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 features a powerful complement of focal-plane instruments: the Mid-Infrared Instrument (MIRI), the Near-Infrared Camera (NIRCam), the Near-Infrared Imager and Slitless Spectrograph (NIRISS), the Near-Infrared Spectrograph (NIRSpec), and the Fine Guidance Sensor (FGS). These instruments offer an exciting suite of scientific capabilities for imaging, high-contrast imaging, and spectroscopy. To bring these capabilities on-line after launch, a carefully scoped and sequenced set of commissioning activities has been developed. These activities will confirm the functionality of the instruments, characterize their performance (optimizing where possible), obtain initial calibrations at a level required to properly plan observations, and demonstrate essential operational sequences such as target acquisition. We present a high-level overview of these activities and the planned commissioning timeline to execute them.
KEYWORDS: Mirrors, James Webb Space Telescope, Optical testing, Adaptive optics, Image segmentation, Space telescopes, Telescopes, Optical components, Wavefronts, Temperature metrology
In 2017, the James Webb Space Telescope Optical Telescope Element and Integrated Science Instrument Module (OTIS) underwent cryogenic optical testing at the Johnson Space Center. In this paper, we summarize the successful execution and results of this 100-day test, which was a major program milestone. We summarize the as-run test configuration and provide a top-level as-run timeline. We also provide the top-level functional, optical, thermal, and operational results from the test. We summarize the key technical issues encountered and the resolution of those issues. The results of the OTIS test indicate that the payload should be fully capable of delivering on JWST’s exciting scientific potential.
KEYWORDS: Stars, Sensors, Mirrors, James Webb Space Telescope, Space operations, Observatories, Control systems, Point spread functions, Space telescopes, Actuators, Acquisition tracking and pointing, Line of sight stabilization, Line of sight pointing
The James Webb Space Telescope uses the Fine Guidance Controller to achieve pointing accuracy to a millionth of a degree needed for its scientific observations. This closed loop controller includes the Fine Guidance Sensor instrument, the Attitude Control System, and the Fine Steering Mirror, all working together to generate precise attitude updates every 64 ms to stabilize and point the Observatory. It was exercised for the first time with the flight hardware during the cryogenic test at Johnson Space Center. We provide a top level summary of the test, the results, and its performance in preparation for on-orbit operations.
KEYWORDS: James Webb Space Telescope, Optical components, Space telescopes, Optical testing, Sensors, Calibration, Data modeling, Human-machine interfaces, Error analysis, Analytical research
NASA’s James Webb Space Telescope (JWST) is a 6.5m diameter, segmented, deployable telescope for cryogenic IR space astronomy. The JWST Observatory includes the Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM), that contains four science instruments (SI) and the Fine Guidance Sensor (FGS). The SIs are mounted to a composite metering structure. The SIs and FGS were integrated to the ISIM structure and optically tested at NASA's Goddard Space Flight Center using the Optical Telescope Element SIMulator (OSIM). OSIM is a full-field, cryogenic JWST telescope simulator. SI performance, including alignment and wavefront error, was evaluated using OSIM. We describe test and analysis methods for optical performance verification of the ISIM Element, with an emphasis on the processes used to plan and execute the test. The complexity of ISIM and OSIM drove us to develop a software tool for test planning that allows for configuration control of observations, implementation of associated scripts, and management of hardware and software limits and constraints, as well as tools for rapid data evaluation, and flexible re-planning in response to the unexpected. As examples of our test and analysis approach, we discuss how factors such as the ground test thermal environment are compensated in alignment. We describe how these innovative methods for test planning and execution and post-test analysis were instrumental in the verification program for the ISIM element, with enough information to allow the reader to consider these innovations and lessons learned in this successful effort in their future testing for other programs.
In late 2015/early 2016, a major cryo-vacuum test was carried out for the Integrated Science Instrument Module (ISIM) of the James Webb Space Telescope (JWST). This test comprised the final cryo-certification and calibration test of the ISIM, after its ambient environmental test program (vibration, acoustics, EMI/EMC), and before its delivery for integration with the rest of the JWST observatory. Over the 108-day period of the round-the-clock test program, the full complement of ISIM flight instruments, structure, harness radiator, and electronics were put through a comprehensive program of thermal, optical, electrical, and operational tests. The test verified the health and excellent performance of the instruments and ISIM systems, proving the ISIM element’s readiness for integration with the telescope. We report here on the context, goals, setup, execution, and key results for this critical JWST milestone.
KEYWORDS: Stars, James Webb Space Telescope, Cryogenics, Sensors, Space telescopes, Observatories, Cadmium sulfide, Point spread functions, Performance modeling, Optical fabrication
The flight model Fine Guidance Sensor (FGS) on the James Webb Space Telescope (JWST) has successfully completed
its performance verification tests. The FGS cryogenic test is described along with some of the key guider performance
results which have been obtained. In particular we describe the noise equivalent angle (NEA) performance as a function
of guide star magnitude for the guider tracking mode. Tracking mode must be able to follow a guide star moving across
the field of view of either guider, primarily to allow the Observatory line of sight to settle in advance of the fine
guidance mode. FGS tracking mode will also be used for JWST’s moving target observing mode. The track testing
made use of the two movable sources within our JWST telescope simulator. The NEA of the FGS-Guiders will in part
determine the ultimate image quality of the JWST Observatory.
KEYWORDS: Point spread functions, Stars, James Webb Space Telescope, Performance modeling, Observatories, Sensors, Electronics, Mirrors, Monte Carlo methods, Data modeling
The Engineering Test Unit (ETU) of the Fine Guidance Sensor (FGS) for the James Webb Space Telescope (JWST) is
currently in fabrication. Extensive modeling of the key FGS-Guider performance parameters has been used throughout
the design process and continues to be used to evaluate the expected performance of the as-built instrument. A key
parameter of interest is the expected Noise Equivalent Angle (NEA) provided by the FGS. The NEA will, in part,
determine the ultimate image quality of the JWST Observatory. In this paper we use updated estimates of the End-oflife
impact of contamination to present the current expected NEA performance of the FGS flight model. As component
test data becomes available this data will be used as input to the FGS NEA performance model to assess the expected
performance of the instrument.
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