We report on the development and testing ofthe adaptive optics system for the 6.5m single-mirror telescope that replaces the Multiple Mirror Telescope (MMT). This system features an adaptive secondary mirror and other innovations intended to give diffraction-limited infrared imaging with full sky coverage and the ultimate in low optical losses and thermal emissivity. In this paper, we first describe the system from a global perspective and then dwell particularly on the optical features of the system, the science package with integral tip/tilt guiding, and the solid-state sodium guide star laser. Other papers in this conference describe in more detail the innovative 300-actuator adaptive secondary, the control processor architecture and performance, and results from Fasttrac II, the immediate precursor to this system

We have studied the performances of natural and laser guide star adaptive optics systems for the ESO Very Large Telescope. Analytical formulae are used to derive the Strehl and FWHM of the PSF obtained with NGS and LGS, for two atmospheric models (20 % of the time and median). Sky coverage was computed with stellar densities, leading to a 75 % sky coverage at a Strehl of 0.3 in K band at average galactic coordinates with LGS and 8 % with NGS. A cross correlation approach was also used: reference stars were searched in the USNO-A catalogue around science objects found in the Veron-Cetty 96 and SIMBAD catalogues. This approach shows that the number of quasars with strehls greater than 0.2 is increased by a factor of 20 with LGS. In J band LGS is much less efficient, but in good atmospheric conditions, it allows to observe 720 quasars with Strehl greater than 0.2. In median conditions, this number drops to 0, mainly because of the cone effect. This leads to the the need for on-line atmospheric turbulence profile measurement and flexible scheduling of the observation, to optimize the LGS performances in J band.

A conceptual design for the WHT Natural Guide Star Adaptive Optics system, now called NAOMI (Nasmyth Adaptive Optics for Multi-purpose Instrumentation) was presented at the 1995 SPIE meeting1. Although the general principle of using off-axis paraboloidal mirrors (OAP) as a collimator- camera combination, with a deformable mirror located between them in the collimated beam, remains the same many features of the design have been improved.

AdOpt@TNG is the Adaptive Optics module for the Italian “Telescopio Nazionale Galileo" (TNG), a 4m? class telescope that will have its first light in late 1997. This module incorporates a number of peculiar features such as the adoption of an electro-magnetically driven tip-tilt mirror, a continuously adjustable offset between the reference and the scientific objects (in a way to deal, for instance, with moving references, like asteroids, or targets, like comets), a built-in speckle camera with an on-line autocorrelator and others. The speckle camera has been tested at the 1.82m telescope of the Astronomical Observatory of Asiago, a short account of the results obtained is given. Most of the elements of the AdOpt@TNG module have been manufactured and tested. The up-to-date status report of the module is briefly given.

ALFA is an adaptive optics system with laser guide star for the 3.5 m telescope on Calar Alto, Spain. ALFA has been developed jointly by the Max-Planck-Institutes for Astronomy (MPIA, Heidelberg) and for Extraterrestrial Physics (MPE, Garching). All core components of ALFA were installed at the telescope in the course of 1996; the system is currently undergoing integration and testing. ALFA uses a continuous-wave dye laser pumped by an argon ion laser to create an artificial guide star in the mesospheric sodium layer. The laser is stabilized to the sodium D2 line at 589 nm with a sodium vapor cell. In normal operation the frequency is locked to the Lamb dip in the absorption cell, but it is also possible to scan the laser under computer control over several GHz for diagnostic purposes. The dye laser has been optimized for stable operation at high power output; 3.75 W are regularly achieved in single-mode operation. The laser system is located in the coud? room of the telescope; the laser beam is transmitted through the coud? path to a 50 cm beam expansion telescope attached to the mirror cell of the 3.5 m telescope. The first measurements of the sodium spot created with the ALFA laser were made in 1996 with the guide camera of the 3.5 m telescope and with a 1.23m telescope located 276m away.

The FASTTRAC II adaptive optics instrument has been used at the Multiple Mirror Telescope (MMT) for the past 2 years to provide improved image resolution in the near infrared. Results have been obtained using both natural guide stars and an artificial sodium laser beacon. With the imminent closure of the MMT prior to its conversion to a single-mirror 6.5 m telescope, FASTTRAC II has come to the end of its life. The instrument has been to the telescope for a total of 8 runs, and during that time it has been of enormous value both as a learning aid, demonstrating the requirements of its successor on the 6.5 m, and as a scientific tool. At this meeting, we present a selection of astrophysical data derived from FASTTRAC II, including the first closed-loop demonstration of an adaptive optics system using a sodium laser beacon. The sodium laser has been used to obtain near diffraction-limited near-infrared images of the core of M13, allowing the construction of a color-magnitude diagram to below the main sequence turnoff. Results have also been obtained from several gravitationally lensed quasars, and the cores of nearby galaxies in the local group. We also summarize work characterizing atmospheric conditions at the site. These studies have proceeded in two areas - understanding the behavior of the phase perturbation with field angle and time, and characterizing the return from the sodium resonance beacon.

Adaptive Optics systems provide a real time compensation for atmospheric turbulence, which severely limits the resolution of large telescopes. However, the correction is often only partial and a deconvolution is required to reach the telescope diffraction limit. The need for a regularized deconvolution is discussed, and a Maximum A Posteriori based deconvolution technique is presented. This technique incorporates a positivity constraint and the knowledge of the object Power Spectral Density. This method is then extended to the case of an unknown PSF. Deconvolution results are presented for both simulated and experimental data.

Post processing of data obtained from the European Southern Observatory’s ADONIS system is presented here. A physically constrained iterative deconvolution algorithm is applied to both bright and faint object data demonstrating both object and point spread function recovery. Comparisons are made with standard linear and non-linear deconvolution algorithms. Simulated data is used to investigate the combined system and post-processing performance on faint extended objects.

In a recent paper,1 we have proposed a new method to estimate the point spread function of an adaptive optics system, based on the data accumulated by the control computer during the acquisition. In this paper, we present some recent developments related to this method. These developments focus on the wave-front sensor noise estimation and the calibration of the so-called hidden aberrations, which are critical steps in the estimation method.

Laser Guide Star tip-tilt recovery is a central and crucial problem in the framework of adaptive optics systems capable of full-sky diffraction limited performances. In the last years a number of techniques involving auxiliary telescopes shooting additional Laser Guide Stars or probing portions of the main Laser Guide Star generated by the main observatory has been outlined and published. Two of these techniques, relying on Natural Guide Stars well away from the isokinetic patch of the observed scientific target, uses some additional telescopes in the neighborouds to be used as differential tilt telescopes or as auxiliary laser projectors. A comparison of the ground impact and of the maximum achievable performances due to conical anisokinetism problems inherent to the additional telescopes is given.

A method for measuring full aperture tilt with a laser guide star (LGS) is described. The method uses a single monochromatic LGS formed by a laser beam transmitted through the main telescope and two arrays of small auxiliary telescopes separated from the main telescope in transverse directions. To achieve the Strehl ratio required for practical adaptive optics (AO) systems, the method uses an averaging of the LGS image over the position of the auxiliary telescope in the telescope array and over position of a subaperture within the auxiliary telescope, along with integration over FOV of the receiver and over time. I show that the contribution of the down propagation path can be considerably reduced by increasing the number of auxiliary telescopes and the number of subapertures, as well as by time integration. With this method Strehl ratio equal to 0.7 can be achieved for various seeing conditions, telescope diameter, and wavelength, including a visible waveband.

The effect of non-Kolmogorov stratospheric turbulence on star image motion is for the first time experimentally investigated with a ground-based telescope. A new approach permitting isolation of star image motion induced solely by atmospheric turbulence is employed. In this technique Polaris image wander is recorded with the telescope bolted in place to minimize uncontrolled telescope motion. High resolution temporal and spatial statistics of wave-front tilt are obtained. The dependencies of tilt variance, tilt power spectra, and tilt temporal correlation on telescope diameter are investigated for five apertures in the range 0.1-1.5 m. The experimental data show the dependence of tilt variance on telescope diameter does not follow the predictions of the Kolmogorov and von Karman models. The graph of the measured dependence has a “knee” which can be explained only by assuming a non-Kolmogorov stratospheric turbulence effect. The difference between tilt components in different axes indicates anisotropy in stratospheric turbulent inhomogeneities. The slopes of the measured tilt power spectra, approximately -1 in the low frequency range and -8/3 in the high frequency range, do not agree with theoretical predictions. The measured tilt temporal correlation scale is in the range 0.1-1.0 s, and the behavior of the correlation coefficients indicates the effect of large scale inhomogeneities not predicted by the conventional model. Uncontrolled telescope motion is manifested as a “bump” in the tilt power spectra in the range 70-90 Hz, but this makes an insignificant contribution to Polaris jitter variance.

We describe the control system which will be used for the adaptive optics system to be placed on the new 6.5-meter MMT. We describe the hardware used to implement the control system, and possible algorithms to accomplish the control. A real-time reconstructor will use a linear matrix multiply to calculate the figure error using input data from a Shack-Hartmann type wave-front sensor. The mirror actuators will be controlled by a servo control-loop which uses the reconstructed wave-front and capacitive position sensors located at each actuator as feedback. The control electronics are based upon commercially available components which conform to the VME standard. We discuss the unique characteristics of the MMT design both for the reconstruction of the wavefront from the WFS measurements and the adaptive secondary servo control-loop. The control system is designed to address two unique problems associated with using an adaptive secondary mirror with voice-coil actuators. The control system must accurately control the relatively large but thin (2mm) secondary. The control system must work in the absence of a guard band around the outside of the pupil. We present timing diagrams for the flow of data through the computational units of the control electronics, and will show measured latencies for each computational component and the control system as whole.

Complex instruments generally have complex control requirements. However theoretical analysis, instrument design, and software planning can allow a simple user interface to control the instrument successfully. A case in point is the Canada-France- Hawaii Telescope Adaptive Optics system. In normal operation one button push is sufficient to begin adaptive correction. During system design much effort was placed in automatic gain optimization for bimorph mirror control. This has allowed the system to run unattended for hours at a time. During software design and development other control points were identified and decision paths formulated to allow their parameters to be adjusted automatically. So, although a complete engineering interface was designed and implemented, in practice it has only been used for initial development and acceptance testing. Normal observing needs only a simple form for the astronomer. We present the resulting interface and trace some of the design decisions that allowed us to simplify this potentially complex interface.

The 349 degree of freedom Keck adaptive optics system will be mapped on to the 36 segment Keck primary mirror. Each telescope segment is independently controlled in piston and tilt by an active control system and each segment also has its own set of aberrations. This presents a unique set of problems for the Keck adaptive optics system, not encountered with continuous primaries. To a certain extent the low order segment aberrations, beginning with focus, can be corrected statically by the adaptive optic system. However, the discontinuous surface at the segment edges present special problems in sensing and correcting wavefront with laser guide stars or natural guide stars.

The upgraded 6.5 m MMT in Arizona will use an adaptive secondary to optimize performance in the near infrared spectral region. The secondary mirror is a 2 mm thick, 640 mm diameter Zerodur shell suspended only by a flexible center hub. Three hundred voice coil actuators installed in an aluminum reference surface deform the shell according to commands from a wavefront sensor. Capacitor position sensors surrounding each actuator provide feedback in an inner servo loop, much faster than the exterior wavefront sensor control bandwidth. A 60 actuator prototype, nearly identical to the final adaptive secondary size, has been built and is currently being tested.

Micro-electro-mechanical deformable mirrors (MEM-DM) are solid state electronic devices with small, movable reflective surface elements which can be used to manipulate the phase of optical wave fronts. MEM-DMs differ from conventional continuous facesheet deformable mirrors in that the movable surface of a MEM-DM consists of a set of segmented moving surfaces. The segmented, reflective surfaces of a MEM-DM give rise to larger diffraction effects than those provided by continous facesheet deformable mirrors. However, MEM-DMs are still attractive due to their low cost and the low drive voltages. In this paper we present laboratory results demonstrating reduction of a fixed aberration using a MEM-DM device.

Liquid crystal (LC) based spatial light modulators (SLM) offer a low cost alternative to the expensive deformable mirrors used in most adaptive optics systems. Real time correction of simulated atmospheric turbulence is demonstrated. The corrective element is a 10 by 10 pixel ferroelectric liquid crystal (FLC) SLM. An improvement in the Strehl ratio from 23% to 33% with a frame rate of 6Hz was demonstrated. A nematic liquid crystal (NLC) SLM was used to generate realistic atmospheric turbulence.

A new tracker is under development at the Starfire Optical Range. The tracker is based on a high-speed CCD camera running at 5000 frames per second. A suite of hardware and software that allows unprecedented flexibility in control system design and implementation delivers the mirror control signals. This paper discusses the design and implementation of the entire tracker system, including the camera design and control algorithms implemented. Two control strategies are investigated: simple integral control and linear quadratic gaussian/loop transfer recovery (LQG/LTR). Experimental results are presented for each of the control system algorithms tested.

Project ULTIMA is an investigation into the feasibility of building ultra-large aperture visible/mid-IR space telescopes. A promising concept found by the study is a freely flying spherical primary mirror, twenty meters or more in diameter, located at the L1 or L2 Earth-Sun libration point. The primary would be passively cooled to 45 K. There would be no metering structure. Instead, using a combination of alignment and steering mirrors, reaction wheels, and microthrusters, the aspherical secondary mirror, active tertiary mirror, and focal plane instruments would be precisely stationed in the correct position above the primary. The primary advanced composition would be either a membrane or ultra-light segmented technology. Preliminary fmdings show that a 20-30 m telescope may be feasible for imaging in the 1-20 µm regime.

This paper reports the application of an adaptive optics (AO) system to our ground-to-satellite laser communication system. We believe that the AO system is one of the best solutions to compensate the degradation of the laser beam due to an atmospheric turbulence and to realize a high bit-rate laser communication system. In the first stage of developing and designing the AO system, we have measured an atmospheric turbulence at Communications Research Laboratory (CRL), located in the west suburb of Tokyo. It shows that Fried's coherence length is about 5 cm in winter and 9 cm in summer at 0.8 µm observing wavelength. We also simulated the AO performance of the laser communication system using our results of the atmospheric measurement.

An adaptive system for active imaging and radiation focusing on extended rough-surfaced objects in the presence of phase distortions was experimentally demonstrated. This technique is based on the use of two laser beams having different wavelengths, here a He-Ne and Argon laser. The Argon laser beam was used for adaptive radiation focusing on the rough target surface. The second laser beam served for target active imaging. The imaging system included the same wavefront correctors used for the radiation focusing system. For adaptive control of the wavefront correctors, a gradient technique based on a radiation focusing quality criterion optimization was applied. Statistical properties of the speckle field scattered by the target surface were used as the radiation focusing quality metric. To measure this criterion a single photodiode with a simple signal analysis method was employed. Adaptive correction was implemented with a nine electrode bimorph mirror and a liquid crystal spatial phase modulator having 127 individually addressed hexagonal liquid crystal cells.

An adaptive imaging system with a liquid crystal phase modulator having 127 individually addressed hexagonal elements is experimentally studied. The system operation is based on direct optimization of an image quality metric dependent on image plane intensity distribution. For optimization of the image quality metric we applied a modified version of a stochastic perturbation gradient descent algorithm. Experimental results demonstrated the efficiency of the algorithm for high- resolution adaptive wavefront correction in an imaging system. A modification of the algorithm that significantly accelerates algorithm convergence is suggested and studied by numerical simulation.

A two deformable mirror concept for correcting amplitude effects in laser beam projection through the turbulent atmosphere is presented. This system uses a deformable mirror and a Fourier transforming mirror to adjust the amplitude of the wave front in the telescope pupil. A second deformable mirror is used to correct the phase of the wave front before it leaves the aperture. The phase applied to the deformable mirror used for controlling the beam amplitude is obtained using a technique based on the Fienup phase retrieval algorithm. One dimensional simulation results are presented which indicate that the average on-axis energy is improved by 15% or more on average, and the 90% encircled energy radius can be reduced by up to 30% on average.

Atmospheric turbulence severely limits the resolution of ground-based telescopes. Adaptive optics can correct for the aberrations caused by the atmosphere, but requires a bright wavefront reference source in close angular proximity to the object being imaged. Since natural reference stars of the necessary brightness are relatively rare, methods of generating artificial reference beacons have been under active investigation for more than a decade. In this paper, we report the first significant image improvement achieved using a sodium-layer laser guide star as a wavefront reference for a high-order adaptive optics system. An artificial beacon was created by resonant scattering from atomic sodium in the mesosphere, at an altitude of 95 km. Using this laser guide star, an adaptive optics system on the 3 m Shane Telescope at Lick Observatory produced a factor of 2.4 increase in peak intensity and a factor of 2 decrease in full width at half maximum of a stellar image, compared with image motion compensation alone. The Strehl ratio when using the laser guide star as the reference was 65% of that obtained with a natural guide star, and the image full widths at half maximum were identical, 0.3 arc sec, using either the laser or the natural guide star. This sodium-layer laser guide star technique holds great promise for the world's largest telescopes.

We present images of the (v= 1 — 0) S(1) transition of molecular hydrogen at 2.121 µm towards T Tauri, observed with the SHARP II camera attached to the ADONIS adaptive optics system at the European Southern Observatory’s 3.6 m telescope on La Silla, Chile. With an image scale of 0"1 per pixel, we could cover a field of 25" in each exposure. Positioning T Tauri successively in the four quadrants of the detector, we obtained a 35" mosaic centered on the star. In the 2.12 µm continuum, the 0"7 binary is easily resolved. In continuum-subtracted images in the emission line, we detect a very complex structure: strong emission at the position of the infrared companion, a very bright knot 2-3" northwest, three thin filaments, strong emission extending 10" south towards Burnham’s nebula, and a faint arc 10" north. While the material north and south of the stars may be associated with an outflow, we argue that the knots and filaments may be shock-excited material in the equatorial plane. They could be either tidal tails caused by the interaction of the two star-disk systems (T Tau N and T Tau S), accretion streams from the inner edge of the circumbinary disk, or spiral shocks in this disk.

The 3.5 meter telescope at the U. S. Air Force’s Starfire Optical Range in Albuquerque, New Mexico, is being equipped with an adaptive optics system for compensation of atmospheric phase disturbances. An optical relay using toroidal mirrors interfaces the telescope to the steering mirror(s), the 941 actuator deformable mirror, and the optical sensors. The methods developed for optimizing this toroidal mirror design are presented. The all reflective design accommodates a wide spectral band and provides near diffraction limited performance over the full one milliradian field of view and for object distances from 12.5 kilometers to infinity. A spectrally flexible and optically efficient design for interfacing the wavefront sensor and science camera(s) to the optical train is used. Alignment techniques have been developed for accurate alignment of the toroidal mirrors and for registration of the deformable mirror and wavefront sensor of the adaptive optics system.

We present laboratory subsystem test results obtained during integration of the Palomar 200” Adaptive Optics System at Jet Propulsion Laboratory. These results pertain to the 241 actively controlled actuator, Shack-Hartmann sensed, initial delivery of a system optimized for near infrared observation with the 5 meter diameter telescope at Palomar Mountain. This system initially exploits natural guide stars. Our intention is to provide a wide-ranging summary of subsystem performance measurements and several lessons learned. Noteworthy among these results is our measurement of 6.3 ± 0.2 electron read noise performance of our initial WFS camera, based upon a 64 x 64 pixel MIT/LL CCD detector, running at 600 kilopixel per sec per output amplifier. This camera was constructed by SciMeasure Analytical Systems, Inc. of Decatur, GA.

We discuss the mathematical approach taken for developing different matrix estimators for zonal adaptive optics systems. The software used to generate these matrices is then summarized.

One of the principle difficulties with popular wavefront reconstruction techniques such as least-squares estimators is that they are computationally intensive. In this paper a least-squares reconstructor is represented in terms of a 4-D wavelet basis. This representation is called the multiresolution wavefront reconstructor (MWR). A thresholding operation is applied to the MWR in order to remove wavelet coefficients of negligible magnitude. The resulting thresholded reconstructor matrix is sparse, leading to an estimate calculated in O(N3) operations, as opposed to O(N4) operations for the standard least-squares wavefront reconstructor. The thresholded multiresolution wavefront reconstructor is compared with other techniques in terms of computational complexity.

Parameters for the design of an optimised Shack-Hartmann wavefront sensor are developed from a high fidelity computer model. A number of estimation techniques are applied, and their relative performance is assessed.

The effect of focus anisoplanatism upon the performance of an astronomical laser guide star (LGS) adaptive optics (AO) system can in principle be reduced if the lowest order wavefront aberrations are sensed and corrected using a natural guide star (NGS). For this approach to be useful, the noise performance of the wavefront sensor (WFS) used for the NGS measurements must be optimized to enable operation with the dimmest possible source. Two candidate sensors for this application are the Shack-Hartmann sensor and “phase-diverse phase retrieval,” a comparatively novel approach in which the phase distortion is estimated from two or more well-sampled, full-aperture images of the NGS measured with known adjustments applied to the phase profile. We present analysis and simulation results on the noise-limited performance of these two methods for a sample LGS AO observing scenario. The common parameters for this comparison are the NGS signal level, the sensing wavelength, the second-order statistics of the phase distortion, and the RMS detector read noise. Free parameters for the two approaches are the Shack-Hartmann subaperture geometry, the focus biases used for the phase-diversity measurements, and the algorithms used to estimate the wavefront. We find that phase-diverse phase retrieval provides consistently superior wavefront estimation accuracy when the NGS signal level is high. For lower NGS signal levels on the order of 103 photodetection events, the Shack-Hartmann (phase diversity) approach is preferred at a RMS detector read noise level of 5 (0) electrons/pixel.

Due the fact that the numerical simulation is the basic method intended for studying the nonlinear effects caused by the high power laser beam interaction with the atmosphere, the accurate solving corresponding equations is the first problem from the applied problems of thermal blooming:

Diffraction-limited spectroscopy with adaptive optics (AO) has several advantages over traditional seeing-limited spectroscopy. First, high resolution can be achieved without a large loss of light at the entrance slit of the spectrograph. Second, the small AO image width allows the cross-dispersed orders to be spaced closer together on the detector, allowing a large wavelength coverage. Third, AO spectrograph optics are slow and small, costing much less than for a traditional spectrograph. Fourth, small AO images provide high spatial resolution. Fifth, scattered light is less problematic. And last, the small entrance slit of the spectrograph can get rid of much of the sky background to obtain spectra of faint objects. We have done theoretical calculations and simulations for infrared spectroscopy at the MMT 6.5 m with laser guide star AO, which provides almost full sky coverage. The results show we can expect 40-60% of the photons from a unresolved source within 0.2 arcsec diameter circle for J, H, K, L and M bands under typical atmospheric seeing condition at 2.2 micron (ro = 1.0 m, to = 21 ms, ?o = 15 arcsec and d0 = 25 m). Therefore, the spectrograph entrance slit size should match the 0.2 arcsec image to obtain high throughput. Higher resolution can be achieved by narrowing down the slit size to match the diffraction-limited image core size of about 0. 1 arcsec in the infrared. However, the throughput will be correspondingly reduced by a factor of two. Due to the limited atmospheric isoplanatic angle in the J, H and K bands, the encircled photon percentage within 0.2 arcsec diameter drops from 40-60% when the object is at the laser pointing direction to 20-40% when the object is about 30 arcsec away from the laser direction. Therefore, the useful field of view for AO multiple object spectroscopy is about 60 arcsec. Further studies of JR background (sky and thermal) and JR detector performance show that spectral resolution of R = 2,000 can take full advantage of AO images without much penalty due to the dark current of the JR detector and JR OH sky emission lines. We have also studied natural guide star AO spectroscopy. Though sky coverage for this kind of spectroscopy at the MMT 6.5 m is very limited, a bright star provides much better performance than the laser guide star AO spectroscopy. About 40-70% photons are concentrated within 0.1 arcsec diameter for guide stars brighter than 13 magnitude. Therefore, higher resolution and high throughput can be obtained simultaneously, given a bright enough natural guide object. The field-of-view for multiobject spectroscopy using a natural guide star is similar to that for laser guiding.

The SFIM Group has been involved for ten years in the fabrication of active and adaptive optics, with such realizations as the 8.2 meter diameter primary active mirror of the Very Large Telescope built by its subsidiary REOSC. During its works in this field, the SFIM Group has developed original algorithms aimed at designing low-cost active deformable mirror with high performance. The purpose of optimization is to reduce the number of actuators used, which is the main driver of the mirror cost. This reduction is first achieved by minimizing high frequency defaults due to the pad located between each actuator and the mirror. Then, an optimization of the actuator location is performed, which allows minimizing defaults due to edge effects. This optimization is discussed in part II of this article. Pad optimization is performed by means of a simple model, based on a spectral analysis of the mirror deformation, which accurately coincides with finite element calculation. This work has led to the design of an optimized pad which allows a dramatic decrease of the sampling error, up to a factor of 5 for some deformations with a wide spectral bandwidth. This allowed designing a low cost active mirror proposed to the Commissariat à l' Énergie Atomique, in the scope of the French Laser MégaJoule program.

In the part I of this publication, we have shown that an optimization of the pad allowed us to decrease the actuator number of the active mirror without degrading the performances requirements. The other way to improve performance is presented and discussed in this paper and consists in fitting the pattern of actuators to the set of deformations required in order to decrease the number of actuators. The method we have developed is based on a modal analysis of the mirror. Finite element calculation produces the first dynamic modes of the mirror. Each deformation is decomposed as a linear combination of the dynamic modes. The first step of the optimization is to estimate for a well defined pattern, the best effort vector to perform each mode and to calculate the error associated. The second step is to calculate a overall error taking into account the statistical characteristics of the deformation. The final step consist in the minimization of this error by moving the pattern of actuators. The method of optimization is fully detailed as well as the numerical process used to find the minimum of the error. Many results as well as the advantages of this method are presented. Application of this method to the design of the active mirror of the Laser MegaJoules (French Ignition Program) is mentioned.

The atmospheric image motion control loop for the 3.6m Telescopio Nazionale Galileo (TNG) has been tested and characterized in lab using a turbulence generator. The tip-tilt sensor is based on four Avalanche Photodiodes and the tilt corrector is a voice coil actuated flat mirror. The feedback network is implemented using a a DSP-based system. We have used the same focal ratios of the beam as it will be experienced by the system at the foci of the TNG telescope. The collected data are time-series useful to perform a Fourier analysis aimed to estimate the effectiveness of the correction. These data can be compared with: theoretical and measured tilt spectra obtained at the telescope site, in order to predict the degree of correction that can be achieved by such a system; measured data on single components, like the transfer function of the tip-tilt mirror, the latency of the DSP calculation and the characterization of the APDs tip-tilt sensor unit. The collected data are also suitable to perform further simulations in order to plan which will be the more effective correction at the telescope foci under several different conditions of reference brightness and seeing parameters.

Bimorph mirrors consist of two layers of piezo-electric ceramic glued together with an electrode pattern between them. They present an inexpensive alternative to more conventional deformable mirrors used in adaptive optics which use discrete piezoelectric actuators. The main technical question in bimorph production is how to produce a high quality mirror surface. Several methods have been tried, including direct polishing of the piezo ceramic and application of replicated surfaces. A prototype bimorph mirror has been developed for the Anglo-Australian Telescope Adaptive Optics system. It uses a layer of silicon dioxide deposited onto the piezo substrate and then polished and coated with silver. This production technique is discussed along with the characteristics of the resulting mirror as determined using a WYKO laser interferometer.

We describe the optical fabrication of thin glass shells which will be combined with rigid active supports for adaptive secondary mirrors and for space optics. These applications require glass shells about 2 mm thick, with diameters up to about 1 m for adaptive optics and possibly 6 m for space optics. The extreme flexibility presents unique fabrication challenges which are overcome by a simple adaptation of traditional glassworking techniques. Here we describe the fabrication of concave spherical shells 20 cm and 55 cm diameter. A method of handling and supporting the thin substrates for loose abrasive grinding and polishing is demonstrated and some variations on this approach are compared. Extension of the technique to aspheric adaptive secondary mirrors and to ultra-light mirrors up to 6 meters in diameter is discussed. The subsequent integration and optical testing of a 55 cm shell with a 36 point active support is reported.

Several load conditions, including gravity, effects of moments due to magnet decentering, and forces applied by the central restraining membrane, have been analyzed for the 632 mm diameter, 2 mm thick adaptive secondary mirror, being developed for the MMT Telescope. All these effects can be kept well within the optical specifications.

For force-actuated, thin facesheet mirrors, structural flexibility within the control bandwidth calls for a new approach to adaptive optics. Dynamic influence functions are used to characterize the influence of each actuator on the entire surface of a deformable mirror. A linearized model of atmospheric distortion is combined with these dynamic influence functions to produce a dynamic reconstructor for providing actuator inputs in response to wavefront sensor measurements. This dynamic reconstructor is recognized as an optimal control problem. A hierarchic control scheme which seeks to emulate the quasi-static control approach that is generally used in adaptive optics is compared to the dynamic reconstruction technique. Although dynamic reconstruction requires somewhat more computational power to implement, it achieves better performance with less power usage, and is less sensitive to errors than the hierarchic technique because it incorporates a dynamic model of the deformable mirror.

Adonis, the Adaptive Optics instrument on use on the ESO 3.6m telescope in la Silla, is available for the astronomical community since April 1993. Since August 1996, a Posix-compatible toolset has been installed to perform the basic steps of infra-red data preprocessing in all imaging modes, and since May 1997, the Fabry-Perot observation mode is also handled. The principles and tools of pipeline processing for adaptive optics infrared data are introduced here, together with examples for Fabry-Perot imaging reduction.

The centroid calculation algorithms used in some tracking systems have an inherent gain associated with them. This gain is dependent on the relationship between the spot size and the field of view of each element in the detector array. The new CCD tracker developed at the Starfire Optical Range is very sensitive to the overall gain of the tracker algorithms, including centroid calculation gain. This paper discusses an automatic gain control system developed to estimate the instantaneous gain of the centroid calculation algorithm. This system bases its estimate only on the information contained in the pixel counts of the tracker’s detector array. This centroid calculation gain estimate is then used to maintain a constant overall gain throughout the entire tracking system. This paper also shows that the parameter used to estimate centroid estimator gain can be used to estimate the image Strehl ratio.

The designs of existing Adaptive Optical System are "unadaptive" themselves, it means the parameters of the systems, such as subaperture diameter d, dynamic range of wavefront distortion correction, frequency bandwidth, etc., are fixed. Since these parameters were mainly decided according to some statistic values of atmospheric disturbance and detected objects, those systems don't have the ability to adapt the variation of observation conditions to obtain optimized operating parameters in real time though it is necessary in many cases. In this paper, a concept of an Adaptation of Adaptive Optical System(AAOS) is proposed. The AAOS has certain real time optimization ability against the variation of the brightness of detected objects m, atmospheric coherence length r0 and atmospheric time constant ? by means of changing subaperture number and diameter, dynamic range, and systems' temporal response. The necessity of AAOS using a Hartmann-Shack wavefront sensor and some technical approaches are discussed. Scheme and simulation of an AAOS with variable subaperture ability by use of both hardware and software are presented as an example of the system.

Segments alignment is one of the fundamental factors affecting the shape of the PSF of an optical segmented surface (composed by mirrors or lenses). Tilt and piston of each segment must be very well and uniformly adjusted in relation with the rest of segments. The Shack-Hartmann sensor is very efficient in detecting the local wavefront tilt, and the Curvature sensor is sensitive enough in order to detect local piston errors in the segmented mirrors (1). We show, with numerical simulations, how suches two sensors work in the presence of tilt and piston aberrations. Then, we propose a combine sensor that simultaneously senses the wavefront by Shack-Hartmann and Curvature techniques. An iterative process should improve the measures.

For wavefront disturbance to be corrected, an adaptive optics system acts as a low pass filter, for noise of wavefront sensor--as high pass filter. For most of AO systems there is a time delay in wavefront sensing and processing, and the bandwidths will be limited by the time delay. In this paper, three kinds of transfer functions (close-loop, open-loop and error transfer functions) for an AO system with time delay are analyzed. The four criteria of bandwidths are defined and compared between a simple proportional-integral (PI) control algorithm and a Smith control algorithm. By using the Smith algorithm, the bandwidths are improved. The experimental results of the transfer functions and bandwidths with our newly developed 61 elements system are also presented.

A class of control algorithms for adaptive optics is presented which does not use or require division in the centroid portion of the control processing. It is shown by theory and observation that the denominator-free algorithms perform significantly better than standard control algorithms, particularly in high- or variable-background situations. The use of this technique made possible the natural-guide-star observation of, among other things, the inner coma of comet Hale-Bopp close to the Sun and of the object GCS 2721:2316 at magnitude V=13.7.

One of the most promising trends in the modem astronomy is a creation of ground-based adaptive telescopes which employ a signal of a laser guide star (LGS) [1,2]. According to scientific literature the diffrent schemes of LGS generation are the most popular nowadays.

We present a new technique aimed to obtain the downward tilt of a Laser Guide Star (LGS). The concept of this technique consists in averaging the LGS tilt signals as seen from many non-contiguous sub-pupils located on the telescope aperture. The sub-pupils tilt signals have two different contributions. One, common to all the various subpupils, is due to the upward beam propagation, the other, different on the various sub-pupils, is due to the downward beam propagation. These downward tilts have a correlation degree dependent on the sub-pupils array geometry and on the value of outer scale of turbulence. Summing up the sub-pupil signals allows to average out the tilt signal component due to the beacon downward propagation, so obtaining an estimation of the upward propagation laser beam tilt. We show here the achievable results in terms of the ratio between residual tip-tilt error variance obtained using this technique and the uncorrected tip-tilt variance. In particular the dependence from outer scale of turbulence and the use of multi-telescope systems are investigated.

We present here a new technique aimed to measure the Laser Guide Star (LGS) upward tilt. As well known, this information together with the LGS tilt measured using the main telescope allow to retrieve, neglecting focus anisokinetism, the Natural Guide Star (NGS) overall tilt. The described technique is similar in its practical implementation to the two auxiliary telescopes methods introduced sometimes ago. However it reduces the most important practical limitations of this methods: namely the use of two different telescopes each one freely moving around the main observatory on distances of the order of several 100m. In its most complete version the techniques uses signal from sodium and Rayleigh LGSs coupled together to avoid geometrical effects that reduce the accuracy of the determinated downward tilt. However a simpler form of the technique uses only the Rayleigh return signal. This configuration has disvantages on the tilt measurement accuracy but result in an easier practical implementation. We report the mathematical description of the tip-tilt retrieval process used in this technique together with numerical result quantifying the tip-tilt determination process accuracy. These results show the achievable performances and existing limitations when using this techniques for tip-tilt estimation from LGSs in the case of large astronomical telescopes.

A new approach for the experimental study of tilt angular anisoplanatism is developed. This method uses measurements of the random motion of a moon edge image to assess wavefront tilt. This technique provides a wide, continuous range of angular separations which are not available in observations of binary stars. It is determined that the brightness of the moon is sufficient to make observations with a high resolution imaging system. Statistical properties of the tilt angular correlation and tilt averaging function are experimentally investigated. It is shown that tilt angular correlation scale increases from 40 arcsec to 118 arcsec by increasing telescope diameter, indicating that the concept of isoplanatic angle is not applicable to tilt-related phenomena.

Among the most interesting open problems in the theory of star formation is the question whether regions of high-mass star formation also form large numbers of low-mass stars. To answer this question reliably, it is necessary to resolve the crowded central clusters of some of these regions. NGC 3603 is the most massive optically visible giant HII region in our Galaxy. It is located at a distance of 7.2 kpc in the Carina spiral arm. We have observed its central cluster with the SHARP II camera attached to the ADONIS adaptive optics system at the 3.6m telescope on La Silla, Chile. From the photometry of more than 1200 stars, we have constructed near-infrared color-color and color-magnitude diagrams. We find a well-defined main sequence above 4 solar masses; the stars with lower masses have not yet reached the main sequence. By comparison with pre-main-sequence evolutionary models we estimate the age for these lower mass stars to be less than 1 million years. This estimate is supported by the shape of the luminosity function as well. Interpreting the luminosity function in terms of stellar masses, we can construct the initial mass function in NGC 3603 down to about 1 solar mass: We do not find a lower mass cutoff down to this value, which shows that there is indeed prodigious formation of low-mass stars in NGC 3603. The initial mass function — referring to logarithmic mass intervals — follows a power law of index ? = 0.74... 1.32.

The possibility of improving the image quality of the Italian “Telescopio Nazionale Galileo (TNG)” by means of on-line image selection via a fast shutter is discussed. The different performances obtainable with different shutter speeds, selection rates and sharpness criteria are derived by the analysis of temporal series of simulated short exposure images of atmospheric speckles. The speckles are obtained by drifting large format phase screens of von Kármán statistics over the telescope pupil. The phase screens are generated by FFT transforming a white noise of controlled quality and by adding subharmonics to approximate the low frequency pattern. Particular attention is devoted to the identification of the most suitable merit function(s) for on-line image selection and to the comparison of the achievable image quality improvements with those obtainable through off-line selection and co-addition post-processing techniques.

Authors obtained the analytical relations allowing one to realize a transformation in expansions of optical wave phase from the Haar and Walsh bases, which are characterized by a simplicity of engineering realization in compensating devices of adaptive optics systems (AOS), to the statistically optimal Karhunen-Loeve-Obukhov basis describing the most completely and accurately the criteria of AOS approximation to ideal diffraction-limited system. The relations obtained and use of the fast Walsh and Haar transform algorithms allow one to approach the real time in wavefront reconstruction. Moreover, the analytical relations obtained allows an influence of the outer scale of turbulence to be taken into account. Results of numerical calculations of the transformation matrix are presented.

We describe Corneirs NIR camera system for the Hale 200” telescope adaptive optics system at Palomar Observatory. The instrument is under construction at this time, and we expect first light at the telescope in December 1997. Here we summarize the camera’s design as well as its expected performance.

CCD read noise is the single most important factor which determines the performance of the Shack-Hartmann wavefront sensor in photon-starved applications. We address the problem of making optimum centroid estimates in sensors employing NxN-pixel centroiding geometries, where N is generally > 2 so that read noise is even more important than in quad-cell- based sensors. Maximum-likelihood and Bayesian estimators are derived and we show that these afford excellent noise suppression whilst relaxing the constraints on alignment tolerances and static aberrations which are demanded in quad-cell applications. The estimators considered are all linear and are shown to be implementable using conventional real-time processing hardware.

The detecting error of Shack-Hartmann (S-H) sensor is one of the main error sources in adaptive pptical systems. In this paper the main sources of S-H sensor for both of the systematic error and random noise are analyzed. These factors are readout noise and background level of CCD camera, number of detecting pixels, and photon shot noise. Setting a threshold level in processing centroid of Hartmann spots is benefit for improving detecting accuracy. The theoretical analysis and experimental results are presented in this paper.

A study has been made of a Shack-Hartman sensor utilising a Charge Coupled Device (CCD) array and of an alternative approach to estimating the centroid. This is based on modelling the Point Spread Function (PSF) by a Gram-Charlier type expansion type, we will present results for different levels of Kolmogorov turbulence with and without CCD read noise. Our high-fidelity computer simulation demonstrates that more accurate estimation of the PSF centroid may be achieved at low light level as compared to the standard center of gravity estimation.

This paper presents a discussion of the trade-offs between hilly and partially compensated adaptive optics systems. The key concept explored was the trade-offs to be made between increasing the system sample spacing by enlarging the size of subapertures and the associated decrease in noise levels at the wavefront sensor. A method to counteract the impact of low light levels on adaptive optics system performance without the use of laser beacons was examined. This method involves the notion of “clustering” smaller wavefront sensor subapertures into larger effective wavefront sensor subapertures via software instruction. However, for the modeled 1.6 meter adaptive optics system, no benefit was seen by switching to a clustering scheme.

We present results of research aimed at optimizing adaptive optics closed loop bandwidth settings to maximize imaging system performance. The optimum closed loop bandwidth settings are determined as a function of target object light levels and atmospheric seeing conditions. Our work shows that for bright objects, the optimum closed loop bandwidth is near the Greenwood frequency. However, for dim objects without the use of a laser beacon, the preferred closed loop bandwidth settings are a small fraction of the Greenwood frequency. In addition, under low light levels, selection of the proper closed loop bandwidth is more critical for achieving maximum performance than under high light levels. We also present a strategy for selecting the closed loop bandwidth to provide robust system performance for different target object light levels.

We report narrow band adaptive optics near infrared imaging of the planetary nebula IC418. Our 25."6 x 25."6 images were recorded with the SHARP 11+ camera at the ESO 3.6m telescope, using the adaptive optics system ADONIS. The high spatial resolution was combined with the high spectral resolution provided by two Fabry-Perot etalons and a circular variable filter. Here we present images in the Br? and He I emission lines at 2.1655µm and 2.0581µm, respectively and discuss their astrophysical implications. Our results show that combining adaptive optics techniques with high spectral resolution opens a wide field of astrophysical studies.

We present a study of the operating characteristics of the Xinetics Inc. deformable mirror and the driver electronics built by the Jet Propulsion Laboratory for Palomar Observatory’s adaptive optics project. This mirror, the first of its type built by Xinetics Inc., contains 349 PMN actuators which control a 2 millimeter thick mirror surface coated with protected silver. Measurements are separated into static and dynamic categories. The static tests determine the unpowered figure of the mirror surface, the influence of solitary actuators on the mirror surface and how the actuators move as a function of voltage applied, including considerations of hysteresis and creep. We have been able to flatten the mirror surface to an rms value of 19 nanometers. In the dynamic tests, we have resolved the motion of individual actuators whose voltages were changed at frequencies up to 1.5 kHz. The purpose of this study is to show that this deformable mirror has specific characteristics that must be determined in order to optimize its control.

Optimal removal of tip-tilt or image motion is essential for Adaptive Optics. The tilt loop residual jitter considerably affects the performance of the AO, given the high noise propagation coefficients of this term. Therefore we have dedicated efforts for the development of a state-of-the-art servo-loop with optimized sensor and loop-control strategies. The System for Tip-tilt Removal with Avalanche Photodiodes (STRAP) is being tested and will be implemented in a number of VLT instruments. It is compliant with the ESO VLT standards in hardware and software. We report here the rationale and a description of STRAP in its components and subsystems.

We report on the calibration and field test results obtained with the ESO/Meudon Adaptive Optics System, Adonis. This 7x7, 50cm subaperture system is now routinely in use at the ESO La Silla 3.6m telescope, offered to the astronomers together with two infrared cameras covering the 1-5 µm wavelength range. The calibration activity summarized here has investigated the field performances achieved so far with Natural Guide Stars (NGS) using the either the Reticon or the EBCCD wavefront sensors.

To function properly, the three key elements of an adaptive optics system must be properly registered. At the Starfire Optical Range (SOR) of the Air Force Phillips Lab we have developed a set techniques which align the Lenslet array to the WFS camera, and register the DM to the Lenslet array. The former uses the spot centroids to factor misalignment into X,Y translation, magnification, and rotation. The latter process, using an insight from Jim Spinheme of RTS, takes advantage of the mirror mode called "waffle." It can generate the same misalignment factors for deformable mirror (DM) misregistration. These two techniques are implemented in the wavefront sensor (WFS) user interface, as a convenient, easy to use tool. Both theory and implementation are presented.

The cassegrain adaptive optics system (ADOPT) on the Mount Wilson 100-inch telescope has been in operation now since June of 1995. This report chronicles the ongoing performance improvements, the drive towards unassisted use by visiting observers, and some significant science results of this system. The last includes: observations Mira with its companion and other red giants, many observations of asteroids, and the satellites of Jupiter and Saturn, the planets Uranus and Neptune, and of course, double stars. Objects with magnitudes from V=0.0 to V=13.7 have served as natural guide stars, with no change in optics or filters needed over this range. Resolution of 0.07 arcsec is routine on a bright object in good seeing, with 0.058 arcsec at B band as the best performance to date. Coronagraphic and spectroscopic additions are planned for the system.

The ideal electromagnetic actuator for an adaptive secondary mirror must be compact and able to deliver a large force with a small power dissipation, in spite of the geometric constraints. The magnetic circuit, including auxiliary components and coils, has been entirely modeled, in order to optimize the efficiency of the actuator, regarded as a linear motor. Forces and efficiencies have been computed in various geometric configurations, among which the final layout was chosen.

Recent advances in adaptive optics support the feasibility of orbital debris removal by laser photoablation impulse, with laser and tracking systems located on the Earth. An in-depth systems analysis shows that individual laser pulses are most effective in lowering perigee and reducing lifetime at a zenith angle of about 40 degrees when the target is approaching the laser. Many pulses are needed to remove debris, and it is important to begin the engagement at the largest zenith angle permitted by tracking and adaptive optics. The 1-10 cm orbital debris hazard will require damage control equipment and procedures as well as on-orbit addition of shielding to the International Space Station. The current risk to satellites in low Earth-orbit is estimated to be $10-100 million per year in replacement costs. These factors suggest an international laser orbital debris removal system could be cost-effective. A demonstration in which radar and optical tracking together with adaptive optics are used to concentrate a laser on a calibrated target in orbit is the next required step in proving the feasibility and cost-effectiveness of such a system.