Based on phase retrieval of defocused point source images from JWST commissioning, routine maintenance, and science data, we characterize components of the JWST OTE wavefront error variations over a wide range of time scales, including the accumulation of segment pose changes (tilt events) over days and weeks of typical wavefront control cycles, smooth drifts over hours and days, oscillation due to thermal cycling of the ISIM Electronics Compartment heaters with periods of a few minutes, and mechanical vibration modes with periods ~1 second and less. We extract the spatial and temporal forms of the detected WFE variations and explore correlation with relevant observatory telemetry data, including reaction wheel rotation speeds, IEC heater panel temperatures, and spacecraft attitude. This analysis extends the initial performance characterization during JWST commissioning and is intended to enhance the understanding and utility of JWST observations, as well as to provide more detailed in-flight characterization of optical stability for evaluation of integrated modeling and insight for the design and development of future observatories.
The James Webb Space Telescope (JWST) launched on December 25, 2021, and its optical performance in orbit has been even better than predicted pre-flight. The static wavefront error (WFE) is less than half the value specified for the requirement of having diffraction-limited image quality at 2 microns in the NIRCam shortwave channel, enabling the observatory to deliver both sharper images and higher sensitivity than anticipated. In addition to the excellent image quality, the optical stability has also exceeded expectations, both in terms of high-frequency dynamic contributions (which would be perceived as part of “static WFE”) and in terms of drifts over minutes, hours, and days. Stability over long timescales is critical for several important science cases, including exoplanet transit spectroscopy and coronagraphy. JWST’s stability success was achieved through detailed design and testing, with several important lessons learned for future observatories, especially the Habitable Worlds Observatory that is expected to need even higher levels of stability. We review the stability architecture, how it was technologically demonstrated, the ground test results and improvements, the on-orbit results, and the lessons learned.
The James Webb Space Telescope (JWST) is a segmented deployable telescope, currently operating at L2. The telescope utilizes 6 degrees of freedom for adjustment of the Secondary Mirror (SM) and 7 degrees of freedom for adjustment of each of its 18 segments in the Primary Mirror (PM). After deployment, the PM segments and the SM arrived in their correct optical positions to within a ~1 mm, with accordingly large wavefront errors. A Wavefront Sensing and Controls (WFSC) process was executed to adjust each of these optical elements in order to correct the deployment errors and produce diffraction-limited images across the entire science field. This paper summarizes the application of the WFSC process.
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