AOA-Xinetics has been developing techniques for shaping grazing incidence optics with surface-normal and surface-parallel electrostrictive Lead magnesium niobate (PMN) actuators bonded to mirror substrates for several years. These actuators are highly reliable; exhibit little to no hysteresis, aging or creep; and can be closely spaced to correct low and mid-spatial frequency errors in a compact package. In this paper we discuss the design and fabrication of a 45cm grazing incidence mirror fitted with 45 PMN actuators and integral strain gauges and temperature sensors that allow sub-nanometer control of the surface figure.
One of the science missions for the next generation of extremely large ground based telescopes (30-42m apertures) is the imaging and spectroscopy of exoplanets. To achieve that goal an Adaptive Optics (AO) subsystem with a very large number of corrected modes is required. To provide contrast ratios in the range of 10-9 or better for a 42m telescope an AO system with 25,000 to 60,000 channels will be needed. This is approximately an order of magnitude beyond the current state of the art. Adaptive Optics Associates Xinetics has developed the Photonex Module Deformable Mirror (DM) technology specifically to address the needs of extreme AO for high contrast applications. A Photonex Module is a monolithic block of electrostrictive ceramic in which a high density of individually addressable actuators are formed by screen printing of electrodes and partial wire saw cutting of the ceramic. The printed electrode structures also allow all electrical connections to be made at the back surface of the module via flex circuits. Actuator spacings of 1mm or less have been achieved using this approach. The individual modules can be edge butted and bonded to achieve high actuator count. The largest DMs fabricated to date have 4096 actuators in a 64X64mm array. In this paper the engineering challenges in extending this technology by a factor of ten or more in actuator count will be discussed. A conceptual design for a DM suitable for XAO will be presented. Approaches for a support structure that will maintain the low spatial frequency surface figure of this large (~0.6m) DM and for the electrical interface to the tens of thousands of actuators will be discussed. Finally, performance estimates will be presented.
High resolution imaging from space requires very large apertures, such as NASA’s current mission the James Webb Space Telescope (JWST) which uses a deployable 6.5m segmented primary. Future missions requiring even larger apertures (>>10m) will present a great challenge relative to the size, weight and power constraints of launch vehicles as well as the cost and schedule required to fabricate the full aperture. Alternatively, a highly obscured annular primary can be considered. For example, a 93.3% obscured 30m aperture having the same total mirror area (91m2) as a 10.7m unobscured telescope, can achieve ~3X higher limiting resolution performance. Substantial cost and schedule savings can be realized with this approach compared to fully filled apertures of equivalent resolution. A conceptual design for a ring-shaped 30m telescope is presented and the engineering challenges of its various subsystems analyzed. The optical design consists of a 20X annular Mersenne form beam compactor feeding a classical 1.5m TMA telescope. Ray trace analysis indicates the design can achieve near diffraction limited images over a 200μrad FOV. The primary mirror consists of 70 identical rectangular 1.34x1.0m segments with a prescription well within the demonstrated capabilities of the replicated nanolaminate on SiC substrate technology developed by AOA Xinetics. A concept is presented for the deployable structure that supports the primary mirror segments. A wavefront control architecture consisting of an optical metrology subsystem for coarse alignment and an image based fine alignment and phasing subsystem is presented. The metrology subsystem is image based, using the background starfields for distortion and pointing calibration and fiducials on the segments for measurement. The fine wavefront control employs a hill climbing algorithm operating on images from the science camera. The final key technology required is the image restoration algorithm that will compensate for the highly obscured aperture. The results of numerical simulations of this algorithm will be presented and the signal-tonoise requirements for its successful application discussed. It is shown that the fabrication of the 30m telescope and all its supporting subsystems are within the scope of currently demonstrated technologies. It is also shown that the observatory can be brought to geosynchronous orbit, in its entirety, with a standard launch vehicle.
AOA Xinetics (AOX) has been at the forefront of Deformable Mirror (DM) technology development for over two
decades. In this paper the current state of that technology is reviewed and the particular strengths and weaknesses of the
various DM architectures are presented. Emphasis is placed on the requirements for DMs applied to the correction of
high-energy and high average power lasers. Mirror designs optimized for the correction of typical thermal lensing effects
in diode pumped solid-state lasers will be detailed and their capabilities summarized. Passive thermal management
techniques that allow long laser run times to be supported will also be discussed.
Beam correction of high power lasers often requires large wavefront deformations with very slow rates of change.
Adaptive Optics Associates, Inc. (AOA) has developed a new type of deformable mirror in which thermal expansion
of tubes control the mirror's figure. This paper is the subject of a patent disclosure. Such a mirror can produce large
stroke at rates compatible with thermal distortions in high power lasers. This paper discusses the design of this
device including range of deformation, ways to measure the temperature of the tubes, and algorithms to control
mirror shape while compensating for the non-symmetrical response of thermal expansion and contraction. The
construction of a prototype device and the associated control electronics and software is also covered. Photographs
of the working device are shown.
The James Webb Space Telescope (JWST) Coarse Phase Sensor utilizes Dispersed Hartmann Sensing (DHS)1 to measure the inter-segment piston errors of the primary mirror. The DHS technique was tested on the Keck Telescope. Two DHS optical components were built to mate with the Keck optical and mechanical interfaces. DHS images were acquired using 20 different primary mirror configurations. The mirror configurations consisted of random segment pistons applied to 18 of the 36 segments. The inter-segment piston errors ranged from phased (approximately 0 μm) to as large as ±25 μm. Two broadband exposures were taken for each primary mirror configuration: one for the DHS component situated at 0°, and one for the DHS component situated at 60°. Finally, a "closed-loop" DHS sensing and control experiment was performed. Sensing algorithms developed by both Adaptive Optics Associates (AOA) and the Jet Propulsion Laboratory (JPL)2 were applied to the collected DHS images. The inter-segment piston errors determined by the AOA and JPL algorithms were compared to the actual piston steps. The data clearly demonstrates that the DHS works quite well as an estimator of segment-to-segment piston errors using stellar sources.
The term Adaptive Optics (AO) describes the active control of an optical device to remove distortions caused by aberrations in an optical beam path. An AO system enables beam forming and image correction in the presence of distortions and atmospheric effects. Major obstacles in imaging through the atmosphere include extended source/target anisoplanatism, distributed strong turbulence, scintillation, and branch points. Many applications have requirements for which the generation of a wavefront sensing source via the projection of a laser is undesirable or unfeasible. A variety of AO compensation techniques exist and have been demonstrated in the field, each with specific merits and disadvantages. A survey of the many types of AO control is presented. Common AO techniques include Classic Adaptive Optics, Multi-Conjugate Adaptive Optics (MCAO), and Extended Source AO (also known as correlation wavefront sensing). More recent applications include Stochastic Parallel Gradient Descent control (SPGD) and a Holographic Phase Conjugate Engine that were developed to advance the state of the art AO control. Innovative variations on the Stochastic Parallel Gradient Descent AO and Extended Source (scene-based) AO algorithms hold significant promise for the future of AO.
Characterization and calibration process for a liquid crystal (LC) spatial light modulator (SLM) containing dual frequency liquid crystal is described. Special care was taken when dealing with LC cell gap non-uniformity and defect pixels. The calibration results were fed into a closed loop control algorithm to demonstrate correction of wavefront distortions. The performance characteristics of the device were reported. Substantial improvements were made in speed (bandwidth), resolution, power consumption and system weight/volume.
A versatile, scalable wavefront control approach based upon proven liquid crystal (LC) spatial light modulator (SLM) technology was extended for potential use in high-energy near-infrared laser applications. The reflective LC SLM module demonstrated has a two-inch diameter active aperture with 812 pixels. Using an ultra-low absorption transparent conductor in the LC SLM, a high laser damage threshold was demonstrated. Novel dual frequency liquid crystal materials and addressing schemes were implemented to achieve fast switching speed (<1ms at 1.31 microns). Combining this LCSLM with a novel wavefront sensing method, a closed loop wavefront controller is being demonstrated. Compared to conventional deformable mirrors, this non-mechanical wavefront control approach offers substantial improvements in speed (bandwidth), resolution, power consumption and system weight/volume.
The design of the Southern African Large Telescope (SALT), which is based closely on the Hobby-Eberly Telescope (HET) at the University of Texas but includes advances incorporating lessons learned from HET, is briefly reviewed. The flowdown of requirements from the optical error budget to the primary mirror control subsystems is presented. The techniques and algorithms used by the Center of Curvature Alignment Sensor (CCAS) to measure segment tilt and piston and estimate the global radius of curvature of the primary are discussed in detail. The steps in the process that allows CCAS to capture and identify segments misaligned by more than 70 arcsec and bring them into alignment with residual errors less than 50milli-arcsec is fully described. Next, the hardware and software designs of CCAS are presented, as well as the results of laboratory performance testing. CCAS has been installed and integrated with the primary mirror control system. Performance results of the integrated system over a range of environmental conditions will be shown. Finally, the overall results of this project are summarized and suggestions for future improvements presented.
Adaptive Optics Associates has designed and built the Southern Astrophysical Research (SOAR) telescope primary mirror calibration wavefront sensor. It will be used to monitor the figure of the active primary mirror during observations. The package also includes an acquisition camera subsystem. The sensor uses many commercial components to control cost while meeting the desired technical specifications. We describe the wavefront sensor system and present results of performance testing obtained in the laboratory.
The Mirror Alignment Recovery System (MARS) is a Shack-Hartmann based sensor at the center of curvature (CoC) of the Hobby-Eberly Telescope (HET) spherical primary mirror used to align the 91 mirror segments. The instrument resides in a CoC tower next to the HET dome, a location which provides a challenging set of problems including wind shake and seeing from two different domes. The system utilizes an internal light source to illuminate the HET and a reference mirror to provide focused spot locations from a spherical surface. A custom lenslet array is sized to the HET pupil image, matching a single hexagonal lenslet to each mirror segment. Centroids of the HET mirror segment spots are compared to the reference spot locations to measure tip/tilt misalignments of each segment. A MARS proof-of-concept (POC) instrument, tested on the telescope in 2001, utilized a commercial wavefront sensor from Adaptive Optics Associates. The final system uses the same concept, but is customized for optimal performance on the HET.
MARS replaces previous burst-antiburst alignment techniques and provides a more intuitive method of aligning the primary mirror for telescope operators. The POC instrument has improved median HET stack sizes by 0.3" EE50, measured at the CoC tower. The current alignment accuracy is 0.14" rms (0.28" rms on the sky), resolution is 0.014", measurement precision is 0.027" rms, and segment capture range is ± 5". With continuing improvements in HET dome ventilation and the addition of software customized for removal of tower motion during measurement, the alignment accuracy is expected to reach approximately 0.04" rms in the final MARS, to be installed in late 2002.
The results of a number of research projects related to the phasing of segmented telescope primaries are presented. The behavior of a segmented mirror controlled using edge position sensors is examined using the results of a numerical simulation. The performance of a novel approach to the optical sensing of piston differences is analyzed. The effect of segmented manufacturing errors on both the phasing control system and telescope performances is discussed. Finally, a concept for an `autonomous segment mirror' is presented and its feasibility assessed.
Atmospheric turbulence over vertical paths or long horizontal paths perturbs phase in the pupil of an optical communications receiver, and also can cause severe intensity scintillation. We describe a mathematical method for predicting a bound for these fluctuations by using the Strehl ratio as a criterion for determining the variations in the intensity fluctuations. Derived is the probability function of the instantaneous Strehl ratio, in addition to methods for computing the lower confidence limit. We show how these functions can vary by the degree of partial wavefront correction via adaptive optics.
Atmospheric turbulence over long horizontal paths perturbs phase in the pupil of an optical communications receiver, and also can cause severe intensity scintillations. We describe a real time wavefront compensation system using PC technology to perform all wavefront control tasks. This system uses a modal correction scheme, and we report the first measurements of residual wavefront taken approximately 1 meter above ground level at 1 km range. The effects of turbulence, scintillations and control bandwidth on the correction are all examined.
Adaptive Optics Associates (AOA) has recently developed a commercial Shack-Hartmann wavefront senor called WaveScope for use in optical testing and adaptive optics. The sensor head and associated wavefront analysis software are a powerful and highly flexible combination. Wavefront data can be manipulated and displayed using Tk/Tcl commands and AOA's own atomic functions. To demonstrate this over the last month we have integrated a deformable mirror manufactured by OKOTechnologies into WaveScope. All the software necessary to control the mirror and perform closed loop adaptive optics was written in the Tk/Tcl scripting language which ships with WaveScope. The result is a low cost integrated adaptive optics system.
Atmospheric turbulence over long horizontal paths perturbs phase and also can cause severe intensity scintillation in the pupil
of an optical communications receiver. This limits the bit error rate over which intensity based modulation schemes can
operate. To quantifi the extent ofthe problem, we built a high speed and high resolution wavefront sensor capable of
measuring both the amplitude and phase over a horizontal turbulent path. We present resulting measurements of the
probability distributions ofboth amplitude and phase as well as Zernike polynomial decomposition ofthe temporal power
spectra of phase fluctuations. These results are compared to existing turbulence models, and are used to determine
requirements for a wavefront correction scheme using adaptive optics.
Adaptive optics systems could be used to maintain the quality of a communication laser beam propagated near
ground over a few kilometers of turbulent atmosphere. Such an adaptive optics system may incorporate a Shack-
Hartmann sensor to measure the wavefront of an arriving beacon laser beam before precompensating that of the
communication laser beam through a deformable mirror. We present experimental measurements of the wavefront
of a laser beam propagated over distances of 0.94 and 2.4 km acquired using a 1200-subaperture Shack-Hartmann
sensor. These data were acquired at 1 kllz frame rate during 2 second time intervals over the span of several days. We
acquired and analyzed 41 sets of data. Our analysis shows that the atmosphere is not stationary over these 2second
intervals, that the statistics of the wavefront may look very different for the same measured value of the atmosphere
structure constant (C) and does not always follow the theoretical predictions based on Kolmogorov turbulence, and
that the subtraction of the first 11 Zernike polynomials (first-order correction) improves the wavefront significantly
unlike the subtraction of next 1 1 polynomials (third-order correction).
This paper presents the electronics, computing hardware, and computing software currently being built to provide real time modal control for a laser guide star adaptive optics system. This approach offers advantages in the control of unobserved modes, the elimination of unwanted modes (e.g. tip and tilt) and automatically handles the case of low resolution lens arrays. In our two step modal implementation, the input vector of gradients is first decomposed into Zernike polynomial modes by performing a least squares. The number of modes is assumed to be less than or equal to the number of actuators. The mode coefficients are then available for collection and analysis or for the application of modal weights. The control loop integrators are at this point in the algorithm. To calculate the DM drive signals, the mode coefficients are converted to the zonal signals via a matrix multiply. At closed loop bandwidths slightly below maximum, it will be possible to do the full two part multiply in real time. Thus the modal weights may be changed quickly without recalculating the full matrix. When the number of gradients measured is less than the number of actuators, the integration in the control loop will be done on the lower resolution grid to avoid growth of unobserved modes. These low resolution data will then be interpolated to yield the DM drive signals.
In this paper we characterize and illustrate Adaptive Optics Control system errors associated with sensor misregistration and present an approach to minimize them by employing nonlinear adaptive control methodologies. Furthermore, through the use of the Extended Kalman Filter methods, we determine if it is feasible that experimental data can be used to drive a complete multi-input multi-output dynamic model of the closed-loop adaptive optics system to estimate parameters directly related to stability margins. Finally, we review some results from an experimental verification of these theories. A closed loop adaptive optics breadboard is used to compare the observed effects of misregistration and other systematic error sources with the predictions of the models described above.
In this paper we characterize and illustrate Adaptive Optics Control system errors associated with sensor misregistration and present an approach to minimize them by employing nonlinear adaptive control methodologies. Furthermore, through the use of the Extended Kalman Filter methods, we determine if it is feasible that experimental data can be used to drive a complete multi-input multi-output dynamic model of the closed-loop adaptive optics system to estimate parameters directly related to stability margins. Finally, we review some aspects of the adaptive reconfiguration capabilities of the Hartmann wavefront sensor to be used for the verification of the Hubble Space Telescope optics. That sensor has the ability to adjust itself to misregistration with the system pupil via the modification of algorithm parameters. It also corrects for misalignment with the image location via active repositioning of the optical head.
As part of the HST repair mission it is necessary to verify the performance of the correction optics before their installation in the telescope. To accomplish this precision testing a Hartmann style wavefront sensor and pupil parameter measurement tool has been designed and built. This instrument, termed the Aberrated Beam Analyzer (ABA), will be used to measure the wavefront of both aberrated HST simulators and the unaberrated output of the correction optics. In addition, the ABA measures the location, size, and obscuration ratio of the exit pupil of the system under test. Parameters such as the chief ray angle, PSF, MTF, encircled energy, and Strehl ratio are calculated from the measured data. Operation of the ABA is fully automated and is controlled via a high level scripting language. All data is permanently archived on optical disks for later analysis. The design and theory of operation of the ABA will be discussed. Particular emphasis will be given to the error budget and the measurement performance of the ABA. Some preliminary data will be presented.
Several wavefront sensing techniques were tested against a common aberrated beam. The error on the beam was predominantly spherical aberration. The techniques included Shack- Hartmann Test, Lateral Shear interferometry, Point Diffraction Interferometry, and a Knife Edge Test. Beam calibration was accomplished using Fizeau Interferometry.
One of the most significant limitations to conventional atmospheric compensation systems is their very restricted field of view (FOV), generally equal to an isoplanatic patch size. A wavefront sensing and compensation concept is proposed that should allow the FOV to be increased in size by factors of ten or more. The kernel of the idea is to use wavefront measurements in several (approximately equals 9) directions separated by 100 - 200 (mu) rad to deduce an estimate of the three dimensional optical path difference (OPD) distribution in the atmosphere. The algorithms are roughly based on those used for medical tomographic imaging. Preliminary analysis indicates that from 9 measurement directions it is possible to estimate the OPD contributions from approximately six altitude layers. Once this 3-D OPD distribution is calculated, it may be used to deconvolve wide FOV short exposure images (i.e., wide FOV speckle holography) or it may be used to derive the drive signals for a suite of deformable mirrors that are conjugate to their respective altitude slices. Initial indications are that the FOV may be increased to 500 (mu) rad for a 3.5 m telescope operating at 0.8 micrometers . Further, since the OPD contribution in each layer is smaller than the full atmosphere, the requirements on the system performance are somewhat relaxed.
The deformable mirror is the single most expensive component in atmospheric correction systems of 100 actuators or less. An innovative design for a membrane mirror is proposed as a cost effective alternative to conventional deformable mirrors. The mirror features a high voltage (250v) bias on the mirror surface, which eliminates many of the disadvantages of past membrane mirror designs. The High Bias Membrane Mirror provides a rugged, reliable, inexpensive unit suitable for low power, low density wavefront correction systems. As such it is ideal for compensated imaging systems for surveillance and astronomy, which may require only partial compensation.
An extensive research program has been conducted to characterize the performance of image-tube components under conditions typical of adaptive optics systems, as well as to develop novel image-tube components and assembly techniques. A comparison is presented of standard image-tube capabilities which identifies the tube components which fall short of requirements. Attention is given to the fiber-optic faceplate, phosphor formulation, gate-electrode pulsers, power supplies, and intensifier packaging that have been developed.