The Integrated Science Instrument Module (ISIM) is the part of JWST that contains the science instruments, the Fine Guidance Sensor, and the data-handling computer.
The Integrated Science Instrument Module (ISIM) is the observatory element that contains the 4 science instruments as well as the Fine Guidance Sensor (FGS). It also houses electronics that (1) control the instrument detectors and mechanisms, (2) maintain the thermal environment, and (3) provide command and data processing for the science instruments and the FGS. See Figures 1 and 2.
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Near Infrared Camera, NIRCam
Mid-Infrared Instrument, MIRI
Near Infrared Spectrograph, NIRSpec
Near Infrared Imager and Slitless Spectrograph, NIRISS
Fine Guidance Sensor, FGS
JWST Acronyms and Abbreviations