A document describes a multi-field wavefront control (WFC) procedure for the James Webb Space Telescope (JWST) on-orbit optical telescope element (OTE) fine-phasing using wavefront measurements at the NIRCam pupil. The control is applied to JWST primary mirror (PM) segments and secondary mirror (SM) simultaneously with a carefully selected ordering. Through computer simulations, the multi-field WFC procedure shows that it can reduce the initial system wavefront error (WFE), as caused by random initial system misalignments within the JWST fine-phasing error budget, from a few dozen um to below 50 nm across the entire NIRCam Field of View, and the WFC procedure is also computationally stable as the Monte-Carlo simulations indicate.
With the incorporation of a Kalman Filter (KF) as an optical state estimator into the WFC process, the robustness of the JWST OTE alignment process can be further improved. In the presence of some large optical misalignments, the Kalman state estimator can provide a reasonable estimate of the optical state, especially for those degrees of freedom that have a significant impact on the system WFE. The state estimate allows for a few corrections to the optical state to push the system towards its nominal state, and the result is that a large part of the WFE can be eliminated in this step. When the multi-field WFC procedure is applied after Kalman state estimate and correction, the stability of fine-phasing control is much more certain.
Kalman Filter has been successfully applied to diverse applications as a robust and optimal state estimator. In the context of space-based optical system alignment based on wavefront measurements, a KF state estimator can combine all available wavefront measurements, past and present, as well as measurement and actuation error statistics to generate a Maximum-Likelihood optimal state estimator. The strength and flexibility of the KF algorithm make it attractive for use in real-time optical system alignment when WFC alone cannot effectively align the system.
This work was done by John Lou, Norbert Sigrist, Scott Basinger, and David Redding of Caltech for NASA's Jet Propulsion Laboratory. NPO-45793
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On-Orbit Multi-Field Wavefront Control With a Kalman Filter
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Overview
The document discusses the on-orbit multi-field wavefront control (WFC) for the James Webb Space Telescope (JWST), focusing on the integration of a Kalman Filter to enhance the telescope's alignment and performance. The JWST, designed as a significant advancement in space science following the Hubble Space Telescope, includes an integrated science instrument module (ISIM) with five instruments, with the Near Infrared Camera (NIRCam) playing a crucial role in wavefront sensing and control.
The primary objective of the WFC operation is to align the telescope by minimizing the differences in root mean square (RMS) wavefront error (WFE) between the current wavefronts and predetermined nominal wavefronts across selected control fields in the NIRCam field of view (FOV). The document emphasizes the importance of accurate wavefront error estimation, as poor estimates can lead to instability and failure in the fine-phasing WFC operation.
The approach described involves using multiple FOV points to reduce field bias during the optics alignment process, resulting in a more balanced optical state that generates uniformly good WFE across the entire FOV. The document outlines a mathematical framework for the WFC operation, employing least-square optimization to address the wavefront error at multiple field points simultaneously. This method aims to improve computational stability and the overall quality of telescope alignment.
Challenges in the alignment process are acknowledged, particularly when initial misalignment is significant or when there is ambiguity in the wavefront as a function of different degrees of freedom (DOF) in the system. In such cases, iterative WFC procedures may struggle to converge to an optimal state, potentially leading to further misalignment. The document describes a testing procedure that incorporates a Kalman state estimator to enhance state estimation and improve alignment outcomes.
Overall, the document highlights the innovative techniques being developed for the JWST's WFC, showcasing the potential of advanced algorithms like the Kalman Filter to address complex challenges in space optics. It serves as a technical support package under NASA's Commercial Technology Program, aiming to disseminate aerospace-related developments with broader technological and scientific applications. For further inquiries, contact information for the Innovative Technology Assets Management at JPL is provided.
