The NSF Center for Adaptive Optics (CfAO) is supporting research on advanced adaptive optics technologies. CfAO research activities include development and characterization of micro-electro-mechanical systems (MEMS) deformable mirror (DM) technology, as well as development and characterization of high-resolution adaptive optics systems using liquid crystal (LC) spatial light modulator (SLM) technology. This paper presents an overview of the CfAO advanced adaptive optics technology development activities including current status and future plans.

The adaptive optics (AO) system for the 6.5 m MMT conversion telescope is the first to compensate the aberrated wavefront at the telescope's secondary mirror. This approach has unique advantages in terms of optical simplicity, high throughput and low emissivity. Its realization presents many technical challenges, which have now been overcome. The deformable mirror is now characterized and accepted. It features a 1.9mm thick 640mm diameter convex aspheric mirror (manufactured at the Steward Observatory Mirror Lab), mounted on a 50 mm thick ULE reference body with 336 actuators, as well as a cluster of 168 DSP's and associated analog circuitry. A wavefront sensor with integrated CCD and lenslet array has also been completed. The complete system is now starting to produce laboratory results, which we present below. Closed loop operation is tested under an auto-collimation illumination system that reflects aberrated artificial starlight from the convex secondary.

In 1999, we presented our plan to upgrade the adaptive optics (AO) system on the Lick Observatory Shane telescope (3m) from a prototype instrument pressed into field service to a facility instrument. This paper updates the progress of that plan and details several important improvements in the alignment and calibration of the AO bench. The paper also includes a discussion of the problems seen in the original design of the tip/tilt (t/t) sensor used in laser guide star mode, and how these problems were corrected with excellent results.

An adaptive optics system was developed for the 8.2-m Subaru Telescope operated by the National Astronomical Observatory of Japan on the summit of Mauna Kea in Hawaii. The system saw first light on 2 December 2000 and achieved diffraction limited imaging in the K- and L- band. The system is located at the Cassegrain focus of Subaru and can feed either IRCS, an infrared camera and spectrograph (used for first light) or CIAO, an infrared coronagraphic imager. The system features a 36-elements curvature sensor using avalanche photo diodes and a 36-electrodes bimorph deformable mirror. The curvature sensor and deformable mirror were custom designed for an optimum match. Wavefront sensing is performed in the visible range while AO-corrected observations are made in the infrared. In this paper the first AO observations are described and the system performances are compared with the design values. A comparison is also made with other existing AO systems. First scientific demonstrations are shown.

In this paper we want to contribute to steer the open, on going design debate of the Extremely Large Telescopes Adaptive Optics. We discuss first from the application point of view whether or not the very existence of ELT projects should be linked to the feasibility and demonstration of Multi-Conjugate AO. We show that ELT will justify their very existence also with scaled conventional AO, especially if inserted in a step-up plan to reach MCAO at a second stage. Then we layout a modular, hybrid AO system conceptual design, optimized for ELTs'. We discuss possible subsystems, expected performances and tradeoffs. The paper reports on research still in progress.

In layer-oriented adaptive optics, multiconjugation is performed in a much more efficient way than conventional wavefront sensing. This improved efficiency is impressive for high altitude layers and moderate for ground ones. On the other hand high altitude layers can be covered with only a limited field of view (where one can search for natural guide stars) while for ground layers the usable field of view is limited essentially by practical reasons. We introduce the further concept of multiple field of view layer oriented where a combination of sampling and covered field leads easily to sky coverages that nearly approach the whole sky with the usage of solely natural guide stars for 8m class telescopes.

Direct detection of photons emitted or reflected by an extrasolar planet is an extremely difficult but extremely exciting application of adaptive optics. Typical contrast levels for an extrasolar planet would be 10⁹ - Jupiter is a billion times fainter than the sun. Current adaptive optics systems can only achieve contrast levels of 10⁶, but so-called extreme adaptive optics systems with 10⁴ -10⁵ degrees of freedom could potentially detect extrasolar planets. We explore the scaling laws defining the performance of these systems, first set out by Angel (1994), and derive a different definition of an optimal system. Our sensitivity predictions are somewhat more pessimistic than the original paper, due largely to slow decorrelation timescales for some noise sources, though choosing to site an ExAO system at a location with exceptional r₀ (e.g. Mauna Kea) can offset this. We also explore the effects of segment aberrations in a Keck-like telescope on ExAO; although the effects are significant, they can be mitigated through Lyot coronagraphy.

NASA's Marshall Space Flight Center, in collaboration with Blue Line Engineering of Colorado Springs, Colorado, is developing a Segment Alignment Maintenance System (SAMS) for McDonald Observatory's Hobby-Eberly Telescope (HET). The SAMS shall sense motions of the 91 primary mirror segments and send corrections to HET's primary mirror controller as the mirror segments misalign due to thermo-elastic deformations of the mirror support structure. The SAMS consists of inductive edge sensors. All measurements are sent to the SAMS computer where mirror motion corrections are calculated. In October 2000, a prototype SAMS was installed on a seven-segment cluster of the HET. Subsequent testing has shown that the SAMS concept and architecture are a viable practical approach to maintaining HET's primary mirror figure, or the figure of any large segmented telescope. This paper gives a functional description of the SAMS sub-array components and presents test data to characterize the performance of the sub-array SAMS.

While the theory behind design of multiconjugate adaptive optics (MCAO) systems is growing, there is still a paucity of experience building and testing such instruments. We propose using the Lick adaptive optics (AO) system as a basis for demonstrating the feasibility/workability of MCAO systems, testing underlying assumptions, and experimenting with different approaches to solving MCAO system issues.

Horizontal path laser communications are beginning to provide attractive alternatives for high-speed optical communications. In particular, companies are beginning to sell fiberless alternatives for intranet and sporting event video. These applications are primarily aimed at short distance applications (on the order of 1 km pathlength). There exists a potential need to extend this pathlength to distances much greater than a 1km. For cases of long distance optical propagation, atmospheric turbulence will ultimately limit the maximum achievable data rate. In this paper, we propose a method of improved signal quality through the use of adaptive optics. In particular, we show work in progress toward a high-speed, small footprint Adaptive Optics system for horizontal path laser communications. Such a system relies heavily on recent progress in Micro-Electro-Mechanical Systems (MEMS) deformable mirrors as well as improved communication and computational components. In this paper we detail two Adaptive Optics approaches for improved through-put, the first is the compensated receiver (the traditional Adaptive Optics approach), the second is the compensated transmitter/receiver. The second approach allows for correction of the optical wavefront before transmission from the transmitter and prior to detection at the receiver.

An adaptive optical system has been built for improving optical beam quality in a new inertial confinement fusion (ICF) system. This system is designed to compensate the static and dynamic wavefront errors in the laser generator and amplifier by pre-compensation manner, which includes a 45-channel deformable mirror, two Shack-Hartmann wavefront sensors with 10x10 sub-apertures, a 45-channel high voltage amplifier and a wavefront control computer. Preliminary principle experiment has been done and the experimental results are reported.

We describe the purpose, theory, implementation and sample results of a wave optics propagation simulation developed to study multi-conjugate adaptive optics (MCAQ) for 4-10m class telescopes. This code was more specifically developed to assess the impact of diffraction effects and a variety of implementation error sources upon the performance of the Gemini-South MCAO system. These errors include: Hartmann sensing with extended and elongated laser guide stars, optical propagation effects through the optics and atmosphere, laser guide star (LGS) projection through the atmosphere, deformable mirror (DM) and wave front sensor (WFS) misregistration, and calibration for non-common path errors. The code may be run in either a wave optics or geometric propagation mode to allow the code to be anchored against linear analytical models and to explicitly evaluate the impact of diffraction effects. The code is written in MATLAB, and complete simulations of the Gemini-South MCAO design (including 3 deformable mirrors with 769 actuators, 5 LGS WFS with 1020 subapertures, 3 tip/tilt natural guide star (NGS) WFS, and 50 meter phase screens with 1/32nd meter resolution) are possible using a Pentium III but require 1 to 6 days. Sample results are presented for Gemini-South MCAO as well as simpler AO systems. Several possibilities for parallelizing the code for faster execution and the modeling of extremely large telescopes (ELT's) are discussed.

The weighted parallel generalized projection algorithm is applied in two steps to the control of two deformable mirrors using multiple wavefront sensing beacons to simultaneously increase the compensated field of view while maximizing point spread function uniformity. Wave optical simulation results are presented that indicate that, even in strong scintillation, not only can the size of the compensated field of view be increased, but also the uniformity of the point spread function over the compensated field of view can be improved by augmentation of constraints enforcing point spread function uniformity.

In this work we have performed very detailed numerical simulations of one of the currently proposed experimental setups of a Multi-Conjugate Adaptive Optics (MCAO) systems devoted to assess under real astronomical conditions and on an 8-m class telescope the performance of MCAO systems. Our simulations, both in open and closed-loop, consider all aspects in a real system except for the modeling of the actual Deformable Mirrors (DMs). Although the performances in the K-band attained using 3 Natural Guide Stars (NGS) when correcting a Field of View (FOV) of 30'' of radius are very promising, one soon realizes on the very low sky coverage attainable with MCAO systems relying only on NGSs. In view of this we propose a MCAO using a constellation of Laser Guide Stars plus a relatively faint NGS. Our simulations of such a system indicate that this technique is feasible and indeed provides a quite large sky coverage even when no optimization has been made. This leaves good hopes that with a more careful choice of parameters our system with a single NGS plus a constellation of LGS at different altitudes should obtain nearly whole sky coverage.

In an adaptive optics (AO) system, noise and detection error can produce errors in the slope measurement of a Hartmann-Shack (HS) wavefront sensor and have further effects on the performance of the AO System. The noise in an AO system can be divided into the readout noise and the photon noise. The detection error in an AO system results from the discrete sampling by using number-limited CCD pixels in the HS sensor and the deadspace between the CCD pixels. A theoretical model for numerically simulating the effects of noise and detection error is presented and a corresponding computer program has been compiled, which is combined with our existing program of numerical simulation of the laser propagation in a turbulent media and an AO system in a stationary state. Taking the long-exposure Strehl ratio and the percentage relative error of the centroid slopes for each subaperture as two evaluation parameters, numerical simulation investigations of the effects of detection error (including the limited sampling density and the deadspace), readout noise and photon noise on a practical AO system have been carried out. Statistics method and formulation method are used to evaluate the effects of readout noise and photon noise in the numerical simulation. It is shown that there is no significant difference between results by using these two methods when the signal to noise ratio (SNR) is larger. However, as SNR gets smaller, the formulation method becomes less accurate than the statistics method. The numerical results are very useful for the design of a practical AO system.

The dynamic control process in an adaptive optics (AO) system is the third important factor to affect the performances of the AO system. A numerical simulation of the dynamic control process in an AO system is carried out for the first time to complete a comprehensive numerical simulation of an AO system. By means of the numerical simulation the long-exposure Strehl ratio and other useful metrics for evaluating the performances of an AO system can be obtained. In comparison to the theoretical analyses of automatic control theory, a numerical simulation of the dynamic control process in an AO system is advantageous. The frequency response characteristics are closely related to the dynamic control performance of an AO system. The bandwidth, phase margin and gain margin which are very important parameters for experimenters can be obtained conveniently from a numerical simulation of the frequency response characteristics. A numerical simulation of the frequency response characteristics in an AO system is presented in this paper for the first time. The numerical simulation results agree with the experimental data excellently. The simulation computational results of frequency response by using composite input of multiple frequencies compare to the single-frequency simulation results very well and can be used to determine the practical parameters of bandwidth and margins. The performances of an AO system can be effectively investigated by means of a combination of a numerical simulation of frequency response characteristics with that of the dynamic control process.

The Gemini Observatory is planning to implement a Multi Conjugate Adaptive Optics System as a facility instrument for the Gemini-South telescope. The system will include 5 Laser Guide Stars, 3 Natural Guide Stars, and 3 Deformable mirrors optically conjugated at different altitudes to achieve near-uniform atmospheric compensation over a 1 arc minute square field of view. The control of such a system will be split in 3 main functions: the control of the opto- mechanical assemblies of the whole system (including the Laser, the Beam Transfer Optics and the Adaptive Optics bench), the control of the Adaptive Optics System itself at a rate of 800 frames per second and the control of the safety system. The control of the adaptive Optics System is the most critical in terms of real time performance. In this paper, we will describe the requirements for the whole Multi Conjugate Adaptive Optica Control System, preliminary designs for the control of the opto-mechanical devices and architecture options for the control of the Adaptive Optics system and the safety system.

The concept of Pyramid Wavefront sensor has been introduced as a more compact and flexible alternative to Shack--Hartmann wavefront sensing. In the past five years, however, such a novel concept promised a much larger sensitivity and an inherent easiness to be implemented in a multiple reference wavefront sensor. AdOptTNG, a natural guide star based adaptive optics module implemented at the 3.5m TNG telescope is equipped with such a sensor. We report here on the updated status, including on-sky experimental verification of various of the several features of such a sensor. We discuss the results obtained, their scalability and the lessons learned in building, aligning and operating it. Some comparison with theoretical and laboratory-based result, is also tentatively reported.

A method for determining the registration of an adaptive optical system is presented. Three deformable mirror actuators are sinusoidally driven at a high rate and the resulting gradients are measured using a Hartmann sensor. The gradient measurements are used to determine the translational, rotational, and magnificational registration of the deformable mirror relative to the Hartmann wavefront sensor. Two configurations are presented. The first is not effected by the compensation system but requires the use of auxiliary beams outside the clear aperture of the adaptive optical system. In the second configuration, the dithering actuators are within the clear aperture of the adaptive optical system. Simulation results are presented that show that the method can be used reliably in either configuration. In the second configuration, the method can be used reliably even in the presence of residual amplitude and phase aberrations.

Active laser-based electro-optical (EO) sensors on future aircraft and spacecraft will be used for a variety of missions and will be required to have a number of demanding technical characteristics. A key challenge to achieving these characteristics is the development of inexpensive, high degree of freedom optical wave front control devices, and the development of effective algorithms for controlling these devices. In this paper we present our research in the development of phase retrieval-based wave front control algorithms that can be used implemented with segmented liquid crystal-based wave front control devices. We have developed a wave front control algorithm that allows dynamic small-angle beam steering and shaping in the presence of an aberrating output window. Our approach is based on a phase retrieval algorithm to determine the optimal figure of a segmented wave front control device. Simulation and experimental results presented here show that this approach allows shaped far field patterns to be created and steered over small angles.

Adaptive optics for the 30-100 meter class telescopes now being considered will require an extension in almost every area of AO system component technology. In this paper, we present scaling laws and strawman error budgets for AO systems on extremely large telescopes (ELTs) and discuss the implications for component technology and computational architecture. In the component technology area, we discuss the advanced efforts being pursued at the NSF Center for Adaptive Optics (CfAO) in the development of large number of degrees of freedom deformable mirrors, wavefront sensors, and guidestar lasers. It is important to note that the scaling of present wavefront reconstructor algorithms will become computationally intractable for ELTs and will require the development of new algorithms and advanced numerical mathematics techniques. We present the computational issues and discuss the characteristics of new algorithmic approaches that show promise in scaling to ELT AO systems.

A technique for profiling of C_n² over an atmospheric propagation path is proposed, developed and analyzed. The technique employs differential-tilt measurements to arrive at statistics which have unique weighting functions over the propagation path. These weighting functions are computed theoretically and used to derive a reconstructor matrix for C_n² values throughout the path to be applied to an appropriate set of differential-tilt statistics. A candidate optical system is presented, and the performance of the profile reconstructor is analyzed. This study indicates that the relative error in the C_n² estimates is approximately 5%. The relative error in estimating key atmospheric parameters such as the Fried parameter, isoplanatic angle, and the Rytov parameter from the reconstructed profiles is approximately 3%.

In many adaptive optical systems the standard wave front sensing and reconstruction process involves a conventional least squares reconstruction of gradient measurements obtained from a Shack-Hartman wave front sensor (WFS). This reconstruction algorithm assumes the WFS measurements are equal to the average phase gradient within each subaperture. However, this assumption does not account for the effects of scintillation in the wave front. As scintillation increases, intensity fluctuations in the wave front increase the disparity between the output of the WFS and what the reconstruction algorithm expects, which in turn causes a degradation in the performance of the reconstructor. We present an algorithm that attempts to mitigate the scintillation problem by reconstructing the real and imaginary components of the wave front using gradient and intensity information obtained from a Shack-Hartmann WFS. This algorithm estimates the wave front by inverting a more precise model of the WFS measurement process. Wave optics simulations over a variety of atmospheric conditions are used to compare the performance of our algorithm against a least squares reconstructor and a complex exponential reconstructor.

Based on the principle of correlation tracking algorithm, the effectiveness of Cross Correlation coefficient and Absolute Difference algorithms for the low contrast extended objects such as solar granulation and the sunspot is studied. The tilt signals computed by computer post- processing are presented for the successively acquired solar granulation images. Moreover, the contrasts of the long exposure images of the solar granulation and sunspot without and with tilt removal are compared.

Rayleigh laser guide stars are an interesting tool to probe the atmospheric turbulence in astronomical adaptive optics. Conventional wavefront sensors can be applied at the expense of a restriction of the beacon range, usually realized by gating techniques, which translates into a very inefficient use of the light. Some alternative solutions to this inconvenient are presented in this paper, leading to the introduction of a new wavefront sensing concept. The idea is to place in the focal plane area an optical element whose section does not change for the conjugation to different ranges from the telescope aperture, hence the z-invariant name of the concept.

We report initial results from a prototype system to generate multiple Rayleigh laser guide stars for adaptive optics from a single pulsed laser at 354 nm wavelength. A 3.2 mW laser pulsed at 630 Hz was used to project three beams on the sky simultaneously, each pulsed at 210 Hz. A spinning mirror was used to direct the pulses in three directions at the vertices of an equilateral triangle 90 arcsec across. Laser pulses were triggered by a synchronising electrical pulse from the motor. Dynamic focusing optics in the receiving telescope will in future be used to hold such beacons from more powerful lasers in focus over a height range of many kilometers. Multiple beacons can be used to derive tomographic information on the vertical distribution of the aberration. We show initial analytical and numerical work on how the unique features of refocused Rayleigh beacons can improve the tomographic wavefront measurement for multi-conjugate adaptive optics.

The use of multi-conjugate adaptive optics (MCAO) for large telescopes will be most efficient with multiple laser guide stars. To implement a single laser guide star (LGS) at 589 nm requires the use of bulk and complex dye lasers at the moment. Solid-state lasers will be necessary to reduce to decent levels the infrastructure impact and operational complexity of multiple guide star adaptive optics systems. Development programs for discrete solid-state lasers are in place at GEMINI, the USAF Starfire Optical Range and University of Chicago. ESO has chosen the path of the development of fiber lasers, which is complementary to the other programs. The advantages of fiber lasers are manifold: they are very compact, inherently rugged, most reliable and have a potential for low cost. In this paper we present the research and development activity started at ESO on fiber lasers for MCAO. The aim is to produce > 10-W continuous-wave (CW) output power at 589 nm with a linewidth < 3 GHz. We report here on our fiber Raman laser conceptual design.

We report in this paper on the design and progress of the ESO Laser Guide Star Facility. The project will create a user facility embedded in UT4, to produce in the Earth's Mesosphere Laser Guide Stars, which extend the sky coverage of Adaptive Optics systems on the VLT UT4 telescope. Embedded into the project are provisions for multiple LGS to cope with second generation MCAO instruments.

Results for a multi-conjugate adaptive optical (MCAO) system with multiple laser beacons at multiple altitudes are presented in this paper. The use of multi-conjugate deformable mirrors (DM's) increases the corrected field of view of an adaptive optical telescope system. This improves the imaging capability for extended astronomical objects such as planets, galaxies, and nebulae. Multiple laser beacons, as opposed to multiple natural guide stars, are needed to achieve a useful degree of sky coverage. The use of laser beacons at multiple altitudes in a hybrid laser beacon configuration has been shown in previous papers to reduce both focus and tilt anisoplanatism. In this study we combine all three of these aspects. The hybrid beacon scenarios used in this study consists of multiple high altitude sodium beacons at 90 km and/or multiple low altitude Rayleigh beacons at 10 to 20 km. We present results for an 8-m class telescope for 2 and 3 different DM conjugate altitudes. For each of these MCAO configurations the following parameters are varied: number of Rayleigh beacons, number of Rayleigh beacon wavefront sensor (WFS) subapertures, Rayleigh beacon altitudes for the Rayleigh/sodium configuration, number of natural beacons for tip/tilt correction, and number of natural beacon WFS subapertures. When the WFS subaperture for the natural beacon is greater than 1 x 1 it contributes to the higher order correction in addition to being used for tip/tilt correction. Results are compared in terms of Strehl Ratio for the J, H, and K band.

In the near future, the Gemini Observatory will offer Laser Guide Star Adaptive Optics (LGS AO) observations on both Gemini North and South telescopes. The Gemini North AO system will use a 10W-class sodium laser to produce one laser guide star at Mauna Kea, Hawaii, whereas the Gemini South AO System will use up to five such lasers or a single 50W-class laser to produce one to five sodium beacons at Cerro Pachon, Chile. In this paper we discuss the similarities and differences between the Gemini North and South Laser Guide Star Systems. We give a brief overview of the Gemini facility Adaptive Optics systems and the on-going laser research and development program to procure efficient, affordable and reliable lasers. The main part of the paper presents the top-level requirements and preliminary designs for four of the Gemini North and South Laser Guide Star subsystems: the Laser Systems (LS), Beam Transfer Optics (BTO), Laser Launch Telescopes (LLT), and their associated Periscopes.

Adaptive optics performance using a sodium laser guide star at the Lick Observatory 3.0 m telescope is presented. In order to accomplish this the residual effects of natural guide star tip-tilt motion is removed. This is measured from 500 short exposure images (t_exp = 57ms). The data show instantaneous Strehl ratios ranging from ~ 0.10 to ~ 0.45 with a mean value of ~ 0.26. Centroid tracking of these data yield residual tip-tilt errors of ~ 21 mas, within specifications for the system. This resdual tip-tilt motion reduces the Strehl ratio of long exposure imaging by only ~ 7%.

8m class telescopes offer an extremely powerful tool for astronomical research. The light collecting power is enormous and with the combination of adaptive optics and laser guide stars astronomy will make a big step forward in knowledge in almost any field. For the creation of artificial laser-based guide stars that make use of the sodium layer in the earth atmosphere, very powerful lasers at 589nm are necessary. We introduce here the PARSEC laser that will be installed at the UT4 telescope of the VLT, in Chile. This laser will emit cw radiation at more then 10W output power and offers a scalability in power for future multi guide star systems.

The Lick Observatory laser guide star adaptive optics system has been significantly upgraded over the past two years in order to establish it as a facility science instrument on the Shane 3 meter telescope. Natural Guide Star (NGS) mode has been in use in regular science observing programs for over a year. The Laser Guide Star (LGS) mode has been tested in engineering runs and is now starting to do science observing. In good seeing conditions, the system produces K-band Strehl ratios >0.7 (NGS) and >0.6 (LGS). In LGS mode tip/tilt guiding is achieved with a V~16 natural star anywhere inside a 1 arcminute radius field, which provides about 50% sky coverage. This enables diffraction-limited imaging of regions where few bright guidestars suitable for NGS mode are available. NGS mode requires at least a V~13 guidestar and has a sky coverage of <1%. LGS science programs will include high resolution studies of galaxies, active galactic nuclei, QSO host galaxies and dim pre-main sequence stars.

The key step for full field correction in adaptive optics with two deformable mirrors is to match the amplitude profile of the main laser with the same of beacon laser on deformable mirror 2. Amplitude and phase corrections for turbulence are studied based on Y-G wave front retrieval algorithms. The simulation result show that amplitude and phase corrections can improve Strehl ratio by a factor of 1.27 to 2.50 compared with phase-only correction.

We discuss the design and implementation of a low-cost, high-resolution adaptive optics test-bed for vision research. It is well known that high-order aberrations in the human eye reduce optical resolution and limit visual acuity. However, the effects of aberration-free eyesight on vision are only now beginning to be studied using adaptive optics to sense and correct the aberrations in the eye. We are developing a high-resolution adaptive optics system for this purpose using a Hamamatsu Parallel Aligned Nematic Liquid Crystal Spatial Light Modulator. Phase-wrapping is used to extend the effective stroke of the device, and the wavefront sensing and wavefront correction are done at different wavelengths. Issues associated with these techniques will be discussed.

Understanding the statistics of remnant speckles in the halo of an adaptively-corrected point-spread function is critically important to using adaptive optics in high-dynamic-range searches for faint companions. It has been clear for some time that photon (Poisson) statistics alone do not adequately account for noise in the halo, as the coherent nature of speckles gives them a temporal persistence that leads to a much larger noise contribution, termed speckle noise. I consider in this paper the physical mechanism for speckle formation, and show that residual speckles, in the case of highly corrected adaptive optics systems, tend to be pinned to secondary maxima (Airy rings) in the underlying diffraction-limited point-spread function, affecting their spatial distribution in an important way. Further, in current practical adaptive optics systems, the structure of the Airy rings will shift over relatively short time scales in response to flexure-induced non-common-path errors, modifying the temporal evolution of the statistics of the speckle distribution as well.

The wavefront sensor (WFS) is perhaps the most critical adaptive-optic subsystem, particularly for astronomical applications with natural guide stars, where current WFS sensitivity limitations seriously restrict sky coverage. In this paper, we discuss the possibility of a WFS based on a phase-contrast principle of the sort employed by Zernike for microscopy. Such a WFS would be implemented by inserting a focal-plane filter with a (pi) /2 phase-shifting central spot having a transverse size of the order of the diffraction limit. The result would be an image of the pupil in which intensity is directly proportional to the seeing- and aberration-induced phase variations over the pupil. In comparison, the signals produced by the two most common current WFS schemes, Shack-Hartmann and curvature sensing, are proportional to the phase slope and to the second derivative, respectively. The phase-contrast approach might derive some advantages stemming from its more natural match to the control eigenvectors of the electrostrictive deformable mirrors that are expected to predominate in high-order adaptive optics systems, in the same way that curvature sensors are currently well matched to bimorph mirrors. It may thus yield substantial performance improvements with simpler hardware and lighter computational loads. We examine this and other possible advantages of the phase-contrast WFS, and investigate some of the practical design issues involved in its implementation.