1.1.1 The Near-Infrared Region

The near-IR spectral region extends from 1 to 5 $\mu$m. The atmosphere is transparent over several reasonably well-defined spectral regions, or windows, for which there are a set of widely accepted standard filters designated: $J$ (1.25 $\mu$m), $H$ (1.65 $\mu$m), $K$ (2.2 $\mu$m), $L$ (3.6 $\mu$m) and $M$ (4.8 $\mu$m). A fairly large gap in atmospheric transmission, due entirely to absorption by water vapour occurs between 5 and 8 $\mu$m, which partly accounts for the treatment of the near-IR as a distinct spectral region. The designation has been reinforced by the development of indium antimonide (InSb) detectors that are very responsive throughout the region but which cut-off sharply at 5 $\mu$m.

Figure 1.1: Atmospheric transmission in the near- and mid-infrared spectral region. Filter names in each spectral window are shown.

Blackbody (i.e. thermal) continuum emission from "room temperature" bodies becomes important in the near-IR at $\lambda \geq 3$ $\mu$m. This means that the radiation emitted by the telescope and the atmosphere (water vapour) is a strong background against which celestial bodies must be detected. As long as the detector noise is small, it is the photon shot noise (i.e., the fluctuations in the number of collected photons) associated with this background that is the principal source of noise in IR observations; such observations are said to be background-limited, and one always wants the detectors to be so good that it is the background shot noise (proportional to the square root of the collected photons), rather than the approximately constant detector noise, that dominates the observational noise. This IR spectral region beyond 3 $\mu$m where the background emission becomes so important is called thermal IR. Because of the large thermal background (and therefore the large associated photon shot noise), observations at thermal IR wavelengths are generally much less sensitive than those at 1-3 $\mu$m. It is partly for this reason that most of near-IR observations are made in the $J$, $H$ and $K$ bands. Indeed, mercury-cadmium-telluride (HgCdTe) detector arrays (such as HST's NICMOS), which have high quantum efficiency only at $\lambda \leq 2.5$ $\mu$m, are optimized for operation in the $J$, $H$ and $K$ bands. The thermal continuum radiation is relatively weak in the 1-3 $\mu$m region, but other types of atmospheric emission play a significant role there. In particular the OH lines are emitted by the atmosphere as a result of its interaction with sunlight. This line emission (see Figure 1.2), which can vary significantly with time, provides the main background against which earth-based 1-3 $\mu$m observations must be made.

Figure 1.2: OH Emission Lines in the Near-IR Spectral Region.