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A single-conjugate adaptive optics (AO) system for the 6.5 m MMT based on an adaptive secondary mirror is now undergoing commissioning at the telescope. In addition to providing the basis for a program of scientific observations, the system will serve as a platform for the development of a dual-conjugate AO system in which a constellation of five refocused Rayleigh laser guide stars (RLGS) will provide the majority of the wavefront information. A SCIDAR campaign at a telescope close to the MMT site is beginning to generate explicit measurements of the atmospheric turbulence profile. In this paper, we describe the current system design, and present the results of initial performance simulations using the measured atmospheric parameters.

The adaptive secondary for the MMT (called MMT336) is the first mirror of its kind. It was designed to allow the application of wavefront corrections (including tip-tilt) directly at the secondary mirror location. Among the advantages of such a choice for adaptive optics operation are higher throughput, lower emissivity, and simpler optical setup. The mirror also has capabilities that are not found in most correctors including internal position feedback, large stroke (to allow chopping) and provision for absolute position calibration. The 336 actuator adaptive secondary for MMT has been used daily for over one year in our adaptive optics testing facility which has built confidence in the mirror operation and allowed us to interface it to the MMT adaptive optics system. Here we present the most recent data acquired in the lab on the mirror performance. By using interferometer measurements we were able to achieve a residual surface error of approximately 40nm rms. Coupling the mirror with a Shack-Hartmann wavefront sensor we obtained a stable closed loop operation with a -3dB closed loop bandwidth of approximately 30Hz limited by the wavefront sensor frame rate. We also present some preliminary results that show a 5Hz, 90% duty cycle, ±5 arcsec chopping of the mirror. Finally the experience gained and the problems encountered during the first light adaptive optics run at the telescope will be briefly summarized. A more extensive report can be found in another paper also presented at this conference.

In this communication, we present the progress of the 6.5m MMT adaptive optics system. During the last part of 2001 and the 1st part of 2002, the system has been validated in the laboratory statically and dynamically with sample frequencies of up to 550 Hz. In June 2002, an attempt has been made to make this system work on the telescope but has been hampered by mechanical failures. However, ease of installation of the system and open-loop operation of the mirror was demonstrated at this occasion and offers reasons to be optimistic on the future of the system. The MMT-AO system is the first AO system 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. Today, the deformable mirror is characterized and accepted. It features a 1.8 mm 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 for position sensing and actuator driving. The system has been characterized in the laboratory at sampling speeds up to 550 Hz and had been integrated on the telescope.

A handful of groups around the world are actively working on the development of the next generation of telescopes of 30 m diameter and more. Present implementations of adaptive optics will be inadequate to realize the full resolving power of these new instruments in imaging and spectroscopy. Instead, multi-conjugate adaptive optics (MCAO) systems are being contemplated. We explore here the application of MCAO using laser guide beacons, to a 30 m telescope. Using a new simulation code, we show that reliance on the expensive lasers needed to generate sodium resonance beacons can be reduced through the use of refocused Rayleigh laser guide stars at much lower cost. We show that in the geometric optics approximation, modes of the phase distortion of order less than or equal to the number of deformable mirrors in the MCAO are correctable with no isoplanatic error. A new figure of merit is derived which predicts the relative ability of a chosen beacon/deformable mirror architecture to sense and correct wavefront aberration, based solely on knowledge of the optical geometry and the statistics of the aberration to be corrected. Numerical simulation can therefore be minimized by avoiding the exploration of unpromising beacon arrangements.

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.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Use of the highly sensitive Hokupa'a curvature wavefront sensor has allowed for the first time direct adaptive optics (AO) guiding on brown dwarfs and VLM stars (SpT=M7-L2). An initial survey of 9 such objects discovered one 0.15" binary (2MASSJ 1426316+155701). The companion is about half as bright as the primary (Delta K = 0.61+/-0.05$, Delta H = 0.70+/-0.05) and has even redder colors H-K=0.59+/-0.14 than the primary. The blended spectrum of the binary has been previously determined to be M9.0. We modeled a blend of an M8.5 template and a L1-L3 template reproducing a M9.0 spectrum in the case of Delta K = 0.61+/-0.05,Delta H = 0.70\pm0.05$. These spectral types also match the observed H-K colors of each star. Based the previously observed low space motion and $H_{\alpha}$ activity we assign an age of $0.8^{+6.7}_{-0.3} Gyr$. Utilizing this age range and the latest DUSTY models of the Lyon group we assign a photometric distance of $18.8^{+1.44}_{-1.02} pc$ and masses of $M_{A}=0.074^{+0.005}_{-0.011} M_\odot$ and $M_{B}=0.066^{+0.006}_{-0.015} M_\odot$. We therefore estimate a system separation of $2.92_{+0.22}^{-0.16}AU$ and a period of $13.3{+3.18}^{-1.51} yr$ respectively. Hence, 2M1426 is among the smallest separation brown dwarf binaries resolved to date.

A small prime focus corrector for the new 6.5 m MMT telescope has been built using small off-the-shelf optical components. Its purpose is to aid in initial test of the primary mirror. At f/1.25 the paraboloidal primary is by far the fastest of any large telescope, and without correction the field and the plate scale are both inconveniently small. The corrector design is based on the Maksutov camera, with a concave spherical relay mirror and a refractive meniscus corrector to subtract the coma introduced by the primary mirror and balance out the spherical aberration from the relay mirror. The result is a 48 arcsec field at f/7 with 80 percent encircled energy within 0.25 arcsec diameter. The instrument includes a CCD with 500 by 500 20 micron pixels, a guide camera and provision for a fast wavefront sensor to measure mirror figure and atmospheric turbulence. The instrument was tested at the telescope before implementation of the active control of mirror support forces or the mirror air conditioning system. Nevertheless images with FWHM 0.53 arcsec were recorded.

The adaptive optics system for the 6.5 m MMT conversion telescope will be 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. We report here the present state of construction, and the results of static and dynamic performance tests of the Cassegrain optical package.

In order to prepare for the adaptive secondary program at the new 6.5 meter MMT, we have begun a campaign to measure atmospheric turbulence with a Shack-Hartmann wavefront sensor (WFS) camera. Our first efforts, prior to second aluminization of the primary, consisted of taking turbulence data with our WFS camera with special coma-correcting optics at the MMT prime focus, without a Cassegrain secondary mirror. Our first measurements consisted of 1000 frames of approximately 10 millisecond duration, taken approximately 3 seconds apart. We measure a 5/3 power law structure function, suggesting Kolmogorov turbulence, with an r0 equals 15 cm, but with possible hints of an outer scale and tracking jitter in the structure function. At the end of our data acquisition, we deliberately put 2 micrometer of astigmatism into the primary mirror with its actuators, and in our analysis, we recover 1.7 +/- 0.3 micrometer of astigmatism. A brief analysis of the low-order modal amplitudes with the 3 second frame delay shows that there are significant self-correlations of the low-order modes even on this long time-scale.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The 336-actuator adaptive secondary unit (MMT336) for the new MMT is being assembled in Italy and will be delivered in June 2000 for the acceptance test at Steward Observatory (Tucson, AZ). The latest results obtained on a reduced-size (36 actuators) prototype called P36 are reported, confirming a settling time less than 1 ms measured in previous tests. The flattening procedure has been successfully tested on the P36 unit, reducing the initial surface error of 1.1 micrometer down to 43 nm rms. Moreover the dynamical tests on the P36 unit show that the system is able to attenuate the atmospheric-induced error from 466 nm to 31 nm rms. This in the case of median seeing condition at MMT (0.75 arcsec) a high wind speed (48 m/s) and a 1 kHz command rate per actuator. Finally, in the same conditions the atmospheric error is effectively attenuated up to a frequency of 100 Hz (OdB attenuation level).

We compare linear zonal predictors of atmospheric turbulence for adaptive optics. Zonal prediction has the possible advantage of being able to interpret and utilize wind-velocity information from the wavefront sensor better than modal prediction. For simulated open-loop atmospheric data for a 2- meter 16-subaperture AO telescope with 5 millisecond prediction and a lookback of 4 slope-vectors, we find that Widrow-Hoff Delta-Rule training of linear nets and Back- Propagation training of non-linear multilayer neural networks is quite slow, getting stuck on plateaus or in local minima. Recursive Least Squares training of linear predictors is two orders of magnitude faster and it also converges to the solution with global minimum error. We have successfully implemented Amari's Adaptive Natural Gradient Learning (ANGL) technique for a linear zonal predictor, which premultiplies the Delta-Rule gradients with a matrix that orthogonalizes the parameter space and speeds up the training by two orders of magnitude, like the Recursive Least Squares predictor. This shows that the simple Widrow-Hoff Delta-Rule's slow convergence is not a fluke. In the case of bright guidestars, the ANGL, RLS, and standard matrix-inversion least-squares (MILS) algorithms all converge to the same global minimum linear total phase error (approximately 0.18 rad2), which is only approximately 5% higher than the spatial phase error (approximately 0.17 rad2), and is approximately 33% lower than the total 'naive' phase error without prediction (approximately 0.27 rad2). ANGL can, in principle, also be extended to make non-linear neural network training feasible for these large networks, with the potential to lower the predictor error below the linear predictor error. We will soon scale our linear work to the approximately 108-subaperture MMT AO system, both with simulations and real wavefront sensor data from prime focus.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present analysis and numerical simulations of a new method to sense atmospheric wavefront distortion in real time with Rayleigh beacons. Multiple range-gated images of a single pulse from the laser are used to determine each phase map, providing an advantage over other methods in that photon noise is substantially reduced for a given brightness of the beacon. A laser at about 350 nm projects collimated pulses of light adjacent to the telescope. Rayleigh-scattered light from each pulse is recorded through the full telescope aperture in a sequence of video frames, each a few microseconds long. Images are captured as the pulse approaches and passes through the height at which the camera is focused. Phase diversity is thus naturally introduced between the frames. An iterative algorithm is used to extract the pupil-plane phases from the recorded intensity distributions. We anticipate that such beacons are likely to be valuable in future advanced systems for adaptive optics on very large telescopes with multiple laser beacons and deformable mirrors that aim to provide a large corrected field of view by tomography of the atmospheric turbulence.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Single sodium beacons will likely be the most convenient for adaptive systems to correct 6 - 10 m class telescopes over a small field of view (the isoplanatic angle), provided reliable, powerful 589 nm lasers become available and affordable. However, when adaptive optics are applied to extended fields of view and correction of telescopes as large as 32 m diameter, it seems likely that laser beacons produced by Rayleigh scattering will be preferred. For these more demanding applications which require atmospheric tomography, Rayleigh beacons come into their own for two reasons. First, the cone effect, which causes the high turbulence to be sampled at a different scale, is no longer problematic when multiple lasers are used and height dependence is solved for explicitly. Second, the tomographic solution can make use of the beacon created by a laser pulse during all of its journey through the upper atmosphere, not just scattering from a thin layer selected by range gating. In this way a laser that costs an order of magnitude less to buy and maintain than a sodium laser of the same power can yield a brighter beacon and more information about the atmospheric turbulence. This is important because both the number and brightness of beacons or stars must increase with the number of layers included in the tomographic solution. For the same reason, tomography with natural stars is unlikely to be valuable for very large telescopes because in general the number and required brightness of each star increase with corrected field angle, while current narrow-field adaptive optics systems relying on natural stars are already very limited in sky coverage. Our method for tomography to take advantage of Rayleigh scattering over a wide range of heights uses short pulses from near diffraction-limited, ultraviolet lasers, projected from a small aperture above the telescope's secondary mirror. Each pulse subtends less than 1 arcsec at any instant as it travels up through many kilometers. An imaging detector at the main telescope focus conjugate to mid-height is used to record fast movies of the rising pulses as they come into and out of focus. Phase diversity analysis of the movies taken together then yields the three-dimensional turbulence of the atmosphere.

We will present a system to perform closed-loop optical tests of the 64 cm diameter, 336 actuator adaptive secondary made at the Steward Observatory Mirror Laboratory. Testing will include Shack-Hartmann wavefront sensing and modal correction of static and dynamic aberrated wavefronts. The test optical system is designed so that experiments can be made with both the focal plane instrument and secondary installed in their normal configuration at the MMT, or with the same 9 m spacing in a laboratory test tower. The convex secondary will be illuminated at normal incidence through two 70 cm diameter lenses mounted just below. The artificial, aberrated star is projected from near the wavefront sensor in the Cassegrain focus assembly. Computer generated holograms correct for spherical aberration in the really optics at the test wavelengths of 0.594 and 1.5 micrometers . Atmospheric turbulence is reproduced by two spinning transmission plates imprinted with Kolmogorov turbulence. The Shimmulator will give us the opportunity to test fully the adaptive optics system before installation at the new MMT, hence saving much precious telescope time.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present the results of a compete set of static and dynamic runs of the FEA model of the MMT adaptive secondary. The thin mirror is the most delicate component of the MMT adaptive secondary unit, as it provides the deformable optical surface able to correct the incoming wavefront. The static performances are evaluated as a function of the various load cases arising form gravitational loads and from the forces deriving from the magnetic interactions between actuators. In addition, computations were performed to assess the dynamic response to the high bandwidth, adaptive correcting force.s In both cases, the performances of the adaptive mirror design are able to accommodate the severe specifications.

High resolution spectroscopy experiments with visible adaptive optics (AO) telescopes at Starfire Optical Range and Mt. Wilson have demonstrated that spectral resolution can be routinely improved by a factor of - 10 over the seeing-limited case with no extra light losses at visible wavelengths. With large CCDs now available, a very wide wavelength range can be covered in a single exposure. In the near future, most large ground-based telescopes will be equipped with powerful A0 systems. Most of these systems are aimed primarily at diffraction-limited operation in the near IR. An exciting new opportunity will thus open up for high resolution IR spectroscopy. Immersion echelle gratings with much coarser grooves being developed by us at LLNL will play a critical role in achieving high spectral resolution with a compact and low cost IR cryogenically cooled spectrograph and simultaneous large wavelength coverage on relatively small IR detectors. We have constructed a new A0 optimized spectrograph at Steward Observatory to provide R = 200,000 in the optical, which is being commissioned at the Starfire Optical Range 3.5m telescope. We have completed the optical design of the LLNL IR Immersion Spectrograph (LISPEC) to take advantage of improved silicon etching technology. Key words: adaptive optics, spectroscopy, high resolution, immersion gratings

A laboratory adaptive optics system has been built for testing the wave front sensor hardware and software for the new Multiple Mirror Telescope adaptive optics system. The system will also allow different wave front reconstruction and prediction schemes to be examined. The laboratory system contains a silicon micromachined adaptive mirror with 37 electro-static actuators as well as a novel approach for generating atmospheric turbulence. The design and initial testing of the system are presented.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have designed a new type of curved deformable mirror with 3000 actuators. It will be used at the 6.5 m MMT telescope in an adaptive optics system to correct the wavefront on a scale of 12 cm, for diffraction limited images in visible light and for high contrast imaging with a coronagraph in the near IR. The mirror's 51 cm diameter glass faceplate, 400 microns thick, will be actuated by tubular piezoceramic actuators on 8 mm centers. The mirror is made concave so it can be used in a simple, compact optical relay with no need for large collimator or camera elements. A prototype deformable mirror with 300 elements that test the support and assembly techniques is being built for lab test and will be tested in a closed loop adaptive optics system at the Steward Observatory.

This paper describes the construction and testing of the Shack-Hartmann wavefront sensor camera for the new MMT adaptive optics system. Construction and use of the sensor is greatly simplified by having the 12 X 12 lenslet array permanently glued to the detector array, obviating the need for any further realignment. The detector is a frame transfer CCD made by EEV with 80 by 80 pixels, each 24 microns square, and 4 output amplifiers operated simultaneously. 3 by 3 pixel binning is used to create in effect an array of quad-cells, each centered on a spot formed by a lenslet. Centration of the lenslet images is measured to have an accuracy of 1 micrometers rms. The maximum frame rate in the binned mode is 625 Hz, when the rms noise is 4.5-5 electrons. In use at the telescope, the guide star entering the wavefront sensor passes through a 2.4 arcsec squares field stop matched to the quall-cell size, and each lenslet samples a 54 cm square segment of the atmospherically aberrated wavefront to form a guide star image at a plate scale of 60 micrometers /arcsec. Charge diffusion between adjacent detector pixels is small: the signal modulation in 0.7 arcsec seeing is reduced by only 10 percent compared to an ideal quad-cell with perfectly sharp boundaries.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An optical instrument for measuring the thickness of a thin gap or transparent film is presented. It will be used to calibrate the adaptive secondary mirror under development for the Steward Observatory 6.5 m Multiple Mirror Telescope. Capacitative sensors in the mirror assembly measure dynamically the thickness of the nominally 50 micrometers air gap between the deformable mirror and a glass reference body. The miniature interferometer has been developed to accurately determine the gap thickness so that the capacitive sensor may be calibrated. Interference fringes are produced by illuminating an air gap, which is between two reflective surfaces, with monochromatic plane waves and observing the reflected light. Intensity variations are measured as the wavelength of illumination is varied over an octave. The film thickness is determined by correlating the observed fringes with those modeled for different gaps. Absolute measurement to an accuracy of a small fraction of a wavelength is possible.

The Multiple Mirror Telescope on Mt. Hopkins will soon be upgraded to a single 6.5 m primary mirror.An adaptive- optical system, featuring a thin-shell, adaptive-secondary mirror with 330 voice-coil actuators is being developed for this new telescope. The thin-shell mirror is supported by a thick, concave aluminum substrate which also holds the actuator control and monitoring electronics and serves as the reference surface. With the actuator electronics dissipating heat into the substrate, the thermal behavior of the aluminum reference plate becomes as important issue. This paper presents results form tow experiments designed to determine the thermal behavior of the reference plate.

The Multiple Mirror Telescope (MMT) is now being replaced by a single 6.5m telescope which will have an integral adaptive optics system optimized for near IR operation. We illustrate key results obtained with low-order adaptive optics at the MMT and report on progress on the major components of the system for the new telescope. We also give a brief update on the status of the telescope itself. First light for the adaptive system is expected in mid 1999.

A description is given for the geometry of the first generation adaptive-optical (AO) system being developed for the 6.5m single-mirror telescope that will replace the Multiple Mirror Telescope on Mt. Hopkins. The AO system consists of an adaptive secondary mirror with 330-actuators and a wave front senor composed of a 13 by 13 square subaperture array. The deformable mirror actuator vibrational modes were determined from a finite element model of the adaptive-secondary mirror. Numerical simulations in based on the finite element results were carried out to determine the expected performance of the system. Two reconstruction algorithms were compared - a least squares reconstructor and a modal equalization technique developed for this AO geometry. Strehl ratios are reported for the two algorithms for various guide star magnitudes and number of corrected wave front modes.

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.

The first images of astronomical objects have been obtained with a telescope exploiting wavefront compensation with adaptive optics where the reference beacon was generated by laser excitation of mesospheric sodium. This was done using the FASTTRAC II low-order adaptive optics system at the multiple mirror telescope (MMT). FASTTRAC II is a prototype for a full-scale adaptive optics system under construction for the 6.5 m telescope that will replace the MMT in late 1997. The 6.5 m system is designed to provide correction to the diffraction limit of resolution in the near infrared (1 - 5 micrometer) with high Strehl ratio and excellent sky coverage. This paper describes the new system and its expected performance in view of the achieved performance of FASTTRAC II.

We present a novel, high performance, and economical design for tip-tilt mirrors. The two- dimensional tilt of each mirror is sensed through capacitive displacement sensors sensitive to approximately 1 nm rms out to 10 KHz. An analogue PID circuit utilizes this positional feedback to lock the mirrors to the commanded tilt via voice coil drivers. These mirrors achieve a 10 - 90% risetime of 3.0 ms and have a critically damped response at 100 - 150 Hz update rates. We have incorporated six of these tip-tilt mirrors to adaptively combine all six beams from the MMT. During first light of the instrument in April 1995, individual beams improved from 1.1 inch to 0.6 inch FWHM in 60 s exposures corrected with 60 Hz update rates.

A new adaptive optics system has been constructed for moderately high resolution in the near infrared at the Multiple Mirror Telescope (MMT). The system, called FASTTRAC II, has been designed to combine the highest throughput with the lowest possible background emission by making the adaptive optical element be an existing and necessary part of the telescope, and by eliminating all warm surfaces between the telescope and the science camera's dewar. At present, only natural guide stars are supported, but by the end of 1995, we will add the capability to use a single sodium resonance beacon derived from a laser beam projected nearly coaxially with the telescope. In this paper, we present a description of FASTTRAC II, and show results from its first test run at the telescope in April 1995.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present an overview of the new adaptive system under development for the conversion of the Multiple Mirror Telescope (MMT) to a 6.5 m continuous primary mirror. The system is optimized for diffraction-limited imaging from 1.6 to 2.2 micrometer wavelength, using an adaptive secondary mirror which directly feeds an infrared science detector at f/15 Cassegrain focus. Nearly full sky coverage will be obtained using a low-power, continuous wave (cw) sodium laser beacon to sense high-order wavefront errors, with image motion sensing using a quadrant detector sensitive to infrared field star photons in the 1.2 - 1.6 micrometer band. Components are currently under development, so that the adaptive instrument can be integrated with the new 6.5 m telescope soon after first light.

The large binocular telescope (LBT) will have two 8.4 m apertures spaced 14.4 m from center to center. Adaptive optics will be used to recover deep, long exposure diffraction-limited images in the infrared. The LBT configuration has a diffraction-limited resolution equivalent to a 22.8 m telescope along the center-to-center baseline. Using simulated LBT images and an iterative blind deconvolution algorithm (IBD--Jefferies and Christou, 1993) a sequence of three exposures, at sufficiently different parallactic angles, allows recovery of imagery nearly equivalent to that of the circumscribing 22.8 m circular aperture. To establish a credibility basis for these simulations we have studied the performance of IBD for image constructions of several examples of atmospherically perturbed and partially corrected stellar and galactic data. IBD is robust against influences of real, non-ideal data obtained from large astronomical telescopes, including partial anisoplanicity and Poisson noise from object, sky, and thermal background. For faint objects, which are sky-background and photon-statistics limited, the use of adaptive optics is presumed in these simulations. IBD removes the dilute aperture point spread function effects in the set of parallactic angle-diverse images linearly combined to produce the circumscribed aperture result. Optimal image combination strategy is considered for multi-aperture imaging array configurations.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Detailed computer simulations have been performed to evaluate the potential for spatio-temporal prediction of wavefront slope data from a Shack-Hartmann sensor to improve the imaging quality of an astronomical adaptive optics system by overcoming the effect of the inherent delay between sensing and correcting the wavefront phase error. Results of the simulations show that an improvement of a factor of at least 2 in the residual mean-square phase error can be achieved by extrapolating into the future on the basis of past wavefront measurements. Experiments will be conducted very shortly with actual data gathered at a 1.6 m telescope using a Shack-Hartmann sensor with 31-cm subapertures to test the simulations. Both the simulations and the experimental arrangement have been chosen to match closely the measured operating conditions of the adaptive optics system under construction for the 6.5 m MMT (Martin & Anderson 1995).

For the past three years, Steward Observatory's adaptive optics program has been directed toward developing and testing techniques required to correct large telescopes to the diffraction limit. Previously, we have reported on a specific design for an 8 m sodium-laser adaptive system capable of achieving a strong diffraction-limited core at imaging wavelengths of 1.6 - 2.2 microns, with very good sky coverage for image motion sensing using infrared field stars. Recent experiments at the 6.9 m Multiple Mirror Telescope (MMT) have provided the first large aperture measurements of off-axis anisoplanatism for natural stars and focus anisoplanatism for an artificial sodium beacon. This paper summarizes predicted performance for adaptive optics planned for the upgraded MMT, after conversion in 1996 to a single-mirror telescope using the 6.5 m primary mirror developed at Steward's Mirror Laboratory. Performance estimates are obtained using the real MMT atmospheric data and lead to component specifications which are realizable in the near term, permitting the system to be developed and integrated for operation soon after the telescope upgrade is completed.

We have obtained the first measurements with a sodium laser beacon of focus anisoplanatism over large aperture, at the Multiple Mirror Telescope. In complementary studies, the atmospheric turbulence at the high altitude and on the large scale responsible for the measured focus anisoplanatism was explored by observations of binary stars of different separations. We confirm the predictions of Kolmogorov theory, and derive an effective height for the turbulence of 5050 m above the telescope. These results confirm that the sodium laser guide star planned for use with the 6.5 m telescope conversion of the MMT in 1996 will allow diffraction limited infrared observations in the H and K bands.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

M. Lloyd-Hart, R. Angel, B. Jacobsen, D. Wittman, D. McCarthySteward Observatory, University of Arizona, Tucson, AZ 85721E. Kibblewhite, B. Carter, and W. Wild,Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637ABSTRACTA sodium guide star has been used to sense and correct atmospheric aberration during two runs at the Multiple Mirror Tele-scope (MMT). For the first run in 1993 May, the artificial star was created by a 0.5 W beam from a continuous-wave dye laser tunedto the D2 resonance line, projected from a telescope centered and coaxial with the main array of six 1 .8 m mirrors. Scattering by themesospheric sodium layer produced an artificial beacon equivalent in brightness to a natural star of visual magnitude 12.5, and ofangular extent 1".2 full width at half maximum (FWHM). During the second run in 1994 February, a 1.7 W dye laser was used togenerate an artificial guide star of visual magnitude 10.4, and 1". 1 FWHM.1 In each case, the beacon was used by the MMT adap-tive optics system to compensate in real time for atmospherically-induced differential image motion between the six mirror ele-ments, at correction rates of up to 76 Hz. In the latter experiment, global wavefront tilt correction using a natural reference star wasadded, giving complete adaptive control. Simultaneously recorded images of a natural star coincident with the laser beacon showsignificantly reduced width and an increase in Strehi ratio of almost a factor of two.1. INTRODUCTIONAdaptive optics promises to revolutionize ground-based optical and infrared astronomy by removing the wavefront aberrationcaused by atmospheric turbulence, thus providing images and spectroscopy of unprecedented resolution. To correct in real time forthe aberration, an adaptive optics system requires a reference light source above the turbulence to be sensed, from which the instan-taneous shape of the distorted wavefront is deduced. By using a deformable optical element to correct the shape of the wavefrontfrom the reference source, one also corrects aberrations in light from nearby sources of interest, within the isoplanatic patch.2 Unre-solved natural stars are ideal as reference sources. However, the atmosphere is far from static, and the figure of the deformable ele-ment must be continuously updated at rates of at least 50 Hz for correction in the near infrared, or 200 Hz at visible wavelengths.Thus, the time available for integration of photons from the reference source is extremely limited. To be observable with adaptiveoptics using a natural field star, an object must be within the isoplanatic patch of a star sufficiently bright to provide adequate signal-to-noise ratio at the wavefront sensor. This requirement is so stringent that for large telescopes, at best only a few percent of the skyis available for imaging at the diffraction limit of resolution by this method, even at the more favorable infrared wavelengths.As a partial solution, an artificial beacon may be created by backscattered light from a ground-based laser beam. A number ofgroups have successfully demonstrated the use of a low-altitude beacon in closed-loop adaptive optics systems, relying on Rayleighscattering of a pulsed laser beam from molecules of atmospheric oxygen and nitrogen.3'4Two problems are associated with the use of laser guide stars. Firstly, because of the finite height of the beacon, backscatteredlaser light does not follow the same path through the turbulence as the light from astronomical objects at infinity. This effect, calledfocus anisoplanatism, introduces an error into the measured wavefront distortion which may be relatively large at the best astronom-ical sites where aberration from high atmospheric layers is often dominant. There is thus a need to place the beacon as high in theatmosphere as possible to minimize this limitation. This motivating force has lead to the development of sodium resonance lasers,which produce a beacon by exciting the D2 line of mesospheric sodium, at 590 nm wavelength.5'6'7'8The second problem arises because light from the artificial beacon undergoes refraction by the atmosphere in both the upwardand downward paths, so the component of wavefront tilt common to the paths is not sensed. Only the differential tilt between thetransmitting and receiving telescopes can be measured. For full adaptive correction therefore, one still requires a natural star tosense global wavefront tilt over the full aperture. Estimates of off-axis anisoplanatism show that guide stars bright enough to sensewavefront tilt errors should be found with around 50% probability for correction in the near infrared.92 EXPERIMENTAL GOALSVirtually all laser beacon experience to date has been with Rayleigh beacons from military sites with strong low level turbu-lence. Our goal in the present experiments, performed at the Multiple Mirror Telescope (MMT) in 1993 May and 1994 February,was to test experimentally the performance of a sodium laser system, with a large telescope and under the conditions prevailing at afirst class astronomical site. To date, the MMT adaptive optics system has been used to demonstrate a number of novel techniques364

Low spatial frequencies of atmospheric turbulence are specially troublesome to astronomers because the phase distortions they cause have large amplitude. We have begun experiments at the Multiple Mirror Telescope (MMT) to remove these errors with tip, tilt, and piston control of pieces of the wave front defined by the telescope's six 1.8 m primary mirrors. We show long exposure images taken at the telescope with resolution as high as 0.08 arcsec under piston control, and 0.26 arcsec under tilt control, using an adaptive instrument designed to restore diffraction-limited imaging in the near infrared. We also present preliminary results from analysis of images of the pre-main sequence star T Tauri taken with tilt control of the six beams only, at three infrared wavelengths. The resolution is between 0.35 and 0.4 arcsec, higher than has previously been achieved with direct imaging. The faint red companion to T Tau is clearly revealed, and is seen to be undergoing an energetic outburst.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The next few years will witness the construction of several new 6.5 - 10 meter class astronomical telescopes. Imaging with these large instruments not only requires us to gain better understanding of the nature of atmospheric phase distortions over large spatial scales but should also motivate the adaptive optics community to rethink the ways in which we sense wavefront structure. At infrared wavelengths, direct interference of spatially coherent infrared starlight affords engineers of adaptive optical systems for filled apertures a ruler against which to measure all wavefront sensor designs. Direct interferometry tells use exactly what we want to known, the relative value of the wavefront phase at various points across the telescope pupil. Direct interferometry, implemented by splitting off a small fraction of the available photons using a suitable pupil mask, pins down phase difference values that can be used as verification for any wavefront sensing scheme. One technique investigated toward the goal of the application of interferometric techniques to filled aperture telescopes is described below. In this Fourier transform-based technique, direct interferometry provides phase information for the control of adaptive optics systems without the necessity for an independent wavefront sensor. Here we describe and analyze the noise properties of one technique for faint stellar sources.

The next generation of 6 to 10 m class telescopes is being planned to include the capability for adaptive wavefront correction. The MMT with its 7-m baseline, provides an ideal testbed for novel techniques of adaptive optics. Using a new instrument based on a six-segment adaptive mirror, a number of wavefront sensing algorithms including an artificial neural network have been implemented to demonstrate the high resolution imaging capability of the telescope. These algorithms rely on a freely available property of starlight, namely, its coherence over large scales, to sense directly atmospheric and instrumental phase errors across large distances. In this paper, we report results obtained so far with resolutions between 0.08 and 0.3 arcsec at 2.2-micron wavelength. We also show data indicating that at the level of 0.1-arcsec imaging in good seeing, the isoplanatic patch at this wavelength is at least 20 arcsec across.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Modeling atmospheric turbulence which plays a critical role in the training of neural network wavefront sensors is discussed in the framework of an adaptive optics program for the multiple mirror telescope. It is concluded that the accuracy of the wavefront correction possible with a neural network directly depends on the similarity of the training images to those seen in the telescope. The image simulations used in the training of neural network wavefront sensors are based on a random mid-point displacement (RMD) algorithm and sine wave summation algorithms. The RMD algorithm is considered to be an extremely fast method of wavefront generation used for very large arrays and image sequences without time evolution. Multiple turbulent layer atmospheric models based on the sine wave summation algorithm create image sequences with temporal structure functions that closely match real structure function data.

Summary form only given. The authors reported the first results from an adaptive system operating online at the Multiple Mirror Telescope (MMT), in which the wavefront is sensed by a neural network, implemented on an array of 20 transputers. Star images were formed at a wavelength of 2.2 mu m by two coherently phased apertures of the MMT, and analyzed by the net. Outputs from the net were used to drive piezoelectric actuators on two segments of an adaptive mirror in the optical beam train, controlling five degrees of freedom of the wavefront. With the servo loop in operation, the corrected image shows significant power at the diffraction limit of 0.1 arcseconds (0.5 mu rad) and a much improved peak intensity. >

The MMT consists of six comounted 1.8 m telescopes from which the light is brought to a combined coherent focus. Atmospheric turbulence spoils the MMT diffraction-limited beam profile, which would otherwise have a central peak of 0.06 arcsec FWHM, at 2 microns wavelength. At this wavelength, the adaptive correction of the tilt and path difference of each telescope beam is sufficient to recover diffraction-limited angular resolution. Computer simulations have shown that these tilts and pistons can be derived by an artificial neural network, given only a simultaneous pair of in-focus and out-of-focus images of a reference star formed at the combined focus of all the array elements. We describe such an adaptive optics system for the MMT, as well as some successful tests of neural network wavefront sensing on images, and initial real-time tests of the adaptive system at the telescope; attention is given to a demonstration of the adaptive stabilization of the mean phase errors between two mirrors which resulted in stable fringes with 0.1 arcsec resolution.

A 1.8 m f/i borosilicate honeycomb primary for the Vatican Advanced Technology Telescope (VATT) is beingpolished at the Steward Observatory Mirror Laboratory. To reduce thermal distortion of large borosiicate honeycombtelescope mirrors to a negligible level during polishing, the glass must be maintained isothermal to within 0.1 °C. It isalso essential to minimize temperature variations in the air while a test is in progress. Testing of the surface is donein air using a laser and interferometer mounted above the mirror. To control refractive index variation in the light pathof the laser, assuming some turbulence, the air is required to be isothermal to 0.2 °C. A system designed to achievethese goals has been installed in the Mirror Laboratory.Thermocouples are employed as temperature sensors in the polishing room, air ducts, and on the mirror itself. Theoutput of each thermocouple is referred to one of two isothermal blocks of aluminum. By repeated reading, measure-ments are made to an accuracy of 0.005 °C at a rate of one thermocouple per second.

We describe results from the first multi-laser wavefront sensing system designed to support tomographic modes of adaptive optics (AO). The system, now operating at the 6.5 m MMT telescope in Arizona, creates five beacons by Rayleigh scattering of laser beams at 532 nm integrated over a range from 20 to 29 km by dynamic refocus of the telescope optics. The return light is analyzed by a Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement the beacon range gate. A separate high-order Shack-Hartmann sensor records simultaneous measurements of wavefronts from a natural star. From open-loop measurements, we find the average beacon wavefront gives a good estimate of ground layer aberration. We present results of full tomographic wavefront analysis, enabled by supplementing the laser data with simultaneous fast image motion measurements from three stars in the field. We describe plans for an early demonstration at the MMT of closed-loop ground layer AO, and later tomographic AO.

We investigate the performance of a general multiconjugate adaptive optics (MCAO) system in which signals from multiple reference beacons are used to drive several deformable mirrors in the optical beam train. Taking an analytic approach that yields a detailed view of the effects of low-order aberration modes defined over the metapupil, we show that in the geometrical optics approximation, N deformable mirrors conjugated to different ranges can be driven to correct these modes through order N with unlimited isoplanatic angle, regardless of the distribution of turbulence along the line of sight. We find, however, that the optimal deformable mirror shapes are functions of target range, so the best compensation for starlight is in general not the correction that minimizes the wave-front aberration in a laser guide beacon. This introduces focal anisoplanatism in the wave-front measurements that can be overcome only through the use of beacons at several ranges. We derive expressions for the number of beacons required to sense the aberration to arbitrary order and establish necessary and sufficient conditions on their geometry for both natural and laser guide stars. Finally, we derive an expression for the residual uncompensated error by mode as a function of field angle, target range, and MCAO system geometry.

We show in benchtop experiments that wave-front phase estimation by phase diversity can be significantly improved by simultaneous amplitude estimation. Processing speed, which will be important for real-time wave-front control applications, can be enhanced by use of small-format detectors with pixels that do not fully sample the diffraction limit. Using an object-independent phase-diversity algorithm, we show that, for both pointlike and extended objects, the fidelity of the phase and amplitude estimates degrades gracefully, rather than catastrophically, as the sampling becomes coarser. We show in simulation that the same algorithm also improves the fidelity of image reconstruction of complex targets.

We describe a new wavefront sensor based on the efficient Shack-Hartmann quad cell system. The key improvement to existing designs is a modified lenslet array. The traditional refractive lenslet array is replaced with a segmented reflective system that allows individual control of each subaperture. This system has produced diffraction limited slope correction of the six 1.83 m mirrors of the MMT using a 9.9 V magnitudeguide star and significant image improvement on guide stars as faint as 11.3 V magnitude, limited by the readout noise of the CCD quad cell detector. Recent experiements with an improved detector diffraction limited imaging on guide stars as faint as 15th magnitude. This optimized wavefront sensor, equally applicable to filled aperture telescopes, promises to extend the amount of sky coverage available for adaptive correction in the near IR.

When equipped with adaptive optics, the coming generation of large 6–10-m telescopes can combine huge light grasp with very sharp images. We describe a specific design concept for recovery of diffraction-limited images in the 1.6- and the 2.2-μm atmospheric windows, yielding 0.05-arcsec resolution for an 8-m telescope. Our goal has been to achieve this performance routinely by not requiring above-average atmospheric conditions or the use of unusually bright nearby stars. Atmospheric blurring is sensed with a sodium laser beacon of a few watts. Image motion is sensed by starlight, with a quadrant detector that is sensitive to the broad infrared band in which photon flux is typically largest and the field star has been sharpened by laser-beacon correction that is shared with the science target. A detailed performance analysis shows that for typical conditions Strehl ratios of >25% are expected at 2.2 μm, with the probability of finding a sufficiently bright field star exceeding 50%.

Low spatial frequencies of atmospheric turbulence are especially troublesome to astronomers because the phase distortions that these frequencies cause have a large amplitude. We have begun experiments at the Multiple Mirror Telescope to remove these errors with tip, tilt, and piston control of pieces of the wave front that are defined by the telescope’s six 1.8-m primary mirrors. We present long-exposure images that were recorded at the telescope with a resolution of as high as 0.08 arcsec under piston control, and 0.32 arcsec under tilt control, by use of an adaptive instrument designed to restore diffraction-limited imaging in the near infrared. Of particular importance for high-resolution imaging is the control of the piston or the mean phase errors between the segments. These errors can be calculated from the Fourier transform of the short-exposure combined-focus image, but the accuracy of the reconstructed wave front depends critically on the signal-to-noise ratio of the data. We present a theoretical analysis of the effects of photon and detector read noise on the derived piston errors and computer simulations of wave-front reconstructor algorithms. We find that a Wiener filter combined with nonlinear weighting of the piston errors minimizes the impact of noise. Finally, we summarize expected improvements to our system and discuss the application of these techniques to forthcoming large telescopes.