NASA’s James Webb Space Telescope (JWST) is a 6.5m diameter, segmented, deployable telescope for cryogenic IR space astronomy. The JWST Observatory architecture includes the Primary Mirror Backplane Support Structure (PMBSS) and Integrated Science Instrument Module (ISIM) Electronics Compartment (IEC) which is designed to integrate to the spacecraft bus via six cup/cone interfaces. Prior to integration to the spacecraft bus, the JWST observatory must undergo environmental testing, handling, and transportation. Multiple fixtures were developed to support these tasks including the vibration fixture and handling and integration fixture (HIF). This work reports on the development of the nominal alignment of the six interfaces and metrology operations performed for the JWST observatory to safely integrate them for successful environmental testing.
NASA’s James Webb Space Telescope (JWST) is a 6.6m diameter, segmented, deployable telescope for cryogenic IR space astronomy. The JWST Observatory architecture includes the Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM) element which contains four science instruments (SI), including a guider. The SIs and guider are mounted to a composite metering structure with outer envelope approximate measurements of 2.2x2.2x1.7m. These SI units are integrated to the ISIM structure and optically tested at NASA Goddard Space Flight Center as an instrument suite using an Optical telescope element SIMulator (OSIM). OSIM is a high-fidelity, cryogenic JWST simulator that features a ~1.5m diameter powered mirror. The SIs are aligned to the flight structure’s coordinate system under ambient, clean room conditions using opto-mechanical metrology and customized interfaces. OSIM is aligned to the ISIM mechanical coordinate system at the cryogenic operating temperature via internal mechanisms and feedback from alignment sensors and metrology in six degrees of freedom. SI performance, including focus, pupil shear, pupil roll, boresight, wavefront error, and image quality, is evaluated at the operating temperature using OSIM. This work reports on the as-run ambient assembly and ambient alignment steps for the flight ISIM, including SI interface fixtures and customization and kinematic mount adjustment. The ISIM alignment plan consists of multiple steps to meet the “absolute” alignment requirements of the SIs and OSIM to the flight coordinate system. In this paper, we focus on key aspects of absolute, optical-mechanical alignment. We discuss various metrology and alignment techniques. In addition, we summarize our approach for dealing with and the results of ground-test factors, such as gravity.
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.
KEYWORDS: Thermal modeling, Performance modeling, James Webb Space Telescope, Thermography, Instrument modeling, Systems modeling, Motion models, Temperature metrology, Finite element methods, Error analysis
The James Webb Space Telescope includes the Integrated Science Instrument Module (ISIM) element that contains four science instruments (SI) including a Guider. We performed extensive structural, thermal, and optical performance (STOP) modeling in support of all phases of ISIM development. In this paper, we focus on modeling and results associated with test and verification. ISIM’s test program is bound by ground environments, mostly notably the 1g and test chamber thermal environments. This paper describes STOP modeling used to predict ISIM system performance in 0g and at various on-orbit temperature environments. The predictions are used to project results obtained during testing to on-orbit performance.
The James Webb Space Telescope (JWST) Integrated Science Instrument Module (ISIM) Structure is a precision optical
metering structure for the JWST science instruments. Optomechanical performance requirements place stringent limits
on the allowable thermal distortion of the metering structure. A significant effort was completed to develop capabilities
to predict and metrologize cryogenic thermal distortion of the ISIM Structure. This paper focuses on thermal distortion
finite element modeling, analysis, and model validation. Extensive thermal distortion analysis was completed during the
design phase for the ISIM Structure to demonstrate that thermal distortion requirements were achieved. Comparison of
measurements from recently completed cryogenic testing and model predictions demonstrate the adequacy of thermal
distortion modeling uncertainty factors adopted during the design phase, and provide bounds on the accuracy of the
model predictions. This paper will provide an overview of the test configurations, describe the thermal distortion models
of the tests, and provide a comparison of test results and analytical predictions from the models.
The James Webb Space Telescope (JWST) Integrated Science Instrument Module (ISIM) Structure is a precision optical
metering structure for the JWST science instruments. Optomechanical performance requirements place stringent limits
on the allowable thermal distortion of the metering structure between ambient and cryogenic operating temperature (~35
K). This paper focuses on thermal distortion testing and successful verification of performance requirements for the
flight ISIM Structure. The ISIM Structure Cryoset Test was completed in Spring 2010 at NASA Goddard Space Flight
Center in the Space Environment Simulator Chamber. During the test, the ISIM Structure was thermal cycled twice
between ambient and cryogenic (~35 K) temperatures. Photogrammetry was used to measure the Structure in the
ambient and cryogenic states for each cycle to assess both cooldown thermal distortion and repeatability. This paper will
provide details on the post-processing of the metrology datasets completed to compare measurements with performance
requirements.
The James Webb Space Telescope (JWST) is a general astrophysics mission which consists of a 6.6m diameter,
segmented, deployable telescope for cryogenic IR space astronomy (~35K). The JWST Observatory architecture
includes the Optical Telescope Element and the Integrated Science Instrument Module (ISIM) element that contains four
science instruments (SI) including a Guider.
The alignment philosophy of ISIM is such that the cryogenic changes in the alignment of the SI interfaces are captured in
the ISIM alignment error budget. The SIs are aligned to the structure's coordinate system under ambient, clean room
conditions using laser tracker and theodolite metrology. The ISIM structure is thermally cycled and temperature-induced
structural changes are concurrently measured with a photogrammetry metrology system to ensure they are within
requirements.
We compare the ISIM photogrammetry system performance to the ISIM metrology requirements and describe the
cryogenic data acquired to verify photogrammetry system level requirements, including measurement uncertainty. The
ISIM photogrammetry system is the baseline concept for future tests involving the Optical Telescope Element (OTE) and
Observatory level testing at Johnson Space Flight Center.
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