The Habitable Worlds Observatory (HWO) is a future NASA flagship mission which will use a segmented telescope and coronagraphic instruments to discover and characterize exoplanets, including exoEarths – Earth-like planets orbiting other stars. HWO will require extraordinary optical stability, with wavefront drift performance measured in the picometers. This paper explores how active control of the telescope optics, using metrology systems that include laser distance gauges, segment edge sensors, and picometer precision actuators, can provide the needed telescope stability. Together with wavefront sensing and deformable mirrors in the coronagraph, this approach can control the entire coronagraphic beam train, to stabilize the electric field in the coronagraph. The HWO Technology Assessment Group is developing three “Exploratory Analytic Cases,” which are conceptual designs for HWO that differ in some respects, to provide a basis for detailed analysis. This paper addresses EAC1, a deployed-aperture concept that draws on JWST heritage. EAC1 uses 19 1.8-meter hexagonal segments to form its off-axis Primary Mirror (PM), as sketched in Figure 1. EAC2 will use fewer, larger “keystone” segments in a non-deployed off-axis PM configuration, and EAC3 will be a larger, on-axis deployed telescope using smaller keystone shaped segments.
The Habitable Worlds Observatory is expected to carry a coronagraph instrument capable of direct imaging of Earth-like exoplanets in the habitable zone of distant stars. Such an instrument requires stability of its wavefront to a few picometers RMS in phase, and 1% in amplitude over one observational cycle of approximately 12 hours. These tight requirements demand an adaptive optics system with extreme long term internal stability. In this paper, we show how phase shifting interferometry helps enable this long term stability by suppressing sources of 1/f noise while also providing measurement of both the phase and amplitude of the beam. Additionally, we show how a new type of noncommon path interferometer with a photonic phase shifter enables inclusion of this type of modulation into existing coronagraph layouts.
The Habitable Worlds Observatory will have uniquely stringent wavefront stability requirements, in the single-digit picometers for observations lasting days, to preserve coronagraph contrast for imaging earth-like exoplanets. This need will be addressed using high-precision Wavefront Sensing and Control methods, including continuous picometerprecision metrology and control of the Optical Telescope Assembly (OTA). This paper reviews methods for initializing and maintaining the OTA wavefront, evolved from those used for the James Webb Space Telescope, but extended to much higher precision. It concludes by identifying performance targets for WFSC technology development, to help guide NASA technology investments.
The next generation of space telescopes will require large, segmented apertures for observations in the near ultraviolet through mid- and far-infrared regions to enable new science ranging from exoplanet characterization to precision astronomical observations that refine astrophysics models. To meet these challenges, we are developing instrumented (strain gauge) surface parallel actuators (SPAs) that are robust and can meet the stringent requirements of mass and cost per m2. We have developed a surface parallel mirror test piece and a set of flexured actuators that maintain compression in the piezoelectric stack elements at all times. The characterization work of these actuators is directed at understanding the performance of flexure piezoelectric multilayer stack actuator operation when embedded in the mirror. To determine the influence functions for each actuator position, we will report the measured stroke/strain and charge/capacitance versus voltage curves for all 42 preloaded actuators. Although designed to operate under close loop control via feedback from the strain gauge initial testing on bare lead zirconate titanate (PZT) stack actuators suggests that by driving the stack to a known domain state we could perform open loop control in the actuators to levels of ± 0.3 μm. We will also report on creep for the actuators and cross actuation for each unique actuator position as well as discuss approaches to mitigating the effect on open loop control error. Thermal studies of flextensional actuators embedded in analog rib structures down to 100K will also be presented.
Coronagraphic space telescopes for imaging Earth-like exoplanets, such as the projected Habitable Worlds Observatory, will require extraordinary optical stability, with wavefront drift performance measured in the picometers. This paper considers how active means, using sensing and control subsystems, can control the entire coronagraphic beam train, from the telescope’s segmented primary mirror, through the coronagraph’s deformable mirrors, to stabilize the electric field in the coronagraph. Integrated telescope and coronagraph models are used to show how this can work to preserve contrast at the 10-10 level and provide important observational efficiencies. In future work, the models will also be used to identify needed performance levels for the various control system components, to help inform NASA’s technology funding priorities.
The next generation of space telescopes will require large segmented apertures for observations in the near ultraviolet through mid and far-infrared regions to enable new science ranging from exoplanet characterization to precision astronomical observations that refine astrophysics models. Recent concept studies, such as LUVOIR, HabEx, and Origins, and the future IR/O/UV Large Strategic Mission telescope for exoplanet characterization and general astronomy discussed in the 2021 Decadal Survey "Pathways to Discovery in Astronomy and Astrophysics for the 2020s” include segmented telescopes that are capable of observations in UV through IR bands and thus drive the need for optical surface performance at cryogenic temperatures. These spaceborne mirror applications require precision control, and these segments will require actuators for controlled surface displacements capable of operation at cryogenic temperatures (<150 K). This paper presents a testbed mirror design to test out new actuators and control strategies. This work is directed at understanding the performance of piezoelectric multilayer stack actuator operation down to 100 K, which will provide actuator designers the critical information needed to model and predict performance. The data reported down to 100 K include: displacement/strain and capacitance as a function of applied voltage, stiffness, hysteresis, blocking force, DC resistance measurements, thermal strains, and the coefficients of thermal expansion as a function of the electrical boundary conditions. The actuators include a strain gauge to allow for closed loop control. This approach allows for a comparison of potential open-loop control drive strategies and associated errors reported in previous work. Surface actuation measurements using flexure-based actuators on aluminum mirror segments at room temperature will also be presented. In addition, we will present techniques to optimize displacement per voltage using amplified piezoelectric flexures and the potential to develop mirrors that can be operated at cryogenic temperatures by carefully choosing flexure materials and geometry.
The Nancy Grace Roman Space Telescope, planned to launch in the mid-2020s, will be the first space-based observatory to demonstrate active wavefront correction at high contrast with its Coronagraph Instrument. As a technology demonstrator, the instrument’s main purpose is to mature the various technologies needed by future flagship mission concepts that aim to image and characterize Earth-like exoplanets. These technologies include two high-actuator-count deformable mirrors (DMs), photon-counting detectors, two complementary wavefront sensing and control loops, and two different coronagraph types. Here we describe the complete set of flight mask designs for the Roman Coronagraph. Multiple mask configurations are required to overcome the challenging pupil obscurations and enable the desired types of imaging, spectroscopy, and polarimetry. In designing each mask configuration, we considered many performance metrics, including spectral bandwidth, field of view, contrast, core throughput, encircled energy, deformable mirror surface height, and low-order aberration sensitivity
NASA’s Nancy Grace Roman Space Telescope (formerly known as WFIRST) is a flagship astrophysics mission planned for launch in 2025. The coronagraph instrument (CGI) on Roman will demonstrate the technology for direct imaging and spectroscopy of exoplanets around nearby stars. It will work with the 2.4-meter diameter telescope to achieve starlight suppression that is 2-3 orders of magnitude deeper than previous space-based and ground-based coronagraphs by using active wavefront control in space with deformable mirrors. CGI has passed its Preliminary Design Review (PDR) in September 2019 and is working toward the instrument Critical Design Review (CDR) in the spring of 2021. We describe the CGI engineering design going into CDR and the operational concept planned for CGI observations.
Occulter mask fabrication for Hybrid Lyot Coronagraph (HLC) at JPL is a relatively mature technology as past successful testbed demonstrations can attest. Nevertheless, as NASA’s WFIRST mission moved into Phase B, new mask design space and fabrication process were explored for new requirements and for better performances for the CoronaGraph Instrument (CGI). To minimize the risks associated with the new explorations, CGI modeling team is tasked with assessing the viability of new designs. In this paper, we describe our HLC modeling effort and results, which identified the potential risks with early exploratory designs and modified fabrication processes. As a result, the traditional (proven) style design is kept for risk aversion. Along the way a standard procedure has been developed for systematic mask evaluation, mask baselining, and general flight performance prediction. In the second part, we describe our model validation effort for the chosen baseline mask’s testbed performance. The focus of the testbed demonstration is to address a major concern related to the CGI’s limited time for wavefront control (WFC) in flight. It includes two stages of WFC: ground seed generation WFC, and (simulated) in-orbit commissioning phase WFC. Good agreements have been achieved in both stages of WFC which affirms that the CGI is capable of digging a dark hole that meets raw contrast requirement within the required time allocation. It also represents a significant improvement in our HLC WFC modeling for an as-built real system.
Typically, the cost of a space-borne imaging system is driven by the size and mass of the primary aperture. The solution that we propose uses a method to construct an imaging system in space in which the nonlinear optical properties of a cloud of micron-sized particles, shaped into a specific surface by electromagnetic means, and allows one to form a very large and lightweight aperture of an optical system, hence reducing overall mass and cost. Recent work at JPL has investigated the feasibility of a granular imaging system, concluding that such a system could be built and controlled in orbit. We conducted experiments and simulation of the optical response of a granular lens. In all cases, the optical response, measured by the Modulation Transfer Function, of hexagonal reflectors was closely comparable to that of a conventional spherical mirror. We conducted some further analyses by evaluating the sensitivity to fill factor and grain shape, and found a marked sensitivity to fill factor but no sensitivity to grain shape. We have also found that at fill factors as low as 30%, the reflection from a granular lens is still excellent. Furthermore, we replaced the monolithic primary mirror in an existing integrated model of an optical system (WFIRST Coronagraph) with a granular lens, and found that the granular lens that can be useful for exoplanet detection provides excellent contrast levels. We will present our testbed and simulation results in this paper.
In this paper, we present some ideas regarding the optics and imaging aspects of granular spacecraft. Granular spacecraft are complex systems composed of a spatially disordered distribution of a large number of elements, for instance a cloud of grains in orbit. An example of this application is a spaceborne observatory for exoplanet imaging, where the primary collecting aperture is a cloud of small particles instead of a monolithic aperture.
Large aperture telescope commonly features segment mirrors and a coarse phasing step is needed
to bring these individual segments into the fine phasing capture range. Dispersed Fringe Sensing
(DFS) is a powerful coarse phasing technique and its alteration is currently being used for JWST.
An Advanced Dispersed Fringe Sensing (ADFS) algorithm is recently developed to improve the
performance and robustness of previous DFS algorithms with better accuracy and unique
solution. The first part of the paper introduces the basic ideas and the essential features of the
ADFS algorithm and presents the some algorithm sensitivity study results. The second part of the
paper describes the full details of algorithm validation process through the advanced wavefront
sensing and correction testbed (AWCT): first, the optimization of the DFS hardware of AWCT
to ensure the data accuracy and reliability is illustrated. Then, a few carefully designed algorithm
validation experiments are implemented, and the corresponding data analysis results are shown.
Finally the fiducial calibration using Range-Gate-Metrology technique is carried out and a
<10nm or <1% algorithm accuracy is demonstrated.
A multiphysics, high resolution simulation of an actively controlled, composite reflector panel is developed to extrapolate from ground test results to flight performance. The subject test article has previously demonstrated sub-micron corrected shape in a controlled laboratory thermal load. This paper develops a model of the on-orbit performance of the panel under realistic thermal loads, with an active heater control system, and performs an uncertainty quantification of the predicted response. The primary contribution of this paper is the first reported application of the Sandia developed Sierra mechanics simulation tools to a spacecraft multiphysics simulation of a closed-loop system, including uncertainty quantification. The simulation was developed so as to have sufficient resolution to capture the residual panel shape error that remains after the thermal and mechanical control loops are closed. An uncertainty quantification analysis was performed to assess the predicted tolerance in the closed-loop wavefront error. Key tools used for the uncertainty quantification are also described.
The Advanced Wavefront Sensing and Control Testbed (AWCT) is built as a versatile facility for developing and
demonstrating, in hardware, the future technologies of wavefront sensing and control algorithms for active optical
systems. The testbed includes a source projector for a broadband point-source and a suite of extended scene targets, a
dispersed fringe sensor, a Shack-Hartmann camera, and an imaging camera capable of phase retrieval wavefront
sensing. The testbed also provides two easily accessible conjugated pupil planes which can accommodate active optical
devices such as fast steering mirror, deformable mirror, and segmented mirrors. In this paper, we describe the testbed
optical design, testbed configurations and capabilities, as well as the initial results from the testbed hardware
integrations and tests.
Phase retrieval is an image-based wavefront sensing process, used to recover phase information from defocused
stellar images. Phase retrieval has proven to be useful for diagnosis of optical aberrations in space telescopes,
calibration of adaptive optics systems, and is intended for use in aligning and phasing the James Webb Space
Telescope. This paper describes a robust and accurate phase retrieval algorithm for wavefront sensing, which has
been successfully demonstrated on a variety of testbeds and telescopes. Key features, such as image preprocessing,
diversity adaptation, and prior phase nulling, are described and compared to other methods. Results demonstrate
high accuracy and high dynamic range wavefront sensing.
Wavefront sensing and control is required throughout the mission lifecycle of large space telescopes such as James
Webb Space Telescope (JWST). When an optic of such a telescope is controlled with both surface-deforming and rigidbody
actuators, the sensitivity-matrix obtained from the exit pupil wavefront vector divided by the corresponding
actuator command value can sometimes become singular due to difference in actuator types and in actuator command
values. In this paper, we propose a simple approach for preventing a sensitivity-matrix from singularity. We also
introduce a new "minimum-wavefront and optimal control compensator". It uses an optimal control gain matrix
obtained by feeding back the actuator commands along with the measured or estimated wavefront phase information to
the estimator, thus eliminating the actuator modes that are not observable in the wavefront sensing process.
An effective multi-field wavefront control (WFC) approach is demonstrated for an actuated, segmented space
telescope using wavefront measurements at the exit pupil, and the optical and computational implications of this
approach are discussed. The integration of a Kalman Filter as an optical state estimator into the wavefront control
process to further improve the robustness of the optical alignment of the telescope will also be discussed. Through a
comparison of WFC performances between on-orbit and ground-test optical system configurations, the connection (and a
possible disconnection) between WFC and optical system alignment under these circumstances are analyzed. Our
MACOS-based [2] computer simulation results will be presented and discussed.
The baseline wavefront sensing and control for James Webb Space Telescope (JWST) includes the Dispersed Hartmann
Sensors (DHS) for segment mirror coarse phasing. The two DHS devices, residing on the pupil wheel of the JWST's
Near Infrared Camera (NIRCam) Short Wavelength Channel (SWC), can sense the JWST segment mirror pistons by
measuring the heights of 20 inter-segment edges from the dispersed fringes. JWST also incorporates two identical
grisms in the NIRCam's Long Wavelength Channel (LWC). The two grisms, designed as the Dispersed Fringe Sensor
(DFS), are used as the backup sensor for JWST segment mirror coarse phasing. The versatility of DFS enables a very
flexible JWST segment coarse phasing process and the DFS is designed to have larger piston capture range than that of
DHS, making the coarse phasing more robust. The DFS can also be a useful tool during JWST ground integration and
test (I&T). In this paper we describe the DFS design details and use the JWST optical model to demonstrate the DFS
coarse phasing process during flight and ground I&T.
KEYWORDS: Coronagraphy, Light sources, Photomasks, Algorithm development, Wavefronts, Modeling and simulation, Near field optics, Phase measurement, Data modeling, Optical components
The performance of the high-contrast imaging testbed (HCIT) at JPL is investigated through optical modeling and
simulations. The analytical tool is an optical simulation algorithm developed by combining the HCIT's optical model
with a speckle-nulling algorithm that operates directly on coronagraphic images, an algorithm identical to the one
currently being used on the HCIT to actively suppress scattered light via a deformable mirror. It is capable of
performing full three-dimensional end-to-end near-field diffraction analysis on the HCIT's optical system. By
conducting extensive speckle-nulling optimization, we clarify the HCIT's capability and limitations in terms of its
contrast performance under various realistic conditions. Considered cases include non-ideal occulting masks, such as a
mask with parasitic phase-delay errors (i.e., a not band-limited occulting mask) and one with damped ripples in its
transmittance profiles, as well as the phase errors of all optics. Most of the information gathered on the HCIT's optical
components through measurement and characterization over the last several years at JPL has been used in this analysis to
make the predictions as accurate as possible. Our simulations predict that the contrast values obtainable on the HCIT
with narrow-band (monochromatic) illumination at 785nm wavelength are Cm=1.58x10-11 (mean) and C4=5.11x10-11(at
4λ/D), in contrast to the measured results of Cm~6×10-10 and C4~8×10-10, respectively. In this paper we report our
findings about the monochromatic light performance of the HCIT. We will describe the results of our investigation
about the HCIT's broad-band performance in an upcoming paper.
The one-meter Testbed Telescope (TBT) has been developed at Ball Aerospace to facilitate the
design and implementation of the wavefront sensing and control (WFS&C) capabilities of the
James Webb Space Telescope (JWST). The TBT is used to develop and verify the WFS&C
algorithms, check the communication interfaces, validate the WFS&C optical components and
actuators, and provide risk reduction opportunities for test approaches for later full-scale
cryogenic vacuum testing of the observatory. In addition, the TBT provides a vital opportunity
to demonstrate the entire WFS&C commissioning process. This paper describes recent WFS&C
commissioning experiments that have been performed on the TBT.
Dispersed Fringe Sensing (DFS) is an efficient and robust method for coarse phasing of a segmented primary mirror
such as the James Webb Space Telescope (JWST). In this paper, modeling and simulations are used to study the effect
of segmented mirror aberrations on the DFS fringe image, its signals, and the piston detection accuracy. The simulations
show that due to the pixilation spatial filter effect from DFS signal extraction the effect of wavefront error is reduced. In
addition, the DFS algorithm is more robust against wavefront aberration when the multi-trace DFS approach is used.
We have also studied the JWST Dispersed Hartmann Sensor (DHS) performance in presence of wavefront aberrations
caused by the gravity sag and we have used the scaled gravity sag to explore the JWST DHS performance relationship
with the level of the wavefront aberration. As a special case of aberration we have also included the effect from line-of-sight
jitter in the JWST modeling study.
Optical State Estimation provides a framework for both separating errors in test optics from the target system and deducing the state of multiple optics in a telescope beam train using wavefront as well as pre-test component measurements including the knowledge of their level of error. Using this framework, we investigate the feasibility of simplifying the interferometric alignment configuration of NASA's James Webb Space Telescope, a large segmented-aperture cryogenic telescope, using a single, static auto-collimating flat instead of six such flats, resulting in a reduced sub-aperture sampling.
Relative to ground-based telescopes, the James Webb Space Telescope (JWST) will have a substantial sensitivity advantage in the 2.2-5μm wavelength range where brown dwarfs and hot Jupiters are thought to have significant brightness enhancements. To facilitate high contrast imaging within this band, the Near-Infrared Camera (NIRCAM) will employ a Lyot coronagraph with an array of band-limited image-plane occulting spots. In this paper, we provide the science motivation for high contrast imaging with NIRCAM, comparing its expected performance to that of the Keck, Gemini and 30 m (TMT) telescopes equipped with Adaptive Optics systems of different capabilities. We then describe our design for the NIRCAM coronagraph that enables imaging over the entire sensitivity range of the instrument while providing significant operational flexibility. We describe the various design tradeoffs that were made in consideration of alignment and aberration sensitivities and present contrast performance in the presence of JWST's expected optical aberrations. Finally we show an example of a two-color image subtraction that can provide 10-5 companion sensitivity at sub-arcsecond separations.
KEYWORDS: James Webb Space Telescope, Wavefronts, Error analysis, Phase modulation, Optical alignment, Telescopes, Space telescopes, Filtering (signal processing), Monte Carlo methods, Control systems
An effective multi-field wavefront control (WFC) approach is demonstrated for the James Webb Space Telescope (JWST) on-orbit optical telescope element (OTE) fine-phasing using wavefront measurements at the NIRCam pupil, and the optical and computational implications of this approach are discussed. The integration of a Kalman Filter as an optical state estimator into the JWST wavefront control process to further improve the robustness of the fine-phasing JWST OTE alignment will also be discussed. Through a comparison of WFC performances between the JWST on-orbit and ground-test optical system configurations, the connection (and a possible disconnection) between WFC and optical system alignment under these circumstances are analyzed. Our MACOS-based [2] computer simulation results will be presented and discussed.
The James Webb Space Telescope (JWST) will use image-based wavefront sensing to align the telescope optics and achieve diffraction-limited performance at 2 µm. The Phase Retrieval Camera (PRC) is a high-accuracy, image-based wavefront sensor that was built for the optical characterization of JWST technology-demonstrator mirrors. Recently, experiments with the PRC were performed at the NASA Marshall Space Flight Center to measure the cryogenic surface figure of the beryllium Advanced Mirror System Demonstrator (AMSD). This paper describes the results of these experiments. Using the Modified Gerchberg-Saxton phase retrieval algorithm (JWST’s baseline method for fine-phasing), the PRC measured wavefront aberrations that were as large as 10 waves peak-to-valley (wavefront) in the optical system. A comparison between the PRC results and measurements acquired with an Instantaneous Phase Interferometer will also be presented.
The Wavefront Control Testbed (WCT) was created to develop and test wavefront sensing and control algorithms and software for the segmented James Webb Space Telescope (JWST). Last year, we changed the system configuration from three sparse aperture segments to a filled aperture with three pie shaped segments. With this upgrade we have performed experiments on fine phasing with line-of-sight and segment-to-segment jitter, dispersed fringe visibility and grism angle; high dynamic range tilt sensing; coarse phasing with large aberrations, and sampled sub-aperture testing. This paper reviews the results of these experiments.
Terrestrial Planet Finder Coronagraph, one of two potential architectures, is described. The telescope is designed to make a visible wavelength survey of the habitable zones of at least thirty stars in search of earth-like planets. The preliminary system requirements, optical parameters, mechanical and thermal design, operations scenario and predicted performance is presented. The 6-meter aperture telescope has a monolithic primary mirror, which along with the secondary tower, are being designed to meet the stringent optical tolerances of the planet-finding mission. Performance predictions include dynamic and thermal finite element analysis of the telescope optics and structure, which are used to make predictions of the optical performance of the system
KEYWORDS: Near field optics, Coronagraphy, Systems modeling, Planets, Mirrors, Wavefronts, 3D modeling, Thermal modeling, Diffraction, Performance modeling
The Terrestrial Planet Finder Coronagraph will rely heavily on modeling and analysis throughout its mission lifecycle. Optical modeling is especially important, since the tolerances on the optics as well as scattered light suppression are critical for the mission's success. The high contrast imaging necessary to observe a planet orbiting a distant star requires new and innovative technologies to be developed and tested, and detailed optical modeling provides predictions for evaluating design decisions. It also provides a means to develop and test algorithms designed to actively suppress scattered light via deformable mirrors and other techniques. The optical models are used in conjunction with structural and thermal models to create fully integrated optical/structural/thermal models that are used to evaluate dynamic effects of disturbances on the overall performance of the coronagraph. The optical models we have developed have been verified on the High Contrast Imaging Testbed. Results of the optical modeling verification and the methods used to perform full three-dimensional near-field diffraction analysis are presented.
Because of the complexity of the Terrestrial Planet Finder (TPF) design concepts, the project will rely heavily on the use of engineering and science simulations to predict on-orbit performance. Furthermore, current understanding of these missions indicates that the 3m to 8m class optical systems need to be as stable as picometers in wavefront and sub-milli arcsec in pointing. These extremely small requirements impose on the models a level of predictive accuracy heretofore never achieved, especially in the area of microgravity effects, material property accuracy, thermal solution convergence, and all other second order modeling effects typically ignored. New modeling tools and analysis paradigms are developed which emphasize computational accuracy and fully integrated analytical simulations. The process is demonstrated on sample problems using the TPF Coronagraph design concept. The TPF project is also planning a suite of testbeds through which various aspects of the models and simulations will be verified.
KEYWORDS: Error analysis, Point spread functions, Wavefronts, Magnesium, Actuators, Wavefront sensors, Data modeling, Spatial frequencies, Coronagraphy, Near field optics
The Terrestrial Planet Finder (TPF) high contrast imaging testbed (HCIT) facilitates the investigation into the diversity of engineering challenges presented by the goal of direct exo-planet detection. For instance, HCIT offers a high-density deformable mirror to control the optical wavefront errors, a configurable coronagraph to control the diffracted light, and translatable cameras for measuring the focal and pupil planes before and after the coronagraph. One of the principle challenges for a coronagraphic space telescope is the extreme level of wavefront control required to make the very faint planet signal reasonably detectable. A key component, the extremely accurate sensing of the wavefront aberrations, was recently shown to be achievable using a sufficiently constrained image-based approach. In this paper, we summarize the experimental performance a focus-diverse phase-retrieval method that uses symmetrically defocus point-spread function measurements that are obtained about the coronagraph occulter focal plane. Using the HCIT, we demonstrate the high level of wavefront sensing repeatability achieved with our particular choices of focus diversity, data fidelity and processing methodologies. We compare these results to traceable simulations to suggest a partitioning of the error sources that may be limiting the experimental results.
A piston sensing and control algorithm for segmented mirror coarse phasing using a dispersed fringe sensor (DFS) has been developed for the Next Generation Space Telescope (NGST) wavefront sensing and control. The DFS can detect residual piston errors as large as the order of a depth-of-focus and can phase the segment mirrors with accuracy better than 0.1 microns, which is well within the capture range of fine phasing for NGST. A series of experiments have been carried out on the NGST's Wavefront Control Testbed (WCT) to validate the modeling results, evaluate the DFS performance, and systematically explore the factors that affect the DFS performance. This paper reports the testbed results for several critical issues of DFS performance, including DFS dynamic range, accuracy, fringe visibility, and the effects of segment mirror aberrations.
KEYWORDS: Computing systems, Control systems, Space telescopes, Human-machine interfaces, Data archive systems, Safety, MATLAB, Deformable mirrors, Wavefronts, Software development
The Wavefront Control Testbed (WCT) is used to demonstrate the wavefront sensing and control algorithms and procedures that will be used on the Next Generation Space Telescope (NGST). The Segmented Telescope Control Software, written in MATLAB®, is the primary development and operational tool used. The software has an extensive graphical user interface that allows the user to interact with the hardware and algorithms.
A variety of additional software programs support the Segmented Telescope Control Software (STCS). Various hardware control software interacts with MATLAB via TCP/IP connections. When access to the hardware is unnecessary or undesirable, we can access the model server that simulates the system. A stand-alone safety monitoring LabVIEW program alerts technicians if a hardware failure occurs. A C program gives the operator a graphical way of monitoring the network connections to the various systems. An Interactive Data Language (IDL) data archiving routine creates a database to monitor and maintain the testbed data and executes the MATLAB to Flexible Image Transport System (FITS) translator. Additionally we have implemented a web-based bug tracking and plan to add experiment scheduling and a document archive.
Due to the nature of the testbed, these software programs are constantly evolving, causing a variety of challenges over the years. This poster will describe these software elements and the issues that have arisen trying to use them together.
The Segmented Telescope Control Software (STCS) uses science camera information to align and phase a deployable segmented optical telescope. It was developed the for the Next Generation Space Telescope (NGST) and has been successfully utilized on the Wavefront Control Testbed (WCT) for NGST and a portable phase retrieval camera (PPRC) system. The software provides an operating environment that will be used for the prime contractor's testbeds for NGST, and will eventually evolve into the Wavefront Sensing and Control (WFS&C) ground support software for NGST. This paper describes the engineering version of the STCS, the algorithms it incorporates, and methods of communicating with the testbed hardware.
The NGST Phase Retrieval Camera (PRC) is a portable wavefront sensor useful for optical testing in high-vibration environments. The PRC uses focus-diverse phase retrieval to measure the wavefront propagating from the optical component or system under test. Phase retrieval from focal plane images is less sensitive to jitter than standard pupil plane interferometric measurements; the PRC's performance is further enhanced by using a high-speed shutter to freeze out seeing and jitter along with a reference camera to maintain the correct boresight in defocused images. The PRC hardware was developed using components similar to those in NGST's Wavefront Control Testbed (WCT), while the PRC software was derived from WCT's extensive software infrastructure. Primary applications of the PRC are testing and experimenting with NGST technology demonstrator mirrors, along with exploring other wavefront sensing and control problems not easily studied using WCT. An overview of the hardware and testing results will be presented.
A method of coarse phasing segmented mirrors using white light interferometry (WLI) has been developed for the Next Generation Space Telescope (NGST) wavefront sensing and control. Using the broadband point PSF of the segmented mirrors taken during a segment piston scan, the WLI can accurately detect small residual piston errors. WLI does not rely on extra optics and uses only the final imaging camera. With its high sensitivity to small segment piston error WLI can be used as a complementary phasing algorithm to the dispersed fringe sensor (DFS) for NGST. This paper will present the results from modeling and experiment on the NGST's Wavefront Control Testbed (WCT).
The NGST Wavefront Control Testbed (WCT) is a joint technology program managed by the Goddard Space Flight Center (GSFC) and the Jet Propulsion Laboratory (JPL) for the purpose of developing technologies relevant to the NGST optical system. The WCT provides a flexible testing environment that supports the development of wavefront sensing and control algorithms that may be used to align and control a segmented optical system. WCT is a modular system consisting of a Source Module (SM), Telescope Simulator Module (TSM) and an Aft-Optics (AO) bench. The SM incorporates multiple sources, neutral density filters and bandpass filters to provide a customized point source for the TSM. The telescope simulator module contains a flip-in mirror that selects between a small deformable mirror and three actively controlled spherical mirror segments. The TSM is capable of delivering a wide range of aberrated, unaberrated, continuous and segmented wavefronts to the AO optical bench for analysis. The AO bench consists of a series of reflective and transmissive optics that images the exit pupil of the TSM onto a 349 actuator deformable mirror that is used for wavefront correction. A Fast Steering Mirror (FSM) may be inserted into the system (AO bench) to investigate image stability and to compensate for systematic jitter when operated in a closed loop mode. We will describe the optical design and performance of the WCT hardware and discuss the impact of environmental factors on system performance.
Experience and infrastructure from NGST's Wavefront Control Testbed (WCT) were utilized to develop a portable wavefront sensor, the Phase Retrieval Camera (PRC). The PRC is useful for the testing of optics in high-jitter environments. The principal uses of the PRC will be testing and experimenting with NGST technology demonstration mirrors as well as exploring other issues of wavefront sensing and control not easily studied using the WCT. This presentation will detail the packaging and hardware chosen for the PRC, the PRC software, and calibration of the instrument.
KEYWORDS: Point spread functions, Magnesium, Error analysis, Space telescopes, Coronagraphy, Telescopes, Signal to noise ratio, Stars, Data modeling, Wavefront sensors
The Eclipse coronagraphic telescope will allow for high contrast imaging near a target star to facilitate planet finding. One key element will be its high accuracy, high authority deformable mirror (DM) that controls the wave front error (WFE) down to an acceptable level. In fact, to achieve the desired contrast ratio of nine orders of magnitude (in intensity) to within 0.35 arcseconds of the target star, the WFE in the telescope must be controlled to level below 1Å rms within the controllable bandwidth of the DM. To achieve this extreme wave front sensing (WFS) accuracy, we employ a focus-diverse phase retrieval method extended from the Next Generation Space Telescope baseline approach. This method processes a collection defocused point-spread functions, measured at the occulting position in the Eclipse optical system, into a high accuracy estimate of the exit-pupil WFE. Through both simulation and hardware experiments, we examine and establish the key data requirements, such as the defocus levels and imaging signal-to noise level, that are necessary to obtain the desired WFS accuracy and bandwidth.
Control algorithms developed for coarse phasing the segmented mirrors of the Next Generation Space Telescope (NGST) are being tested in realistic modeling and on the NGST wavefront control testbed, also known as DCATT. A dispersed fringe sensor (DFS) is used to detect piston errors between mirror segments during the initial coarse phasing. Both experiments and modeling have shown that the DFS provides an accurate measurement of piston errors over a range from just under a millimeter to well under a micron.
We have developed a focus-diverse phase retrieval algorithm to measure and correct wavefront errors in segmented telescopes, such as the Next Generation Space Telescope. These algorithms incorporate new phase unwrapping techniques imbedded in the phase retrieval algorithms to measure aberrations larger than one wave. Through control of a deformable mirror and other actuators, these aberrations are successfully removed from the system to make the system diffraction limited. Results exceed requirements for the Wavefront Control Testbed. An overview of these techniques and performance results on the Wavefront Control Testbed are presented.
This paper describes the results of a few of the initial series of tests being conducted with the first configuration of the Next Generation Space Telescope Wavefront sensing and Control Testbed (WCT1). WCT1 is a 1:1, f/16.6 re-imaging system, incorporating two deformable mirrors located at pupil conjugate positions with 6 actuators/diameter (SM/DM) and 20 actuators/diameter (AO/DM). A CCD on a precision stage is used for obtaining defocused images providing phase diversity for wavefront determination using phase retrieval. In a typical experiment, wavefront error is injected into the optical path with the SM/DM and then corrected using the more densely actuated AO/DM. Wavefront analysis is provided via a phase retrieval algorithm, and control software is used to reshape the AO/DM and correct the wavefront. A summary of the results of some initial tests are presented.
A telescope simulator was built as part of the Nexus wavefront control testbed, an NGST technology experiment at NASA's Goddard Space Flight Center. This testbed was designed to demonstrate complete control of a segmented telescope, from initial capture of light, through coarse alignment and phasing, to fine phasing and wavefront control. The existing telescope simulator allows testing of the fine phasing and wavefront control steps. A small deformable mirror in the simulator allows generation of an unobscured aberrated wavefront, for use in exploring the range of measurement and correction using the testbed's image-based wavefront sensor and larger deformable mirror. An alternate path under development for the simulator will create a segmented wavefront using three spherical mirrors; three-degree-of-freedom mounts under each mirror enable alignment and phasing experiments that will cover most of the operation sequence. Details of the hardware design and performance will be presented.
By segmenting and folding the primary mirror, quite large telescopes can be packed into the nose cone of a rocket. Deployed after launch, initial optical performance can be quite poor, due to deployment errors, thermal deformation, fabrication errors and other causes. We describe an automatic control system for capturing, aligning, phasing, and deforming the optics of such a telescope, going from initial cm-level wavefront errors to diffraction-limited observatory operations. This system was developed for the Next Generation Space Telescope and is being tested on the NGST Wavefront Control Testbed.
KEYWORDS: Interferometry, Fourier transforms, Sensors, Near field diffraction, Direct methods, Diffraction, Detection and tracking algorithms, Near field optics, Optical components, Calibration
This work reports on the computation of the average phase of a beam over an optical element via discrete Fourier transform techniques. The objective is to develop accurate diffraction models for the Space Interferometry Mission (SIM). Applications related to SIM include calibration of metrology measurements, evaluation of cornercube diffraction effects, and others. The algorithms that are used to compute the field are described and numerical tests that assess their accuracy are presented.
Control laws for an adaptive optics system for the Palomar Mountain Hale Telescope are described. These are derived using a linear matrix model of the optics, which gives the Hartman-sensor centroids and the science camera wavefront as functions of deformable mirror (DM) commands and atmospheric phase. The matrices defining this system can be directly measured from the optics, some at finer spatial resolution than the wavefront sensor can resolve. A minimum-wavefront compensator feeds back both the wavefront sensor centroids and previous DM commands, using the atmospheric covariance to smooth the response at the finer spatial scale. The DM command feedback provides direct observability of waffle, piston and other modes unobservable in the wavefront sensor. Compensator gains can be updated using Kalman filtering techniques to track the evolution of the atmospheric covariance matrix.
The Next Generation Space Telescope will depart from the traditional means of providing high optical quality and stability, namely use of massive structures. Instead, a benign orbital environment will provide stability for a large, flexible, lightweight deployed structure, and active wavefront controls will compensate misalignments and figure errors induced during launch and cool-down on orbit. This paper presents a baseline architecture for NGST wavefront controls, including initial capture and alignment, segment phasing, wavefront sensing and deformable mirror control. Simulations and analyses illustrate expected scientific performance with respect to figure error, misalignments, and thermal deformation.
An 85 cm aperture beryllium mirror was fabricated as part of the IR Telescope Technology Testbed (ITTT), a facility to which the SIRTF flight telescope will be traceable. The ITTT was developed to demonstrate that diffraction-limited performance at a wavelength of 6.5 micrometers is attainable from an ultra-lightweight meter-class beryllium telescope operating at a temperature of 5.5K. Cryo-null figuring was employed to meet the requirements for the shape of the primary mirror at its operating temperature over an aperture of 79cm. The results of this process will be presented, including the repeatability of the surface through cryogenic temperature cycling. Modeling of system performance using the residual figure error will be described. Image-based methods were used to characterize a turned up edge that is too steep to be measured with an interferometer.
We consider interferometric techniques for capturing ultra-fast pulsed images. We analyze the signal-to-noise performance and information capacity of pulsed image detection systems and we briefly discuss the possibility of improving detection systems using spectral holographic image capture.
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