how they may differ with the environment of the cluster (Haramaya et al. 2008, Bastian et al. 2010). However, studies of star clusters are still very limited due to the high spatial resolution needed to resolve individual stars, confusion with the background and foreground sources that can only be resolved using astrometry or spectroscopy, and high extinctions that require working in the infrared. The new WFAO systems are perfectly suited for these kinds of studies, as they provide a uniform and unprecedented angular resolution over fields covering large portion of clusters. The resolution provided by the WFAO instruments is also an ideal complement to the radio observations that will be delivered by ALMA, the newly commissioning Atacama Large Millimeter Array. With higher angular resolution and higher sensitivity, astronomers will be able to observe the faintest, least massive stars, allowing to close the current huge gap in our knowledge concerning star and planet formation.

Star formation science will be dramatically boosted in the next decade, with the deployment of the ELTs equipped with WFAO. The immense sensitivity and exquisite angular resolution of these future instruments will offer the exciting prospect of reconstructing the formation and evolution histories of a representative sample of galaxies in the nearby Universe by studying their resolved stellar populations (Melbourne et al. 2010). A galaxy's stellar populations carry a memory of its entire star formation history, and decoding this information offers detailed insights into the galaxy's past, and how galaxies form and evolve.

Position and objectives: We aim at bringing strong constraints on the processes at work in star formation in galactic and extra-galactic young clusters. For this we have selected a sample of nearby young clusters to understand how WFAO can open new windows in this research field. These clusters are dense, associated with extended emission in the K-band and contain both bright and faint stars (Zavagno et al. 2010, Deharveng et al. 2012). The image of Fig. 4 is an example of such cluster, called RCW41, observed by WFAO. This star-forming region is embedded within the Vela molecular Ridge, hosting a massive stellar cluster surrounded by a conspicuous HII region (Santos et al. 2012). WFAO allows resolving the individual population and access to the faint, low mass members of the cluster that strongly constrain its age, and evolutionary state. Thanks to the high-resolution, we expect that the new WFAO images will bring significant improvements on the accuracy of the IMF estimation, and provide insights on the hotly debated topic of whether or not there is a universal IMF, or whether the IMF does systematically vary with environment.

As a technical objective, these clusters data set will be used to derive the photometric precision reachable by current WFAO systems, and how it compares with space instruments. This is a fundamental parameter for the star formation science case, which needs to be assessed properly. We will derive a complete description of the main advantages and limitations of the photometry precision obtained by WFAO, and draw the limits of the different methods. Finally, and in preparation for the future instruments, we will analyze how the unique astrometry precision brought by WFAO systems could benefit to the star formation science case.

Figure 4: GeMS J/H/Ks composite image of RCW41.


EDITION 2013

Projet WASABI



Galaxy formation and evolution

State-of-the-art: Trying to understand galaxy formation and evolution has become one of the most active fields of astrophysics over the last few decades. When and how does the Hubble sequence build up? Does merging play a major role, as expected from hierarchical ΛCDM scenario? Are ellipticals assembled mainly via mergers or are there other ways to build such galaxies? Is cold gas accretion along cosmic filaments an efficient process to form bulges in spirals? Are these different mechanisms dominant at different cosmic times?

To understand the physical processes taking place in galaxy formation and evolution and to differentiate between intrinsic and environmental effects, the ability to obtain resolved spectroscopy and images across the objects is a must. Distant galaxies are marginally resolved in seeing-limited conditions and AO is required. Over the past years, one of the most remarkable applications of AO has been in spatially resolving the internal structure and kinematics of star forming galaxies at z ~ 1.5 - 3, the epoch of peak mass assembly (e.g. Cresci et al. 2009, Law et al. 2009, Wright et al. 2009, Förster Schreiber et al. 2009). In particular, these studies revealed a population of large disks, rapidly forming stars in giant star-forming complexes or clumps, probably triggered by high gas accretion rates through cold flows. These observations, although still relatively few, have already led to fundamental developments in our understanding of galaxy evolution.

Most of the current extra-galactic AO studies are however constrained by the number of targets available to AO correction (the so-called sky coverage), and the need for statistics, that requires observing many objects across the largest possible field. These constraints are now significantly reduced by the new WFAO systems.

In the next decade, the unique angular resolution of the ELTs equipped with WFAO will revolutionize this field, as we will be able to reproduce the observation of the galaxies’ structure and internal motions out to distances of tens of millions of light-years. This will certainly modify our understanding of the galaxy mass assembly mechanisms and the Hubble sequence build-up.

Position and objectives: The program that we propose aims at shedding light on galaxy evolution processes, and prepare the next-generation of instruments dedicated to extra-galactic studies.

In a first stage, we will combine deep HST images obtained in the visible with the NIR images obtained from ground-based WFAO systems. At these redshifts, HST visible images probe the young stellar population, dominated by the clumps of the star-forming regions. On the other hand, the NIR images provided by WFAO are sensitive to the old, underlying stellar populations, that trace the bulk of the galaxy mass and provide an unbiased morphology estimation. Note that the resolution brought by WFAO for ground 8-m telescope is a perfect match for HST visible images, and allows for direct comparison of the information at different wavelengths. As an illustration, the image of Fig. 5 shows a galaxy cluster with galaxy as distant as z~7 (when the Universe was < 10% of its current age) observed by HST, and GeMS. The same angular resolution is achieved. Our program will hence highlight the complementarity between Space and ground instrumentation, and prepare such future synergies, in the era of ELTs and JWST. The morphology information will then be compared with resolved internal kinematics derived from AO-assisted 3D spectroscopy. The combination of high-angular images with the galaxy kinematics has proven to be a powerful tool to accurately describe the internal dynamics, and bring strong constraints on the process at work in galaxy evolution (Neichel et al. 2008).

A parallel key technical objective of our study will be to compare the performance of ground-based NIR morphological studies with the results obtained with HST, and especially with the WFC3 camera (also working in the NIR). For this, WFAO images of a set of distant galaxies will be compared with HST-WFC3 images obtained from the CANDELS program. The direct comparison of these images is crucial to understand the limitations of both instrumentations. In particular, and in connection with Sect. 2.2.2, we want to understand the impact of the AO-PSF on the morphology analysis accuracy. The outcome of this study will be used to define the requirements of future WFAO instruments dedicated to extra-galactic studies.


EDITION 2013

Projet WASABI



2.2.2 DEDICATED REDUCTION TOOLS AND INNOVATIVE AO CONCEPTS

The introduction of the new WFAO systems will open an incredible amount of new astronomical discoveries. It also brings new challenges in terms of data reduction, and system optimization.

In order to get the best science results out of the WFAO images, and to fully optimize the return of such complex systems, dedicated and optimized reduction tools are needed. These tools must be built with a deep understanding of the system performance and limitations. These tools must be adapted to each science objectives. Based on the unique skills and knowledge gathered in the team of this project, we propose to develop a set of data reduction and analysis tools for WFAO systems. These tools will be made accessible to the astronomical community, and should pave the way for the preparation of the ELTs.

A second critical aspect is the optimization of future AO-systems. AO ought to be accessible to a wide range of astronomical targets, to cover a wide range of science case. This directly results on the notion of sky coverage, or the fraction of the sky that can benefit from AO corrections. In this project, we will propose new technological development to reduce this limitation, and boost the use of AO for the future generation of instruments.

New tools for new astronomical challenges: State-of-the-art: One crucial aspect of the science cases describe above is the knowledge of the PSF. Knowledge of the PSF directly impacts the accuracy of the science that can be derived from astronomical images. Separating the PSF from the intrinsic structure in observed data is a key part of most AO-assisted astronomical programs. Searches for binaries or planets around bright stars represent the extreme case where every detail is crucial in order to distinguish between small faint objects and the PSF structure. Similarly, in densely populated fields, because the sensitivity is limited by the crowding rather than photon and detector noise, the key to extracting accurate photometry and astrometry is in obtaining a good estimate of the PSF with which to fit the sources (Britton 2006, Davies et al. 2010).

Figure 5: Bullet cluster as seen in the visible by HST (left) and in the NIR by GeMS (right)


EDITION 2013

Projet WASABI



For extragalactic science, requirement on the PSF knowledge are less demanding. Most of the analyses are done via galaxy fitting algorithms, where an analytical model of the intrinsic structure of the galaxy is convolved with the PSF. The accuracy of the analysis is usually limited by the model and/or signal-to-noise. However, the AO PSF is complex, and variable across the field and over time, and extragalactic fields are usually empty of reference stars that could be used to calibrate the models (Cresci et al. 2009, Huertas-Company et al. 2008). In this respect the large field of view offered by WFAO is an asset, regardless of whether the science target fills it, because it becomes highly likely that suitable empirical PSFs can be found within the field even at high galactic latitudes.

Different PSF estimations methods have been considered over the past years (Steinbring et al. 2005, Britton 2006, Cresci et al. 2009, Jolissaint et al. 2012). One approach could be to derive the PSF using analytical models which are fed with some basic parameters (turbulence profile, AO system parameters, guide star parameters) and heuristically tuning it to the actual system PSF. Another approach to reconstruct PSFs would be to make use of point sources in the field and estimate the PSFs lying in between by interpolating the few parameters of an empirically determined analytical PSF profile (e.g. Moffat or Lorentzian distribution). A more complete, but challenging approach, is to actually reconstruct the PSF over the field based on the AO loop parameters. The method, introduced by Véran et al. (1997) was successfully applied to a low-order curvature AO system, called PUEO, and installed at the CFHT telescope. The extrapolation of this method to more complex cases (low signal-to-noise, off-axis guide star, or when using a LGS) has never been demonstrated yet, and accurate PSF reconstruction remains a challenge.

Position and objectives: In this project, we propose to explore the different approaches that would eventually provide the best-suited PSF estimation for WFAO observations. We will make use of the science programs described previously in order to define, validate and develop the best strategy for each of them. We propose to openly distribute all the tools that will be developed during the project.

Toward a broader coverage of AO for astronomy: State-of-the-art: All planned future WFAO instruments are relying on LGS in order to get the wider sky coverage. The use of multi-LGS in astronomy is a young technology, and the extrapolation of the technique to the ELTs sizes is consequent. Some effects, negligible for the 8m class telescopes will introduce strong limitations for an ELT. For instance, the dynamical evolution of the properties of the sodium layer induces measurement errors, which reduce significantly the performance (Pfrommer et al. 2012, Neichel et al. 2013). A second fundamental aspect concerns the sky coverage. Any WFAO system needs to have Natural Guide Stars [NGS] information to complement the Wave-Front Sensor [WFS] data coming from Laser Guide Stars [LGS]. Indeed, LGS data suffer from tip-tilt and focus indetermination related to the nature of the incoherent light coming back from the laser illuminated sodium layer located at 90km from the ground. Therefore, the final sky coverage, and yet ultimate performance, of such WFAO systems are very related to the final performance of the low order NGS sensor. Sky-coverage particularly impacts extra-galactic observations, for which the fields are usually chosen away from the plane of the Milky Way to avoid absorption, and away from any bright stars to allow for deep follow-up by space instruments (Damjanov et al. 2011). As an example, only 5 to 10% of the most studied galactic fields (e.g. COSMOS) can benefit from AO corrections.

Position and objectives: This part of the project will concern development and validation of innovative concepts toward a significantly improved sky coverage for AO-systems. We are first planning to investigate new developments in order to optimize the measurements provided by LGSs. In particular, we will focus our efforts on a fine analysis of the sodium layer physical properties, their temporal variations, and their impact on low order modes (focus, astigmatisms and others). The effect of the Rayleigh background on specific sub-apertures (the so-called fratricide effect) and its impact on low order modes measurements will also be studied. The second aspect of this study will concentrate on a detailed study of the use of two-dimensional imaging detector, with a fair comparison of different technology (EMCCD, CMOS, APDs arrays) for astronomical applications. These new detectors will be tested with innovative concepts and new optimized algorithms to get an efficient use of any single available photon.

executive summary of the proposal - lam1. executive summary of the proposal over the 20 past-years,...

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