Background and stray radiation from sources other than the target increase the noise and thus affect the detectability of a continuum or line flux. Stray radiation comes into the focal plane in the form of thermal self-emission of the optical subsystem or as spurious light from bright infrared emitters (Sun, Earth, Moon, Jupiter). The level of stray radiation is kept below 10% of the astronomical background radiation with several baffles surrounding the optical subsystem (Fig. 3). The major background sources are the celestial background and high energy particles. These are discussed in this chapter.
There are other aspects which are related to background radiation and which constraint the sensitivity. These are: