Although many of the instruments planned for the TMT (Thirty Meter Telescope) have their own closely-coupled adaptive optics systems, TMT will also have a facility Adaptive Optics (AO) system feeding three instruments on the Nasmyth platform. For this Narrow-Field Infrared Adaptive Optics System, NFIRAOS (pronounced nefarious), the TMT project considered two architectures. One, described in this paper, employs conventional deformable mirrors with large diameters of about 300 mm and this is the reference design adopted by the TMT project. An alternative design based on MEMS was also studied, and is being presented separately in this conference. The requirements for NFIRAOS include 0.8-5 microns wavelength range, 30 arcsecond diameter output field of view (FOV), excellent sky coverage, and diffraction- limited atmospheric turbulence compensation (specified at 133 nm RMS including residual telescope and science instrument errors.) The reference design for NFIRAOS includes multiple sodium laser guide stars over a 70 arcsecond FOV, and an infrared tip/tilt/focus/astigmatism natural guide star sensor within instruments. Larger telescopes require greater deformable mirror (DM) stroke. Although initially NFIRAOS will correct a 10 arcsecond science field, it uses two deformable mirrors in series, partly to provide sufficient stroke for atmospheric correction over the 30 m telescope aperture, but mainly to partially correct a 2 arcminute diameter "technical" field to sharpen near-IR natural guide stars and improve sky coverage. The planned upgrade to full performance includes replacing the groundconjugated DM with a higher actuator density, and using a deformable telescope secondary mirror as a "woofer." NFIRAOS incorporates an instrument rotator and selection of three live instruments: a near-Infrared integral field Imaging spectrograph, a near-infrared echelle spectrograph, and after upgrading NFIRAOS to full multi-conjugation, a wide field (30 arcsecond) infrared camera.

We describe an exploratory optical design for the Narrow Field InfraRed Adaptive Optics (AO) System (NFIRAOS) Petite, a proposed adaptive optics system for the Thirty Meter Telescope Project. NFIRAOS will feed infrared spectrograph and wide-field imaging instruments with a diffraction limited beam. The adaptive optics system will require multi-guidestar tomographic wavefront sensing (WFS) and multi-conjugate AO correction. The NFIRAOS Petite design specifications include two small 60 mm diameter deformable mirrors (DM's) used in a woofer/tweeter or multiconjugate arrangement. At least one DM would be a micro-electromechanical system (MEMS) DM. The AO system would correct a 10 to 30 arcsec diameter science field as well as laser guide stars (LGS's) located within a 60 arcsec diameter field and low-order or tip/tilt natural guide stars (NGS's) within a 60 arcsec diameter field. The WFS's are located downstream of the DM's so that they can be operated in true closed-loop, which is not necessarily a given in extremely large telescope adaptive optics design. The WFS's include adjustable corrector elements which correct the static aberrations of the AO relay due to field position and LGS distance height.

Adaptive Optics (AO) will be essential for at least seven of the eight science instruments currently planned for the Thirty Meter Telescope (TMT). These instruments include three near infra-red (NIR) imagers and spectrometers with fields of view from 2 to 30 arc seconds, a mid-IR echelle spectrometer, a planet formation imager/spectrometer, a wide field optical spectrograph, and a NIR multi-object spectrometer with multiple integral field units deployable over a 5 arc minute field of regard. In this paper we describe the overall AO reference design that supports these instruments, which consists of a facility AO system feeding the first three instruments and dedicated AO systems for the remaining four. Key design challenges for these systems include very high-order, large-stroke wavefront correction, tip-tilt sensing with faint natural guide stars to maximize sky coverage, laser guidestar wavefront sensing on a very large aperture, and achieving extremely high contrast ratios for the detection of extra-solar planets and other faint companions of bright stars. We describe design concepts for meeting these challenges and summarize our supporting plans for AO component development.

A ground-based stellar interferometer appears to be a potentially useful research tool in studying stellar astrophysics and synthesizing a high resolution stellar image; however, its short-exposure performance is easily degraded by atmospheric turbulence. Even though adaptive optics has been recognized as a promising technology to improve image quality for a large aperture telescope, the question is often asked: "Is adaptive optics needed in a ground-based stellar interferometer?" In this paper, we develop the appropriate theory and provide simulation results to show why adaptive optics is needed in a ground-based optical interferometer. We also present a novel adaptive optics testbed including a tip-tilt error compensation system and a higher-order phase aberration compensation system to verify our theoretical simulation results.

Adaptive Optics (AO) requires three elements: a Shack Hartmann wave- front sensor, Deformable Mirrors (DM) and computer control to compensate or correct for the distortion of the wavefront caused by turbulence in the atmosphere. These methods are limited by their massive scale, comparable expense and hardware constraints relating to the number of mirror element. We are developing a new approach to astronomical AO which uses an interferometric sensor, phase determination based on wavelet ridge extraction, and phase conjugate distortion correction.

The term Adaptive Optics (AO) describes the active control of an optical device to remove distortions caused by aberrations in an optical beam path. An AO system enables beam forming and image correction in the presence of distortions and atmospheric effects. Major obstacles in imaging through the atmosphere include extended source/target anisoplanatism, distributed strong turbulence, scintillation, and branch points. Many applications have requirements for which the generation of a wavefront sensing source via the projection of a laser is undesirable or unfeasible. A variety of AO compensation techniques exist and have been demonstrated in the field, each with specific merits and disadvantages. A survey of the many types of AO control is presented. Common AO techniques include Classic Adaptive Optics, Multi-Conjugate Adaptive Optics (MCAO), and Extended Source AO (also known as correlation wavefront sensing). More recent applications include Stochastic Parallel Gradient Descent control (SPGD) and a Holographic Phase Conjugate Engine that were developed to advance the state of the art AO control. Innovative variations on the Stochastic Parallel Gradient Descent AO and Extended Source (scene-based) AO algorithms hold significant promise for the future of AO.

Atmospheric turbulence corrupts astronomical images formed by ground-based telescopes. Adaptive optics (AO) systems allow the effects of turbulence-induced aberrations to be reduced for a narrow field of view (FOV) corresponding approximately to the isoplanatic angle θ₀. For field angles larger than θ₀, the point spread function (PSF) gradually degrades as the field angle increases. In this paper, we present a technique to predict the PSF as function of the field angle. The predicted PSF is compared to the simulated PSF and the mean square (MS) error between the predicted and the simulated PSF never exceeds 2.7%. Simulated anisoplanatic intensity images of a star field are reconstructed by mean of a block-processing method using the predicted PSF. Two methods for image recovery are used: the Tikhonov regularization and the expectation maximization (EM) algorithm. The deconvolution results using the space-varying predicted PSF are compared to deconvolution results using the space-invariant on-axis PSF. The reconstruction technique using the predicted PSF shows an improvement of the MS error between the reconstructed image and the object of 7.2% to 84.8% compared to the on-axis PSF reconstruction.

Even though the wavefront distortion introduced by atmospheric turbulence is a dynamic process, its temporal evolution is usually neglected in the adaptive optics (AO) control design. Most AO control systems consider only the spatial correlation in a separate wavefront reconstruction step. By accounting for the temporal evolution of the wavefront it should be possible to further reduce the residual phase error and enable the use of fainter guide stars. Designing a controller that takes full advantage of the spatio-temporal correlation in the wavefront requires a detailed model of the wavefront distortion. In this paper we present a dedicated subspace identification algorithm that is able to provide the required prior knowledge. On the basis of open-loop wavefront slope data it estimates a multi-variable state-space model of the wavefront disturbance. The model provides a full description of the spatio-temporal statistics in a form that is suitable for control. The algorithm is demonstrated on open-loop wavefront data.

The problem of finding the closed-loop optimal controller is formulated in an Η₂-optimal control framework. This provides a natural way to account for the fact that in many AO systems the wavefront phase cannot be measured directly. Given a multi-variable disturbance model of both wavefront slopes and wavefront phases, this provides a general procedure to compute the closed-loop controller. If the wavefront sensor and deformable mirror are static and the only dynamics in the system is a unit-sample delay between measurement and correction, an analytical expression for the optimal controller can be derived. This results in a control approach, in which both identification and computation of the optimal controller are exclusively based on standard matrix operations. No Riccati equation needs to be solved to compute the optimal controller. The proposed Η₂-control approach is numerically validated on open-loop wavefront sensor data and its performance is compared with the common approach. Also the sensitivity to measurement noise is considered.

Non common path aberrations (NCPA) are one of the main limitations to achieve ultimate performance of an adaptive optic system. These static optical aberrations are unseen by the wave front sensor and therefore not corrected by closed AO loop. We present experimental results of a new procedure of measurement and pre-compensation of the AO non-common path aberrations. This new procedure has been studied with simulations, and tested on the AO bench (BOA) at ONERA. Strehl Ratio (SR) obtained on the imaging path reaches 93 % @ 632.8 nm even for low SNR.

The MMT adaptive optics system has been in operation for over 2 years. Besides being a technological demonstration, it has achieved remarkable success. However, the system is presently limited by a few factors, one of which is the lack of an optimised controller. In this paper, we review the optimised modal integrator and evaluate its potential for MMT-AO. We find that it can indeed increase the system sensitivity by one magnitude but that a careful analysis of wave front sensor data must be performed to remove artefacts that can severely bias the outcome of the optimization.

FPGA (Field Programmable Gate Array) technology has become a very powerful tool available to the electronic designer, specially after the spreading of high quality synthesis and simulation software packages at very affordable prices. They also offer high physical integration levels and high speed, and eases the implementation of parallelism to obtain superb features. Adaptive optics for the next generation telescopes (50-100 m diameter) -or improved versions for existing ones- requires a huge amount of processing power that goes beyond the practical limits of today's processor capability, and perhaps tomorrow's, so FPGAs may become a viable approach. In order to evaluate the feasibility of such a system, a laboratory adaptive optical test bench has been developed, using only FPGAs in its closed loop processing chain. A Shack-Hartmann wavefront sensor has been implemented using a 955-image per second DALSA CA-D6 camera, and a 37-channel OKO mirror has been used for wavefront correcting. Results are presented and extrapolation of the behavior for large and extremely large telescopes is discussed.

Durham University's Centre for Advanced Instrumentation (CfAI) are currently producing a generic high-performance low-cost real-time control system (RTCS) for adaptive optics (AO) based on Field Programmable Gate Array (FPGA) technology. This platform, labelled DARTS, 'Durham Adaptive optics Real Time System', will primarily be used as the controller for Durham's enhanced Rayleigh Technical Demonstrator (RTD) system. However, DARTS could be used as a low latency control system for existing AO instruments or could be used for future 'budget' AO Natural Guide Star (NGS) and/or Laser Guide Star (LGS) RTCS. DARTS uses an FPGA device to host an end-to-end modular real-time AO pipeline connected to a Wishbone control bus. The FPGA takes advantage of the pipeline's highly parallel computationally intensive tasks which usually are calculated in series by a system processor. DARTS hopes to increase the obtainable control loop frequency and reduce the computational latency of the RTD's RTCS. DARTS is capable of high bandwidth I/O due to the implementation of the serial Front Panel Data Port (sFPDP) industrial protocol. The hardware's I/O design is modular, allowing for the future connection of various WFSs and DMs via signal converters. Various communications architectures are suggested to allow non real-time configuration and visualisation data to flow between the wishbone control bus and a processing device, either externally or internally to the FPGA device. This paper reveals the current status of the project.

An adaptive optics system using multiple deformable mirrors and an array of guidestars can correct over a wider field of view than traditional single DM systems and can also eliminate the cone-effect error due to the finite altitude of laser guidestars. In large telescope systems, such as the envisioned 30-meter telescope, or TMT, the extraordinarily large amount of computation needed to implement multi-conjugate adaptive optics at atmospheric turnover rates is prohibitive for ordinary CPUs, even when another ten years of computer development is taken into account. We present here a novel approach, implementing a fast iterative version of the key inverse tomography calculations in an array of parallel computing elements. Our initial laboratory experiments using field-programmable gate arrays (FPGAs) are promising in terms of speed and convergence rates. In this paper we present the theory and results from simulations and experiments.

The European Southern Observatory (ESO) and Durham University's Centre for Advanced Instrumentation (CfAI) are currently designing a standard next generation Adaptive Optics (AO) Real-Time Control System. This platform, labelled SPARTA 'Standard Platform for Adaptive optics Real-Time Applications' will initially control the AO systems for ESO's 2^nd generation VLT instruments, and will scale to implement the initial AO systems for ESO's future 100m telescope OWL. Durham's main task is to develop the Wavefront Sensor (WFS) front end and Statistical Machinery for the SPARTA platform using Field Programmable Gate Arrays (FPGA). SPARTA takes advantage of a FPGA device to alleviate the highly parallel computationally intensive tasks from the system processors, increasing the obtainable control loop frequency and reducing the computational latency in the control system. The WFS pixel stream enters a PMC hosted FPGA card contained within the SPARTA platform via optical fibres carrying the VITA 17.18/10 standard 2.5Gbps^-1 serial Front Panel Data Port (sFPDP) protocol. Each FPGA board can receive a maximum of 10Gbs^-1 of data via on-board optical transceivers. The FPGA device reduces WFS frames to gradient vectors before passing the data to the system processors. The FPGA allows the processors to deal with other tasks such as wavefront reconstruction, telemetry and real-time data recording, allowing for more complex adaptive control algorithms to be executed. This paper overviews the SPARTA requirements and current platform architecture, Durham's Wavefront Processor FPGA design and it concludes with a future plan of work.

We have implemented a testbed to demonstrate wavefront sensing and control on an extended scene using Shack-Hartmann and MGS phase retrieval simultaneously. This dual approach allows for both high sensitivity and high dynamic range wavefront sensing. Aberrations are introduced by a silicon-membrane deformable mirror. The detailed characterization of this mirror and its sensitivity matrix are presented. The various Shack-Hartmann algorithms, including a maximum likelihood approach are discussed and compared to phase retrieval results using a point source. The next phase of the testbed will include results with extended scenes.

The main noise source in detection of faint companions such as extrasolar planets near bright stars with AO is speckle noise--residual PSF structure caused by wavefront errors due to the atmosphere, the AO system, and static optical effects. Of these, the most fundamental are atmospheric speckles--even given infinite wavefront SNR and a perfect DM, timelag between sensing and correction will always lead to a residual atmospheric speckle pattern. There have been several suggestions as to the lifetime of these atmospheric speckles, none strongly supported by theory or simulation. We have carried out a systematic series of simulations and analysis to explore this question. We show that speckles have different behavior in the regime in which diffraction is significant (first-order speckles, which are rapidly modulated as a phase error translates across the aperture) and in the coronagraphic regime (second-order speckles, which evolve only as the phase screen completely clears the aperture.). We use simulations to analyze the behavior of speckles in a variety of regimes, showing that the second-order atmospheric speckle lifetime is almost constant irrespective of the properties of the AO system, and is set primarily by the atmospheric clearing time of the telescope aperture.

In the frame of the VLT Planet-Finder project, the phase A system study has demonstrated the feasibility of an extreme adaptive optics system aimed at the direct detection of extrasolar giant planets. The main results of this study are presented in this paper.

The achievable contrast level for space-based detection of exo-planets will be limited by the stability of the optics. As a consequence, active amplitude and phase compensation will be needed. In order to mitigate these wavefront instabilities, we suggested, as an alternative to classical adaptive optics, the use of a Michelson interferometer equipped with two deformable mirrors. Simulations showed that this set up is able to create a symmetric "dark hole" in an appropriate area of the image plane. However, increasing the bandwidth of the incident light critically alters this nulling performance. A quantitative analysis of this effect will first be presented. An alternative to circumvent this problem is to introduce a dispersive element in one of the legs of the interferometer so that the path length difference does not exhibit the one over wavelength dependence. In the case of the insertion of a gaseous cell, the OPD could then be controlled by pressure variations. The last section of this paper will present a simulation-oriented proof of concept relying on the dispersive properties of nitrogen.

Future wavefront sensors for AO on large telescopes will require a large number of pixels and must operate at high frame rates. Unfortunately for CCDs, there is a readout noise penalty for operating faster, and this noise can add up rather quickly when considering the number of pixels required for the extended shape of a sodium laser guide star observed with a large telescope. Imaging photon counting detectors have zero readout noise and many pixels, but have suffered in the past with low QE at the longer wavelengths (> 500 nm). Recent developments in GaAs photocathode technology, CMOS ASIC readouts and FPGA processing electronics have resulted in noiseless WFS detector designs that are competitive with silicon array detectors, though at ~ 40% the QE of CCDs. We review noiseless array detectors and compare their centroiding performance with CCDs using the best available characteristics of each. We show that for sub-aperture binning of 6x6 and greater that noiseless detectors have a smaller centroid error at fluences of 60 photons or less, though the specific number is dependent on seeing conditions and the centroid algorithm used. We then present the status of a 256x256 noiseless MCP/Medipix2 hybrid detector being developed for AO.

Fully depleted, backside illuminated pnCCDs with an integrated frame store area and an anti-reflective-coating for the optical and near infrared region have been fabricated. Measurement results of a 51 μm pixelsize device with an imaging area of 264 × 264 pixel will be presented. The devices, which feature a doublesided readout, allow to be operated at frame rates higher than 1000 frames per second. The electronic noise contribution of the entire detector system is slightly above two electrons at fastest readout modes. We will also present the concept of a data acquisition system being able to handle pixel rates of more than 70 megapixel per second. Decentral data reduction and analysis units allow for a centroid determination of sub--images with a very low latency time. The high speed, low noise and high quantum efficiency makes this camera system an ideal instrument for wavefront sensors in adaptive optics systems.

We present the optical setup, reconstruction scheme and observational results of the Multi-conjugate Adaptive Optics (MCAO) system at the German 70cm Vacuum Tower Telescope, Observatorio del Teide, Tenerife. The system serves as a testbed for the future MCAO of the new 1.5m GREGOR solar telescope and is an extension of the conventional Adaptive Optics (CAO) system. We demonstrate that the use of one additional MCAO wavefront sensor and one additional deformable mirror increases the corrected field of view from 10 to 35 arcseconds.

We present a laboratory demonstration of open loop Off-Axis Adaptive Optics with optimal control. The control based on a Minimum Mean Square Error Estimator brings a noticeable performance improvement. The next step will be to close the Off-Axis Adaptive Optics loop with a Kalman based optimal control. While this last experiment is currently under progress, a classic Adaptive Optics loop has already been closed recently with a Kalman based control and experimental results are presented. We also describe the expectable performance of the Kalman based off-axis closed loop thanks to an end-to-end simulator. Last minute notice: the Kalman based Off-Axis Adaptive Optics loop has been closed and very first results are given.

Adaptive optics systems typically include an optical relay that simultaneously images the science field to be corrected and also a set of pupil planes conjugate to the deformable mirror of the system. Often, in the optical spaces where DM's are placed, the pupils are aberrated, leading to a displacement and/or distortion of the pupil that varies according to field position--producing a type of anisoplanatism, i.e., a degradation of the AO correction with field angle. The pupil aberration phenomenon is described and expressed in terms of Seidel aberrations. An expression for anisoplanatism as a function of pupil distortion is derived, an example of an off-axis parabola is given, and a convenient method for controlling pupil-aberration-generated anisoplanatism is proposed.

Adaptive optics enable large telescopes to provide diffraction limited images, but their corrected field is restrained by the angular decorrelation of the turbulent wave-fronts. However many scientific goals would benefit a wide and uniformly corrected field, even with a partial correction. Ground Layer Adaptive Optics systems are supposed to provide such a correction by compensating the lower part of the atmosphere only. Indeed this layer is in the same time highly turbulent and isoplanatic on a rather wide field. In such a system the wave-front analysis is a critical issue. Measuring the ground layer turbulence requires multi-object wave-front analysis. Two multi-object wave-front sensing concepts have been proposed so far, derived from multi conjugate adaptive optics. They are the star oriented and the layer oriented approaches. A criterion for the analytical study of both concepts performance had been proposed in a previous presentation. First results on the behavior one can expect from one concept or the other had been given then. Here is presented a study made by improving the analytical model and completing its results with the ones of a numerical model which accounts for AO limitations that are uneasy to insert in an analytical formalism. Results are presented that highlight the advantages and drawbacks of each wave-front sensing concepts and the interest of optimizing them.

It is well-known that cone effect or focus anisoplanatism is produced by the limited distance of a laser guide star (LGS) which is created within the Earth atmosphere and consequently located at a finite distance from the observer. In this paper, the cone effect of the LGS for different vertical profiles of the refractive index structure constant is numerically investigated by using a revised computer program of atmospheric propagation of optical wave and an adaptive optics (AO) system including dynamic control process. According to the practice, the overall tilt for the tilt-correction mirror is obtained from a natural star and the aberrated wavefront for phase correction of the deformable mirror is obtained from a LGS in our numerical simulation. It is surprisingly found that the effect of altitude of the LGS on the AO phase compensation effectiveness by using the commonly-available vertical profiles of C²_n and the lateral wind speed in the atmosphere is relatively weak, and the cone effect for some C²_n profiles is even negligible. It is found that the cone effect does not have obvious relationship with the turbulence strength, however, it depends on the vertical distribution profile of apparently. On the other hand, the cone effect depends on the vertical distribution of the lateral wind speed as well. In comparison to a longer wavelength, the cone effect becomes more obvious in the case of a shorter wavelength. In all cases concerned in this paper, an AO system by using a sodium guide star has almost same phase compensation effectiveness as that by using the astronomical target itself as a beacon. Effect of dynamic control process in an AO system on the cone effect is studied in this paper for the first time within our knowledge.

FALCON is an original concept for a next generation instrument which could be used on the ESO Very Large Telescope (VLT) and on the future Extremely Large Telescopes (ELT). It is a multi-objects integral field spectrograph with multiple small integral field units (IFUs). Each of them integrates a tiny adaptive optics system coupled with atmospheric tomography to solve the sky coverage problem. This therefore allows to reach spatial (0.15 - 0.25 arcsec) and spectral (R>=5000) resolutions suitable for distant galaxy studies in the 0.8-1.8 μm wavelength range. In the FALCON concept, the adaptive optics correction is only applied on small and discrete areas selected within a large field. This approach implies to develop miniaturized devices for wavefront correction such as deformable mirrors (DM) and wavefront sensors (WFS). We draw up here the main high level specifications for this instrument, that we derive in a first set of opto-mechanical DM requirements including the state of the art of DM technologies.

In this paper, the theoretical base of the covariance approach for generating random phase screens is analyzed, and corresponding computer program is compiled. Preliminary numerical simulation investigation of this new approach is carried out. We propose to use three methods to evaluate the generated phase screens in a combining way. It is found that a comparison of the phase structure function of generated phase screens with the theoretical one is not enough and often inefficient. Open loop results and close loop results by using phase screens generated by the covariance approach are obtained and compared with those by using phase screens generated by the spectral approach for the first time within our knowledge. It is shown that the phase screens generated by covariance approach include more abundant frequency components than those generated by the spectral approach and these frequency components have obvious influences on the open loop results and the close loop results.

The most common wavefront sensor for real-time use in high-order adaptive optics systems is the Shack-Hartmann, in part because it is sensitive to a broad optical band. An alternative possibility is based on Zernike's phase contrast technique. Though quite sensitive in principle, at least for monochromatic light, there had been no simple way to obtain the broadband performance needed for competitive sensitivity in an actual adaptive optics system. Recently, we proposed a general achromatization scheme that relies upon the innate π/2 phase shift between the transmitted and reflected beams in a beam splitter. Here, a more detailed study of this broad-band phase contrast wavefront sensor is presented, along with some practical issues concerning component tolerances. These results offer encouraging indications that broad-wavelength-band implementations will be feasible in practice.