JWST’s main science payload module is called the Integrated Science Instrument Module, or ISIM.
ISIM is the telescope’s instrument-carrying heart. It houses Webb’s four science instruments—NIRCam, NIRSpec, MIRI and NIRISS—as well as the Fine Guidance Sensor. The module supplies structural support, electrical power, computing, thermal control and connections needed to turn faint infrared light into scientific data.
Webb is often described as three major sections: the Optical Telescope Element, or OTE; the Spacecraft Element, or SE; and ISIM. The OTE contains the primary and secondary mirrors, while the Spacecraft Element includes the bus and sunshield. Thus, OTE and SE are important parts of the observatory, but neither is the main science payload module.
ISIM had to remain extremely cold so the telescope’s own heat would not overwhelm its infrared observations. MIRI requires especially low temperatures and uses an additional cryocooler. After extensive testing, ISIM and the telescope optics were integrated into the combined OTIS assembly before launch.