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 Ultraviolet/Optical/Infrared (UVOIR) flagship astrophysics architectures proposed by the Astro2020 Decadal Survey fundamentally challenge the current test-like-you-fly approach to space systems, because of their physical scale, multiple stages of on-orbit deployment, and extremely stringent optical performance requirements unique to visible-light coronagraphy. These limitations elevate the importance of integrated control, structural dynamics, and optical modeling, particularly in early system architecture studies. A unique non-contact observatory control architecture called Disturbance Free Payload (DFP) for next-generation large astrophysics observatories involves physically isolating the segmented telescope structure from the supporting spacecraft by means of a non-contact interface. In this control architecture, rigidbody telescope pointing is achieved by actuating the payload with non-contact voice coil actuators and maintaining positive interface gaps using spacecraft inertial actuators and interface non-contact sensors. This architecture presents distinct advantages over current state-of-the-art spacecraft vibration isolation approaches, particularly for large flexible spacecraft, but also introduces unique disturbance and coupling mechanisms that must be analyzed. In this paper, development of an integrated model is described, consisting of a 6.7-meter inscribed segmented optical system, and an unobscured telescope with 55 primary mirror segments. The paper starts with an overview of the models that directly predict time-domain lineof-sight and wavefront error dynamic stability (optics, dynamics, control system, error sources). Next, key dynamic stability performance metrics for coronagraph contrast performance are described and a systematic methodology for realizing an accurate but computationally feasible truncated modal model is presented. Finally, an exemplar point design that is compliant to 10-picometer RMS wavefront error is developed, and the necessary component errors to achieve this performance are presented.
An approach is developed for the alignment and stability maintenance of the LUVOIR segmented primary mirror using a segment state estimation and wavefront control method based on a hybrid segment motion sensing architecture of laser truss metrology and segment edge sensors. Our current computer model was generated for LUVOIR Architecture Option A with a 15-meter aperture, 120-segment primary mirror. The methodology and simulation results will be presented and analyzed. JPL has a long history of technology development in laser metrology and edge sensors, including work in SIM [7], Keck and TMT [8], CCAT [3] and LUVOIR [1]. We will discuss our current efforts of LUVOIR laser metrology and edge-sensor models development, showing sensitivities of sensor measurements to various LUVOIR mirror eigen-modes, removing global modes and strengthening weak modes by performing joint (hybrid) lasermetrology and edge sensing. We will define and derive an important performance metric called wavefront error multiplier (WEM), and show that WEM provides a simple link between sensor errors and the closed-loop (controlled) system wavefront error. We will show WEM values for several hybrid sensor configuration options studied. We will discuss an algorithm for mirror shape control and maintenance through segment state and wavefront estimations using joint edge-metrology sensing. We will compare simulated performance of mirror state estimation, wavefront estimation and wavefront control based on joint edge-metrology sensing among several sensor configurations, and show the impact of sensor error distributions on the segmented mirror alignment performance. Mirror shape control performance will also be evaluated in the context of imaging contrast between inner working angles (IWA) and outer working angles (OWA) of a LUVOIR coronagraph.
CCAT will be a 25 m diameter telescope operating in the 2 to 0.2 mm wavelength range. It will be located at an altitude
of 5600 m on Cerro Chajnantor in Northern Chile. The telescope will be equipped with wide-field, multi-color cameras
for surveys and multi-object spectrometers for spectroscopic follow up. Several innovations have been developed to
meet the <0.5 arcsec pointing error and 10 μm surface error requirements while keeping within the modest budget
appropriate for radio telescopes.
The 25-m aperture CCAT submillimeter-wave telescope will have a primary mirror that is divided into 162 individual
segments, each of which is provided with 3 positioning actuators. CCAT will be equipped with innovative Imaging
Displacement Sensors (IDS) - inexpensive optical edge sensors - capable of accurately measuring all segment relative
motions. These measurements are used in a Kalman-filter-based Optical State Estimator to estimate wavefront errors,
permitting use of a minimum-wavefront controller without direct wavefront measurement. This controller corrects the
optical impact of errors in 6 degrees of freedom per segment, including lateral translations of the segments, using only
the 3 actuated degrees of freedom per segment. The edge sensors do not measure the global motions of the Primary and
Secondary Mirrors. These are controlled using a gravity-sag look-up table. Predicted performance is illustrated by
simulated response to errors such as gravity sag.
The 25 meter aperture Cornell Caltech Atacama Telescope (CCAT) will provide an enormous increase in sensitivity in
the submillimeter bands compared to existing observatories, provided it can establish and maintain excellent image
quality. To accomplish this at a very low cost, it is necessary to conduct accurate engineering trades, including the most
effective segment and wavefront sensing and control approach, to determine the best method for continuously
maintaining wavefront quality in the operational environment. We describe an integrated structural/optical/controls
model that provides accurate performance prediction. We also detail the analysis methods used to quantify critical design
trades.
The 25-m aperture Cornell Caltech Atacama Telescope (CCAT) will have a primary mirror that is divided into 162
individual segments, each of which is equipped with 3 positioning actuators. This paper presents a mathematical
description of the telescope, its actuators and sensors, and uses it to derive control laws for figure maintenance. A
Kalman Filter-based Optical State Estimator is used to continuously estimate the aberrations of the telescope; these are
used in a state-feedback controller to maintain image quality. This approach provides the means to correct for the optical
effects of errors that occur in un-actuated degrees of freedom, such as lateral translations of the segments. The control
laws are exercised in Monte Carlo and simulation analysis, to bound the closed-loop performance of the telescope and to
conduct control design trades.
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.
KEYWORDS: Point spread functions, Wavefronts, Error analysis, Actuators, Near field optics, Coronagraphy, Control systems, 3D modeling, Apodization, Mirrors
We have investigated the dependence of the High Contrast Imaging Testbed (HCIT) Phase Induced Amplitude
Apodization (PIAA) coronagraph system performance on the rigid-body perturbations of various optics. The structural
design of the optical system as well as the parameters of various optical elements used in the analysis are drawn from
those of the PIAA/HCIT system that have been and will be implemented, and the simulation takes into account the
surface errors of various optics. In this paper, we report our findings when the input light is a narrowband beam.
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.
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.
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.
KEYWORDS: Error analysis, James Webb Space Telescope, Wavefronts, Telescopes, Phase modulation, Space telescopes, Optical testing, Interferometers, Actuators, Monte Carlo methods
The use of wavefront measurements to deduce the state of multiple optics in a telescope beam train - their misalignments and figure errors - can be confused by the fact that there are multiple potential sources for the same measured error. This talk applies Kalman filtering techniques as a tool for separating true telescope errors from artifactual testing errors in the alignment and testing of NASA's James Webb Space Telescope, a large segmented-aperture cryogenic telescope to be launched after 2010.
We discuss the implementations of parallel programming tools, mathematical tools, and some signal and image processing applications on the Intel iWarp system. The paper starts with a discussion on parallel processing for signal and image processing applications. Some issues related to programming on the iWarp system are addressed. The paper presents implementations of efficient parallel programming tools on the iWarp system and discusses mapping applications to the iWarp system using these programming tools. The applications mapped to the iWarp system include a two-dimensional fast Fourier transform (2-D FFT), a few matrix computation algorithms, two low-level image processing schemes, and an acoustic signal processing algorithm. Performance results from our implementations are presented and analyzed.
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