1.1.3 The Far-Infrared Region

The far-IR region extends from 30 to about 300 $\mu$m. Primarily because of water vapour, the Earth's atmosphere is completely opaque to far-IR radiation. The atmosphere is partially transparent at $\lambda > 300$ $\mu$m, which marks the beginning of the sub-millimetre region. Far-IR astronomy must be carried out using airborne, balloon-borne, or space telescopes. The latter have one major advantage that justifies the enormous expense and effort that are required to deploy them: they are extremely sensitive. This very high sensitivity is achieved by being above the IR-absorbing and IR-emitting atmosphere, and cooling the telescope so that its IR emission is negligible.