Allan Wirth

Applying Hartmann wavefront-sensing technology to precision optical testing of the Hubble Space Telescope correctors

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.

Verification of the James Webb Space Telescope coarse phase sensor using the Keck Telescope

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.

Wavefront control with a spatial light modulator containing dual-frequency liquid crystal

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.

Deformable mirror technologies at AOA Xinetics

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.

Wide Field of View Adaptive Optics

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.

Mirror Alignment Recovery System (MARS) on the Hobby-Eberly Telescope

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.

Survey of Adaptive Optic Techniques

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.

Developing and testing an optical alignment system for SALT’s segmented primary mirror

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.

A deployable, annular, 30m telescope, space-based observatory

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.

Cophasing methods for segmented mirrors

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.