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.
We have published the optical design and early test results of the Roman Space Telescope grism spectrometer in previous SPIE proceedings. We report the follow-on activity of the spectral and radiometric calibrations, including the calibration methods, experiment designs, and test equipment calibration, such as the light source and detectors used in the test. The grism calibration includes the throughput versus wavelength, which is largely determined by the diffraction efficiency of the two diffractive surfaces. It also includes spectral resolution, point spread function, and relative radiometric measurements. The measured results are presented. The comparisons between the test data and the theoretical simulations are also presented. The tests and results presented are from the engineering test unit in ambient room temperature environment. The thermal/vacuum tests are planned to verify the results when the flight unit is ready.
Double-pass optical testing will play a key role in the alignment and verification of the Roman Space Telescope (RST), NASA’s upcoming flagship astrophysics mission, which was formerly known as the Wide Field Infrared Survey Telescope or (WFIRST). In this test configuration, optical fibers adjacent to the focal plane detector array will send light through the entire optical system in reverse, to be reflected off an auto-collimating flat (ACF) mirror and back through the system a second time (at a different angle) before reaching the detector. The ACF will be tilted through a range of angles to provide samples across the entire field of view, which is exceptionally large for a space telescope and will exhibit measurable field-dependent aberration. Phase retrieval analysis of the resulting point-spread functions will provide wavefront estimates; however, these wavefronts will include aberrations that were accumulated on both passes through the system at their respective field angles. In order to evaluate the alignment and verification of the system, we must predict the performance of the observatory when operating in single-pass. Therefore, we present an algorithm that will separate the aberrations from the two passes to provide a prediction of the telescope’s performance in single-pass. This is done by fitting the data to a polynomial representation of the underlying field aberrations and requiring that the aberrations accumulated on the two passes be consistent with that model.
An important question in the development of the Roman Space Telescope (RST) is how to optically test it at the highest levels of assembly, after the instruments have been integrated with the telescope. Our current strategy is double-pass testing using image-based wavefront sensing (phase retrieval). In this paper, we consider alternative strategies based on Hartmann testing, using either a pupil-plane mask or a mirror array. We developed first-order design considerations for the implementation of such Hartmann tests in the context of RST and designed two specific Hartmann tests to evaluate in further simulations. One of the major suggested benefits of Hartmann testing is insensitivity to vibrations that induce either line-of-sight jitter or dynamic changes in wavefront aberration. In order to understand whether this is true (and under what conditions), we developed a physical optics simulation of our two Hartmann tests under both Gaussian line-of-sight jitter and sinusoidal dynamic secondary mirror motion. We then also developed a data reduction process for fitting these Hartmann test images and estimating system wavefronts.
We have presented the optical design and early test result of WFIRST grism spectrometer in previous SPIE conferences. This paper reports the follow-on activity of the spectral and radiometric calibrations, including the calibration methods, experiment designs, and the light source and detector calibration. The real grism calibration includes the throughput versus wavelength, which is largely determined by the diffraction efficiency of the two diffractive surfaces. It also includes spectral resolution, point spread function, and encircle energy measurements. The measured data are presented. The comparisons between the test data and the simulation from theory, or optical model, are also presented.
This work describes a design process that greatly increases the depth of field of a simple three-element lens system intended for biometric iris recognition. The system is optimized to produce a point spread function that is insensitive to defocus so that recorded images may be deconvolved without the knowledge of the exact object distance. This is essentially a variation on the technique of wavefront encoding; however, the desired encoding effect is achieved by aberrations intrinsic to the lens system itself, without the need for a pupil phase mask.
This work describes a design process which greatly increases the depth of field of a simple three-element lens system intended for biometric iris recognition. The system is optimized to produce a point spread function which is insensitive to defocus, so that recorded images may be deconvolved without knowledge of the exact object distance. This is essentially a variation on the technique of wavefront encoding, however the desired encoding effect is achieved by aberrations intrinsic to the lens system itself, without the need for a pupil phase mask.
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