2.2.3 Thermal

The necessity to cool the CCD detector is determined by the requirement for low noise. At ambient temperatures, thermal excitations cause signal charges to be stored in every pixel. The random fluctuations of these processes impose a shot noise that is many times larger than the intrinsic on-chip amplifier noise. The actual optimum temperature is defined by a trade-off of engineering difficulty and performance. The lowest temperatures we require are actually dictated by the minimization of radiation damage effects later in the mission.