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Over the last decade, adaptive optics has become essential in different fields of research including medicine and industrial applications. With this new need, the market of deformable mirrors has expanded a lot allowing new technologies and actuation principles to be developed. Several E-ELT instruments have identified the need for post focal deformable mirrors but with the increasing size of the telescopes the requirements on the deformable mirrors become more demanding. A simple scaling up of existing technologies from few hundred actuators to thousands of actuators will not be sufficient to satisfy the future needs of ESO. To bridge the gap between available deformable mirrors and the future needs for the E-ELT, ESO started a development program for deformable mirror technologies. The requirements and the path to get the deformable mirrors for post focal adaptive optics systems for the E-ELT is presented.

The pyramid wavefront sensor has been introduced in the field of astronomical adaptive optics a few years ago. An important issue characterizing this wavefront sensor is how to reach high dynamical range, a task realized so far by either vibrating the pyramid or oscillating a tip-tilt mirror in a plane conjugated to the exit pupil of the telescope. A new method is proposed here to achieve the same result, without any moving part: the new approach is based on a light diffusing plate placed in an intermediate pupil plane. Some practical implementations of this concept are presented and the relevance to multi-conjugate adaptive optics is discussed.

Since the very beginning of 2008, the Large Binocular Telescope (LBT) is officially equipped with it's first binocular instrument ready for science observations: the Large Binocular Camera (LBC). This is a double CCD imager, installed at the prime focus stations of the two 8.4m telescopes of LBT, able to obtain deep and wide field images in the whole optical spectrum from UV to NIR wavelengths. We present here the overall architecture of the instrument, a brief hardware review of the two imagers and notes how observations are carried on. At the end we report preliminary results on the performances of the instrument along with some images obtained during the first months of observations in binocular mode.

We present a 900 sec, wide-field U image of the inner region of the Andromeda galaxy obtained during the commissioning of the blue channel of the Large Binocular Camera mounted on the prime focus of the Large Binocular Telescope. Relative photometry and absolute astrometry of individual sources in the image was obtained along with morphological parameters aimed at discriminating between stars and extended sources, e.g. globular clusters. The image unveils the near-ultraviolet view of the inner ring of star formation recently discovered in the infrared by the Spitzer Space Telescope and shows in great detail the fine structure of the dust lanes associated with the galaxy inner spiral arms. The capabilities of the blue channel of the Large Binocular Camera at the Large Binocular Telescope (LBC-Blue) are probed by direct comparison with ultraviolet GALEX observations of the same region in M31. We discovered 6 new candidate stellar clusters in this high-background region of M31. We also recovered 62 bona-fide globulars and 62 previously known candidates from the Revised Bologna Catalogue of the M31 globular clusters, and firmly established the extended nature of 19 of them., 6 pages, 3 figures, accepted for pubblication on A&A

The European Southern Observatory together with external research Institutes is building a Multi-Conjugate Adaptive Optics Demonstrator (MAD) to perform wide field of view adaptive optics correction. The aim of MAD is to demonstrate on the sky the feasibility of the MCAO technique and to evaluate all the critical aspects in building such kind of instrument in the framework of both the 2nd generation VLT instrumentation and the 100-m Overwhelmingly Large Telescope (OWL). The MAD module will be installed at one of the VLT unit telescope in Paranal to perform on-sky observations. MAD is based on a two deformable mirrors correction system and on two multi-reference wavefront sensors capable to observe simultaneously some pre-selected configurations of Natural Guide Stars. MAD is expected to correct up to 2 arcmin field of view in K band. MAD has just started the integration phase which will be followed up by a long period of testing. In this paper we present the final design of MAD with a brief report about the status of the integration.

On the way to the Extremely Large Telescopes (ELT) the Large Binocular Telescope (LBT) is an intermediate step. The two 8.4m mirrors create a masked aperture of 23m. LINC-NIRVANA is an instrument taking advantage of this opportunity. It will get, by means of Multi-Conjugated Adaptive Optics (MCAO), a moderate Strehl Ratio over a 2 arcmin field of view, which is used for Fizeau (imaging) interferometry in J,H and K. Several MCAO concepts, which are proposed for ELTs, will be proven with this instrument. Studies of sub-systems are done in the laboratory and the option to test them on sky are kept open. We will show the implementation of the MCAO concepts and control aspects of the instrument and present the road map to the final installation at LBT. Major milestones of LINC-NIRVANA, like preliminary design review or final design review are already done or in preparation. LINC-NIRVANA is one of the few MCAO instruments in the world which will see first light and go into operation within the next years.

LINC-NIRVANA is an imaging interferometer for the Large Binocular Telescope (LBT) and will make use of multi-conjugated adaptive optics (MCAO) with two 349 actuators deformable mirrors (DM), two 672 actuator deformable secondary mirrors and a total of 4 wavefront sensors (WFS) by using 8 or 12 natural guide stars each. The goal of the MCAO is to increase sky coverage and achieve a medium Strehl-ratio over the 2 arcmin field of view. To test the concepts and prototypes, a laboratory setup of one MCAO arm is being built. We present the layout of the MCAO prototype, planned and accomplished tests, especially for the used Xinetics DMs, and a possible setup for a test on sky with an existing 8m class telescope.

While this paper is written, the Blue channel of the double prime focus camera for the Large Binocular Telescope is being commissioned at the telescope. We report here on the optical alignment of the prime focus corrector, a rather challenging activity, due to the tight alignment tolerances and to the size of the lenses. Furthermore we describe the current plans about the alignment of the prime focus corrector with the primary mirror of the telescope, which is foreseen in the next few months.

The Prime Focus for the Large Binocular Telescope are a couple of Prime Focus stations each equipped with four 4kx2k CCDs and a six lenses corrector with an aspheric surface and the first lens as large as roughly 800mm in diameter. These cameras will cover almost half degree of Field of View on 8m-class telescopes with unprecedented velocity of F/1.4. The two units are optimized for the Red and Blue portions of the visible wavelength and additionally an extension to J and H bands is foreseen. An overview of the project, including the optomechanics, the cryogenics, the electronics, and the software is given along with a preliminary account of lessons learned and on how much the second unit, the Red one, the schedule of which is shifted with respect to the Blue one by several months, will take advantage from the experience gained in the Blue unit assembly and integration.

Wide-Field imaging at visible wavelengths with seeing of the order of 0.1-0.2arcsec is believed to be possible from the high Antarctic plateau site of Dome-C by the removal of ground layer effects only. We present a proposal for a 2m-class telescope specifically designed for the science case of short duration (~10s or greater) microlensing events in the crowded central regions of Galactic Globular Clusters and nearby galaxies where the achievement of a spatial resolution of the order of a fraction of arcsec is essential. The philosophy behind the telescope proposal is discussed in detail. It is emphasized that this is a project with a specific unique science goal in mind and not a large scale facility instrument. A preliminary design for the optics, ground layer removal using a deformable secondary and "static" wavefront sensor and telescope structure is presented. In particular, it is shown that substantial simplification in the design can be achieved by having a specific science goal in mind, so reducing the complexity and increasing reliability. Transport and logistics for the successful deployment and operation of the telescope at the Dome-C site are discussed.

LINC-NIRVANA is a Fizeau interferometer which will be built for the Large Binocular Telescope (LBT). The LBT exists of two 8.4m mirrors on one mounting with a distance of 22.8m between the outer edges of the two mirrors. The interferometric technique used in LINC-NIRVANA provides direct imaging with the resolution of a 23m telescope in one direction and 8.4m in the other. The instrument uses multi-conjugated adaptive optics (MCAO) to increase the sky coverage and achieve the diffraction limit in J, H, K over a moderate Field of View (2 arcmin in diameter). During the preliminary design phase the team faced several problems similar to those for an instrument at a 23m telescope. We will give an overview of the current design, explain problems related to 20m class telescopes and present solutions.

We present the optical design of the layer-oriented wavefront sensors for two 8m class telescopes. By a combination of fast optics and "stars enlargers" it is possible to shrink the pupil image in a way to fit the available detectors. These concepts are then extended to the design of a wavefront sensor for a 100 m class telescope.

Layer Oriented represented in the last few years a new and promising aproach to solve the problems related to the limited field of view achieved by classical Adaptive Optics systems. It is basically a different approach to multi conjugate adaptive optics, in which pupil plane wavefront sensors (like the pyramid one) are conjugated to the same altitudes as the deformable mirrors. Each wavefront sensor is independently driving its conjugated deformable mirror thus simplifying strongly the complexity of the wavefront computers used to reconstruct the deformations and drive the mirror themselves, fact that can become very important in the case of extremely large telescopes where the complexity is a serious issue. The fact of using pupil plane wavefront sensors allow for optical co-addition of the light at the level of the detector thus increasing the SNR of the system and permitting the usage of faint stars, improving the efficiency of the wavefront sensor. Furthermore if coupled to the Pyramid wavefront sensor (because of its high sensitivity), this technique is actually peforming a very efficient usage of the light leading to the expectation that, even by using only natural guide stars, a good sky coverage can be achieved, above all in the case of giant telescopes. These are the main reasons for which in the last two years several projects decided to make MCAO systems based on the Layer Oriented technique. This is the case of MAD (an MCAO demonstrator that ESO is building with one wavefront sensing channel based on the Layer Oriented concept) and NIRVANA (an instrument for LBT). Few months ago we built and successfully tested a first prototype of a layer oriented wavefront sensor and experiments and demonstrations on the sky are foreseen even before the effective first light of the above mentioned instruments. The current situation of all these projects is presented, including the extensive laboratory testing and the on-going experiments on the sky.

Layer Oriented wavefront sensors can be made with a reasonable compact detector by the adoption of several stars enlargers, increasing only locally the focal ratio on the reference stars. The main opto-mechanical requirement in this kind of device is represented by the tolerances in tip and tilt of these star enlargers, which have to be moved over the Field Of View and aligned with the reference stars. A differential tip-tilt among the star enlargers leads to a mismatch between the different pupil images related to the reference stars. This misalignment eventually translates into a blurring of the measured wavefront, reducing the sensing quality. We describe a conceptual layout for an active control of the wavefront sensor, in order to reach the best mechanical positioning of these stars enlargers. In particular we discuss an algorithm to determine the effective pupils positions by simple movements and apply the requested displacement through commercially available piezoelectric actuators, shown in a preliminary opto-mechancial design of such wavefront sensor.

We are currently working on four projects employing Multi Conjugate Adaptive Optics in a Layer-Oriented fashion. These ranges from experimental validations, to demonstration faci lity or full instrument to be offered to an astronomical community and involves telescopes in the range of 4m to 24m equivalent telescope aperture. The current status of these projects along with their brief description is here given. : Multi-Conjugate Adaptive Optics systems, Layer oriented MCAO, Wavefront Sensors.

In order to achieve moderate Field of View (2 arcmin in diameter) and nearly diffraction limited capabilities, at the reddest portion of the visible spectrum in the interferometric mode of LBT, two sophisticated MCAO channels are required. These are being designed to perform a detailed correction of the atmospheric turbulence through three deformable mirrors per telescope arm: the secondary adaptive mirror and two commercial piezostack mirrors, leading to an overall number of degree of freedom totaling ~ 3000. A combination of numerical and optical coaddition of light collected from natural reference stars located inside the scientific Field of View and in an annular region, partially vignetted, and extending up to ≈ 6 arcmin in diameter, allows for such a performance with individual loops characterized by a much smaller number of degree of freedom, making the real-time computation, although still challenging, to more reasonable levels. We implement in the MCAO channel the dual Field of View layer-oriented approach using natural guide stars, only allowing for limited, but significant, sky coverage.

Multiconjugate adaptive optics employing several deformable mirrors conjugated at different altitudes has been proposed in order to extend the size of the corrected field of view [FOV] with respect to the size of the corrected FOV given by a classical adaptive optics system. A three dimensional measurement of the turbulent volume is needed in order to collect the information to command the several deformable mirrors. Given a set of guide stars in the field of view, this can be done both using tomography, in which several wavefront sensors are used, each of them coupled to one of the guide stars, or layer oriented techniques, in which wavefront sensors are coupled to a given layer in the atmosphere, and collect light from the whole set of guide stars. We will call this type of measurements optical layer oriented . This type of measurements can be also obtained combining in a numerical way, tomographic measurements. This hybrid approach is called numerical layer oriented . In order to compare their performance, we present an analytical study of the signal to noise ratio [ SNR ] in the measurements for the two techniques. Optical layer oriented is shown to be more efficient in the range of faint flux and large number of guide stars, while low detector noise will allow numerical layer oriented schemes to be more efficient in terms of SNR .

A demonstrator of the multi-conjugate adaptive optics concept is under construction at ESO and will be installed on the Nasmyth focus of the VLT. This demonstrator called MAD will have two different wavefront sensor channels: Shack-Hartmann and Layer-Oriented; in this article we only describe the Layer-Oriented one. The Layer-Oriented wavefront sensor can select eight reference stars in the two arc-minutes corrected field of view in order to have a maximum of two references in each quarter of the field. XY stages will remotely adjust the position of each reference star selector. The starlight will be fed onto two detectors and two completely independent loops will drive the deformable mirrors, one conjugated to the ground and the other to an altitude of approximately 8 km. The Layer-Oriented wavefront sensor will use the same CCDs than the Shack-Hartmann channel and the pupil will be divided into 9×9 subapertures both for the high and for the ground layer. The spatial sampling of the subapertures will be different for the two CCDs and their integration time will be tuned to typical values of the conjugated altitudes characteristic wind speed. The overall status of the instrument with respect to optics, mechanics, electronics and software is given hereafter. We also summarize the progress on the procurement phase and give the time schedule for the assembling, integration and testing phases.

The multi-conjugate adaptive optics (MCAO) technique allows to correct the vertical distribution of atmospheric turbulence, extending the isoplanatic angle and the sky coverage. In the Layer-Oriented (LO) approach, each wavefront sensor (WFS) is conjugated to a given atmospheric layer and is linked in closed loop to a deformable mirror, conjugated to the same height. This technique benefits from the co-adding of light coming from the guide stars which can be arbitrarily numerous and faint. In the Multiple Field of View (MFoV) scheme, each LO WFS is looking through a different field-of-view (FoV) in order to extend even further the sky coverage by increasing the photon density in the various layers. With the help of the Software Package CAOS, upgraded with numerical tools dedicated to MCAO and LO, we simulate the whole wavefront sensing process of a LO system based on pyramid sensors. We describe the modelization of the phenomena occurring during the sensing, such as diffraction effects, and detector noise. Furthermore we also use an independent ad hoc code in order to analyze the performance of the MFoV scheme. The performances are given considering realistic natural guide star (NGS) configurations.

Multi-Conjugate Adaptive Optics (MCAO) is working on the principle to perform wide field of view atmospheric turbulence correction using many Guide Stars located in and/or surrounding the observed target. The vertical distribution of the atmospheric turbulence is reconstructed by observing several guide stars and the correction is applied by some deformable mirrors optically conjugated at different altitudes above the telescope. The European Southern Observatory together with external research institutions is going to build a Multi-Conjugate Adaptive Optics Demonstrator (MAD) to perform wide field of view adaptive optics correction. The aim of MAD is to demonstrate on the sky the feasibility of the MCAO technique and to evaluate all the critical aspects in building such kind of instrument in the framework of both the 2nd generation VLT instrumentation and the 100-m telescope OWL. In this paper we present the conceptual design of the MAD module that will be installed at one of the VLT unit telescope in Paranal to perform on-sky observations. MAD is based on a two deformable mirrors correction system and on two multi-reference wavefront sensors capable to observe simultaneously some pre-selected configurations of Natural Guide Stars. MAD is expected to correct up to 2 arcmin field of view in K band.

In this paper, we describe a concept of multiple laser guide stars system based on tomographic reconstruction and multi- conjugate correction for 8-m class telescopes. We show that this type of adaptive optics (AO) systems can be considered as the next generation of AO systems for 8-m telescopes and represents a necessary intermediate step toward AO systems for Extremely Large Telescopes. Multiple guide stars allow to correct the cone effect which affects single LGS systems and prevents from going to wavelengths shorter than approximately 1 micrometer. With 4 LGS plus 1 NGS, it is expected that correction with a high Strehl can be obtained at least in the R and I bands, with an extended corrected field of view (FOV). An analytic AO model is used to assess the expected on-axis performance. Using recent results on 3D mapping of turbulence (i.e. tomography), we estimate the sky coverage of such a system. We also discuss the implications of a large corrected field of view on the system design: large wavefront sensor field, constrains on the optics and deformable mirrors, and size of the science detector. With an MCAO system, large telescopes would be able to observe faint extragalactic objects and wide crowded fields (2 arcmin) at the diffraction limit.

Adaptive optics will be a key element of future Extremely Large Telescopes (ELTs) which need sharp images both to avoid object confusion and to boost sensitivity to point sources against sky background. A challenging task of correcting turbulence in the visible part of the spectrum is addressed. With an R approximately 10 natural guide star a Strehl ratio (SR) of 0.6 can be achieved at 500 nm, at the cost of an extremely low sky coverage. Thus, use of Multi-Conjugate Adaptive Optics techniques will be essential to widen the field of view (FOV). We summarize our recent theoretical results and show that with 3 deformable mirrors it is realistic to obtain a FOV diameter of 30' - 60' in the visible, or 3' - 6' in the K band. Contrary to the common belief, the layered structure of the turbulence vertical profile is not needed to achieve this limit, neither do we need to know the profile exactly. The wide corrected FOV ensures a reasonably good sky coverage in the near infrared using 4 natural guide stars (60% in the J band at medium Galactic latitudes). Artificial Laser Guide Stars will be needed to correct in the visible, using in addition a faint natural star to measure low-order wavefront aberrations. On ELTs this star can be faint, up to R equals 23, so a good sky coverage can also be obtained. A significant fraction of guide sources at these magnitudes are extragalactic.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper describes the effects of Laser Guide Star spot elongation and Rayleigh scattering on wavefront sensing performances. An analytical model of Rayleigh scattering and a numerical model of laser plume generation at the altitude of the atmospheric sodium layer were developed. These models, integrated into a general Adaptive Optics (AO) simulation, provide information about the non-uniform centroid measurement accuracy on the sensor sub-apertures. The effects of laser power, laser type, laser launching position, sensor sampling and sensor field of view on the AO loop performances are analyzed and discussed.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We describe in this paper the development of a simulation tool for modeling any kind of adaptive optic system with laser guide star within the framework of the European network Laser Guide Star for 8-m Class Telescopes. This is intended to support investigations about laser guide stars problems line cone effect and tip-tilt determination. The different kinds of software modules to be developed are listed and the libraries already distributed among the network are described: phase screen generation, downward propagation, wavefront sensing and reconstruction. The future studies for solving the cone effect are also presented.

Funding: Comisión Nacional de Investigación Científica y Tecnológica (CONICYT) (72160371); UK Research and Innovation Science and Technology Facilities Council (STFC) (ST/L002213/1, ST/L00075X/1, ST/P000541/1, ST/T000244/1). Point-diffraction interferometers are a class of wavefront sensors which can directly measure the phase with great accuracy, regardless of defects such as vortices and disconnected apertures. Due to these properties, they have been suggested in applications such as cophasing of telescope segments, wavefront sensing impervious to the island effect and high-contrast AO and imaging. This paper presents an implementation of this class of interferometer, the Calibration & Alignment~WFS (CAWS), and the results of the first on-sky tests in the visible behind the SCAO loop of the CANARY AO experiment at the William Herschel Telescope. An initial analysis of AO residuals is performed in order to retrieve the SNR of interference fringes and assess the instrument's performance under various observing conditions. Finally, these results are used to test the validity of our models, which would allow for rapid implementation-specific modelling to find minimum-useful flux and other CAWS limits. Publisher PDF