Adaptive Optics Concepts Research Articles

Optique adaptative : correction des effets de la turbulence atmosphérique sur les images astronomiques

Today, adaptive optics (AO) is installed on all very large astronomical telescopes. It allows to overcome the limitation in angular resolution imposed on large ground-based telescopes by the atmospheric turbulence perturbations. This review paper presents the state of the art of the field for astronomical applications. It gives the principles of AO and describes its main components which are the deformable mirrors, the wavefront sensors and the control algorithms. The paper also presents the main recent achievements and ongoing projects. First, for the direct imaging of extra-solar planets by extreme AO and coronography, taking as example the SPHERE instrument. Then to solve the problem of the very limited sky coverage by using laser guide star and to extend the AO correction beyond the isoplanatic patch. Thus, the various laser-assisted tomographic AO concepts are presented with examples of each realization as for the laser tomography AO, the multi-conjugate AO and the multi-object AO. Finally, the challenges of the new generation of giant telescopes are discussed at the end of the paper with a particular focus on the European Extremely Large Telescope (ELT) project.

Active control of bright electron beams with RF optics for femtosecond microscopy.

A frontier challenge in implementing femtosecond electron microscopy is to gain precise optical control of intense beams to mitigate collective space charge effects for significantly improving the throughput. Here, we explore the flexible uses of an RF cavity as a longitudinal lens in a high-intensity beam column for condensing the electron beams both temporally and spectrally, relevant to the design of ultrafast electron microscopy. Through the introduction of a novel atomic grating approach for characterization of electron bunch phase space and control optics, we elucidate the principles for predicting and controlling the phase space dynamics to reach optimal compressions at various electron densities and generating conditions. We provide strategies to identify high-brightness modes, achieving ∼100 fs and ∼1 eV resolutions with 106 electrons per bunch, and establish the scaling of performance for different bunch charges. These results benchmark the sensitivity and resolution from the fundamental beam brightness perspective and also validate the adaptive optics concept to enable delicate control of the density-dependent phase space structures to optimize the performance, including delivering ultrashort, monochromatic, high-dose, or coherent electron bunches.

Linear Quadratic Gaussian Control Applied to WFAO Systems: Simulation and Experimental Results

Adaptive optics (AO) is a proven technique to correct turbulence on ground-based astronomical telescopes. The corrected Field of View (FoV) is however limited by the anisoplanatism effect. To extend the FoV, new Wide Field AO (WFAO) concepts have been recently developed. They aim at providing a wide FoV correction through the use of multi-guide-star WaveFront Sensors (WFSs) and several Deformable Mirrors (DMs). Such WFAO systems require a tomographic reconstruction of the atmospheric turbulence, leading to a multivariable problem of much higher complexity than in classic AO. They also raise new questions in terms of calibration and control. The Linear Quadratic Gaussian (LQG) control formalism is a natural way to cope with this issue. It enables both tomographic reconstruction and distinction between controlled output and measurements. In this paper, LQG control is presented and applied to WFAO systems. Performance is evaluated thanks to simulations based on theoretical studies, but also through laboratory experiments. We present the first results of the implementation of LQG control on a WFAO system, the HOMER bench, that has been developed recently at ONERA and which is devoted to WFAO laboratory research.

Wide field adaptive optics laboratory demonstration with closed-loop tomographic control

HOMER, the new bench developed at ONERA devoted to wide field adaptive optics (WFAO) laboratory research, has allowed the first experimental validations of multi-conjugate adaptive optics (MCAO) and laser tomography adaptive optics (LTAO) concepts with a linear quadratic Gaussian (LQG) control approach. Results obtained in LTAO in closed loop show the significant gain in performance brought by LQG control, which allows tomographic reconstruction. We present a calibration and model identification strategy. Experimental results are shown to be consistent with end-to-end simulations. These results are very encouraging and demonstrate robustness of performance with respect to inevitable experimental uncertainties. They represent a first step for the study of very large telescope (VLT) and extremely large telescopes (ELT) instruments.

Tomographic reconstruction for wide-field adaptive optics systems: Fourier domain analysis and fundamental limitations

Several wide-field-of-view adaptive optics (WFAO) concepts such as multi-conjugate AO (MCAO), multi-object AO (MOAO), and ground-layer AO (GLAO) are currently being studied for the next generation of Extremely Large Telescopes (ELTs). All these concepts will use atmospheric tomography to reconstruct the turbulent-phase volume. In this paper, we explore different reconstruction algorithms and their fundamental limitations, conducting this analysis in the Fourier domain. This approach allows us to derive simple analytical formulations for the different configurations and brings a comprehensive view of WFAO limitations. We then investigate model and statistical errors and their effect on the phase reconstruction. Finally, we show some examples of different WFAO systems and their expected performance on a 42 m telescope case.

Open-loop correction of horizontal turbulence: system design and result

Adaptive optics systems often work in a closed-loop configuration due to the hysteretic and nonlinearity properties of conventional deformable mirrors. Because of the high-precision wavefront generation and nonhysteretic properties of liquid-crystal devices, the open-loop control becomes possible. Open-loop control is a requirement for advanced adaptive optics concepts. We designed an open-loop adaptive optics system with a liquid-crystal-on-silicon wavefront corrector. This system is simple, fast, and can save much more light compared to conventional liquid-crystal-based closed-loop systems. The detailed principle, construction, and operation are discussed. The 500 m horizontal turbulence correction experiment was done using a 250 mm telescope in the laboratory. The whole system can reach a 60 Hz correction frequency. Evaluation of the correction precision was done at closed-loop configuration, which is 0.2 lambda (lambda=0.633 microm) in peak to valley. The dynamic image under open-loop correction got the same resolution compared to closed-loop correction. The whole system reached 0.68 arc sec resolution capability at open-loop correction, which is slightly larger than the system's diffraction-limited resolution of 0.65 arc sec.

Ground Layer Adaptive Optics

We will introduce the present knowledge of the turbulence profile C n 2 and in particular we will emphasise the existence of a turbulence layer close to the ground. Then we will present the concept of Ground Layer Adaptive Optics and will provide estimates of performance expected from such systems and their potential for astronomical applications. Finally we will provide practical implementation concepts for two instruments at the VLT, MUSE and HAWK-I using multi-Laser Guide Stars and a large Deformable Secondary Mirror. The latter will also be described as its use is optimum for GLAO systems. To cite this article: N. Hubin et al., C. R. Physique 6 (2005).

Adaptive optics concept for multi-object 3D spectroscopy on ELTs

In this paper, we present a first comparison of different Adaptive Optics (AO) concepts to reach a given scientific specification for 3D spectroscopy on Extremely Large Telescope (ELT). We consider that a range of 30%–50% of Ensquarred Energy (EE) in H band (1.65m) and in an aperture size from 25 to 100mas is representative of the scientific requirements. From these preliminary choices, different kinds of AO concepts are investigated: Ground Layer Adaptive Optics (GLAO), Multi-Object AO (MOAO) and Laser Guide Stars AO (LGS). Using Fourier based simulations we study the performance of these AO systems depending on the telescope diameter.

Real-time Control of ESO Adaptive Optics Systems (Echtzeitsteuerung der ESO Adaptive Optik Systeme)

Abstract The light coming from a star travels undisturbed for billions of kilometers until it reaches the boundaries of the Earth atmosphere: there small differences in the refraction index distort the spherical wavefront, creating phase errors in the image-forming ray paths. Even at the best sites, ground-based telescopes observing at visible wavelengths cannot, because of atmospheric turbulence alone, achieve an angular resolution better than telescopes of 10- to 20-cm diameter. Adaptive Optics (AO) is a technique to remove the effects of these distortions in real-time by operating a deformable mirror that rapidly adapts its shape to the atmospheric disturbance canceling it. ESO now operates several AO systems in the Paranal observatory, where the four telescopes of the VLT (Very Large Telescope) are installed. Several of them are the result of a common project called MACAO, for Multi Application Curvature Adaptive Optics. Four MACAO systems, one for each VLT telescope, are now serving the VLT-Interferometer (VLTI) increasing the capability of the interferometer. In this paper we will present the main concept of adaptive optics and then we will derive a model for a generic AO system. We will then tailor the model to MACAO and study some interesting features of MACAO as the control of the piston mode and the control of the tip/tilt modes.

The Application of MEMS Technology for Adaptive Optics in Vision Science

Recent years have seen remarkable results in high-resolution imaging and testing of the human visual system. The ability to obtain in vivo images of the human fundus on the micrometer scale will allow for much earlier diagnosis and treatment of a range of retina diseases. These advances have been made possible in part through the use of adaptive optics (AO). In order to utilize the full numerical aperture and, hence, maximize the resolution of the eye, it is necessary to provide a means of correcting all of the ocular aberrations. The concept of AO has been used successfully to overcome this difficulty. A key component of such a system is an optical element that can be deformed to provide high-order correction of these aberrations. Existing AO systems make use of deformable mirrors that were developed for large ground-based telescopes. In a drive to reduce their size and cost, fabrication of this element using microelectromechanical systems (MEMS) has been actively pursued. This paper explains the challenges in high-resolution imaging of the human eye and details how MEMS technology has been used to further research in this area.

Adaptive Optics in Star Formation

Over the past ten years, the concept of adaptive optics has evolved from early experimental stages to a standard observing tool now available at almost all major optical and near-infrared telescope facilities. Adaptive optics will also be essential in exploiting the full potential of the large optical/infrared interferometers currently under construction. Both observations with high-angular resolution and at high contrast, and with a high point source sensitivity are facilitated by adaptive optics. Among the areas which benefit most from the use of adaptive optics are studies of the circumstellar environment (envelopes, disks, outflows), substellar companions and multiple systems, and dense young stellar populations. This contribution highlights some of the recent advances in star formation studies facilitated by adaptive optics, and gives a brief tutorial on optimized observing and data reduction strategies.

Low-order adaptive optics for free-space optoelectronic interconnects

The concept of adaptive optics for improving the cost-performance of free-space optoelectronic interconnects is discussed. Adaptive optics as a design option for optical interconnect systems is presented and discussed. A practical demonstrator that performs low-order correction was built and tested. Slowly varying misalignments, including thermal effects, were compensated for in a 622-Mbit/s free-space optical data link.

Differential adaptive optics for astronomical interferometry

Sometimes it suffices to make the wave-front distortions across individual telescopes equal rather than zero. This leads to the concept of differential adaptive optics.

Intracavity adaptive optic compensation of phase aberrations III: Passive and active cavity study for a large N_eq resonator

The results of a numerical study of both the passive and the active mode-structure properties and far-field behavior of a linear, positive-branch, confocal unstable resonator with a large equivalent Fresnel number in the ideal unaberrated, intracavity phase-aberrated, and intracavity phase-corrected states are presented. A simple, saturable, gain-medium model of a homogeneously broadened CO2–electron-beam discharge laser system is employed in the active cavity study. The active cavity results presented here show that, at least for the level of gain saturation considered, the passive cavity theory of phase-aberration sensitivity remains applicable in this more-physical situation. Furthermore, the ideal correction to phase-tilt and curvature aberrations, derived from passive cavity considerations, remains exact in the presence of saturable gain. Finally, the influence of a multidithered, zonal deformable mirror on the cavity-mode structure and far-field behavior is analyzed in order to account for recent experimental tests of the intracavity adaptive optics concept.

Target loop adaptive optics: thermal blooming correction

To evaluate adaptive optics (AO) concepts for the correction of nonlinear atmospheric phenomena occurring in high energy laser beam propagation and to perform AO system trade offs, a simple, but reasonably accurate, model of the entire AO loop is required. Heretofore, no such model existed which takes into account the temporal response of the atmosphere and yet is simple enough to permit extensive system tradeoffs. We have developed such a model. Optimum thermal blooming correction dictates that the atmospheric response time must be accounted for properly, especially for strong blooming strength. Experimental verification of the above model was carried out using an argon-ion laser with an iodine blooming cell and a closed AO loop to provide corrections for tilt, focus, and astigmatism. Reasonable agreement between theory and experimental data was achieved.

Target loop adaptive optics: Thermal blooming correction

To evaluate adaptive optics (AO) concepts for the correction of nonlinear atmospheric phenomena occurring in high energy laser beam propagation and to perform AO system trade offs, a simple, but reasonably accurate, model of the entire AO loop is required. Heretofore, no such model existed which takes into account the temporal response of the atmosphere and yet is simple enough to permit extensive system tradeoffs. We have developed such a model. Optimum thermal blooming correction dictates that the atmospheric response time must be accounted for properly, especially for strong blooming strength. Experimental verification of the above model was carried out using an argon-ion laser with an iodine blooming cell and a closed AO loop to provide corrections for tilt, focus, and astigmatism. Reasonable agreement between theory and experimental data was achieved.