Since infrared is primarily heat radiation, detectors are most sensitive to infrared light when they are kept extremely cold. The Cryo-Telescope Assembly, shown in blue, is cooled to within a few degrees above absolute zero with liquid helium. The warmer spacecraft, shown in red, is uncooled. The telescope is surrounded by an outer shell that radiates heat to cold space in the anti-Sun direction, and is shielded from the Sun by the solar panel assembly.
Intermediate shields intercept heat from the solar panel and the spacecraft bus.
The outer shell and inner, middle, and outer shields are vapor cooled, i.e. , the cold helium vapor from the helium tank is used to carry away the heat from these structures. The telescope is attached to the top of the vapor cooled cryostat shell. The telescope and cryostat shell are launched warm, and cool down once in orbit. The star tracker and gyro package is mounted on the spacecraft bus. The main antenna is located at the rear of the spacecraft bus.