JWST’s NIRCam instrument operates at about 40 K.
NIRCam—the Near-Infrared Camera—is designed to detect light from roughly 0.6 to 5 micrometers. Cooling it to only a few dozen kelvins suppresses thermal infrared emission from the instrument itself, preventing that glow from overwhelming the extremely faint astronomical signals it is built to measure.
NIRCam is passively cooled by Webb’s large sunshield and radiators. NASA commonly describes the near-infrared instruments as operating at about 39–40 K, while technical descriptions can give narrower design ranges for particular parts, such as the optical bench or detector assemblies. Thus 40 K is the standard rounded quiz answer, not a claim that every component has exactly the same temperature.
NIRCam does more than take pictures. Its ten mercury-cadmium-telluride detector arrays also help measure wavefront errors so engineers can align Webb’s 18 primary-mirror segments. MIRI is the major temperature-related contrast: it requires an active cryocooler to reach roughly 7 K.