1.1.2 The Mid-Infrared Region

The mid-IR region extends from about 5 $\mu$m to the atmospheric cut-off near 30 $\mu$m, the latter being the longest IR wavelength observable from the ground. The best mid-IR atmospheric windows extends from about 8 to 14 $\mu$m, with another important, but weaker, window extending from about 16 $\mu$m to somewhat above 25 $\mu$m. The broadband filter spanning the 8-14 $\mu$m window (referred to as the "10 $\mu$m window") is called the $N$ band filter, and the broadband filter spanning the 18-25 $\mu$m window (the "20 $\mu$m window") is usually called the $Q$ band filter. At the best (i.e. highest and driest) ground-based observing sites, like Mauna Kea in Hawaii, the atmosphere is somewhat transparent even out to 30 $\mu$m.

Atmospheric CO$_2$ causes deep absorption around 15 $\mu$m. Water vapour is responsible for the short-wavelength cut-off of the mid-IR region near 8 $\mu$m and for absorption lines throughout the mid-IR region. The atmospheric pressure drops roughly exponentially with altitude, i.e.

$$P_z=P_0 \exp^{-z/H}~,$$

where $z$ and $H$ are the altitude and scale height, respectively. The scale height of air is about 8,000 m, whereas the scale height of water is about 3,000 m. Note that OH lines, which are a special problem for 1-3 $\mu$m observations, originate at an altitude of about 90 km, so observing from even the highest ground-based sites does not reduce their influence.

The entire mid-IR region is in the thermal IR regime; the Planck spectral intensity for a 300 K temperature object peaks at 17 $\mu$m. Depending on the telescope's IR emission properties, the telescope thermal emission may dominate over the atmosphere's thermal emission at wavelengths where the atmosphere is especially transparent and therefore less emissive; the 10 $\mu$m window is such a region, but emission in the 20 $\mu$m window, which is much less transparent, is dominated by the atmosphere.