JWST ISIM Integration and Testing at Goddard Space Flight Center
It is an exciting year for integration and testing of the JWST Integrated Science Instrument Module (ISIM) at the Goddard Space Flight Center (GSFC). The ISIM comprises the four JWST science instruments and fine guidance sensor, integrated into a common structure at the telescope’s focal plane, as well as power and command and data handling electronics, mounted in a warm electronics module nearby (see Greenhouse, M. A. 2013 in AAS Newsletter 168 and Greenhouse, M. A. et al. 2010, Proc. SPIE, 7731 for more details). In important program milestones, two of the four JWST Science Instruments were delivered last year: the Mid-Infrared Instrument (MIRI) and the Fine Guidance Sensor/Near Infrared Imager and Slitless Spectrograph (FGS/NIRISS); see Figure 1. After completing post-shipment functional tests, optical metrology, and electrical checkouts with ISIM electronics systems, these instruments have since been installed into the flight ISIM structure (Figure 2). Preparations are well under way for the first of three planned ISIM cryo-vacuum tests; that first test is scheduled to begin later this summer, following on the recent successful completion of two important precursor cryo-testing activities.
Figure 1a: The MIRI undergoing incoming inspection at the Goddard Space Flight Center, after delivery of the instrument by the MIRI Consortium.
Figure 1b:The FGS/NIRISS undergoing incoming inspection at the Goddard Space Flight Center, after delivery of the instrument by the Canadian Space Agency.
Figure 2: The flight ISIM structure, after installation of the MIRI and FGS/NIRISS instruments and some MIRI cryocooler hardware. Note the 2m scale bar provided for reference.
One of those precursors was the cryo-certification and calibration of a critical piece of optical test equipment that will be utilized throughout the ISIM cryo-vacuum test program: the Optical Telescope Element Simulator (OSIM). As its name implies, the OSIM (see Figure 3) is intended to simulate the optical beam that will be delivered to each of the Science Instruments when the ISIM is mounted behind the JWST telescope — delivering, with its suite of light sources, an input beam of the proper f/#, chief ray angle, and absolute position, with a precise and well-calibrated wavefront, anywhere in an instrument’s field of view. An initial cryo-vacuum test of the OSIM in 2012 demonstrated excellent optical performance and stability of the simulator, as well as proper operation of its various mechanisms and light sources; the second test, recently completed, performed the detailed cryo-calibration of OSIM pointing control to enable it to place its beam accurately with respect to a set of alignment references, in all six degrees of freedom. The OSIM hereafter will remain in place in Goddard’s largest vacuum chamber throughout the upcoming series of ISIM tests (over roughly the next two years), minimizing the potential for disturbance to its optical calibration. It is important to note, however, that the OSIM does contain suitable alignment diagnostic equipment to enable its pointing calibration to be adjusted, as required, for any small shifts induced by subsequent cryo-cycles or the installation of the ISIM onto the system.
Figure 3: The JWST telescope simulator, the OSIM, being lowered into Goddard’s largest vacuum test chamber (the opening visible at the top is ~8 meters in diameter), for its (since completed) cryo-vacuum certification and calibration test. The OSIM will remain undisturbed in the chamber throughout the upcoming ISIM cryo-test campaigns.
A second noteworthy cryo-vacuum test completed recently was the verification of an important thermal shield assembly that surrounds the MIRI when mounted to the ISIM. The MIRI Shield provides thermal isolation of that mechanically cryo-cooled Mid-Infrared instrument (whose wavelength coverage extends to 28 µm, with detectors operating at 6-7K) from the rest of the ISIM assembly, which is passively cooled in flight to the 35-40K requirements of the observatory’s near-infrared instrument suite. An initial cryo-test of the MIRI Shield demonstrated vulnerability to unwanted thermal contacts and excessive parasitic heat loads into that system; after small modifications were made to the Shield, it passed a second cryo test with excellent performance and has now also been installed onto the ISIM structure.
With these important milestones behind us, the focus now turns to a first cryo-vacuum checkout of the OSIM/ISIM system. Cryo-vacuum testing of the ISIM will involve a test setup of unusual complexity, as the test will include: (1) the ISIM, mounted on top of the OSIM, within a suite of cooling panels intended to emulate the 35-40K thermal environment that the ISIM will see in the JWST Observatory, (2) the ISIM Electronics Compartment (a large assembly of electronics boxes that operates at room temperature) within its own thermal control enclosure two meters away, (3) a ground-test refrigeration system to provide the additional MIRI cooling required, (4) this large suite of hardware to be controlled and operated at temperature, and (5) a complex set of operational and contamination requirements to be obeyed during the cooldown and warmup periods. The full-up test assembly (the flight payload plus the associated ground test equipment) is nearly 8m across and 10m tall and will take several weeks to reach stable operating temperature for each test.
The formal verification program for the ISIM will be carried out in two cryo-vacuum tests with the full complement of science instruments and systems, bracketing the ISIM-level vibration and acoustics tests (for confirmation of stability of the system against those mechanical loads). That verification sequence will begin in 2014, after the delivery of the final two flight instruments, the Near-Infrared Camera (NIRCam) and Near-Infrared Spectrograph (NIRSpec), which are currently undergoing their final instrument-level integration and test (I&T) activities before delivery to the ISIM I&T flow later this year.
In the meantime, utilizing the substantial portion of the ISIM assembly currently in hand, the JWST Project will take the opportunity to carry out a “risk-reduction” cryo-vacuum test of the combined OSIM/ISIM system beginning later this (northern) summer of 2013. Goals of the test include:
- evaluating the performance of the various thermal control systems involved in cooling the ISIM hardware to the desired flight-like temperatures, so that any issues with that test equipment can be identified and corrected;
- operating the ISIM systems and instruments together in the flight-like cryo-vacuum environment;
- using the calibrated OSIM to conduct initial optical checkout of the MIRI and FGS/NIRISS as mounted on ISIM, assessing their 6 degree-of-freedom alignments as well as evaluating image jitter and in-band stray light backgrounds;
- carrying out some important instrument-level verifications and calibrations that can be executed more efficiently with the OSIM test setup than could have been done with additional testing at their home institutions before delivery.
Finally, execution of this risk reduction test will provide invaluable experience in test operations, including: fine-tuning the process of developing and executing OSIM + ISIM optical test scripts; performing quick-look analysis of and efficiently logging, distributing, and archiving the data acquired; and practicing the significant human logistics (communications, roles and responsibilities) involved in a test of this level of complexity. The benefits to the smooth, safe, and efficient execution of the later ISIM verification tests will be enormous.
The ISIM testing happening now at GSFC is a major component of the overall JWST integration and test plan to prepare the observatory for its scheduled October 2018 launch. To see the ISIM coming together in real time, watch the online JWST webcam. To learn more about future milestones leading up to the launch, please visit our webcasted talk from the January 2013 AAS Town Hall on the JWST.