Observations in the near-IR are challenging for a number of reasons. Compared to the optical, the near-IR sky is dominated by the vibrationally excited OH molecules which are created when hydrogen and ozone react. The highly variable OH emission contributes significantly to the sky background making it variable on time scales of tens of seconds to minutes. Beyond a wavelength of ~700 nm, the OH emission lines start to dominate and continue to do so until wavelengths longward of 2.3 \mu m where a steep increase in flux density is observed due to thermal black body radiation from the sky and the telescope itself. This effect is amplified by the presence of thin clouds (e.g. cirrus clouds) which have the effect of reflecting heat of Earths surface.

Although the OH emission lines can be a nuisance for astronomical observations, especially low resolution spectroscopy and imaging of faint objects, they can also serve as good wavelength calibrators (Osterbrock et al. 1996, ,Rousselot et al. 2000). At near-IR wavelengths (1 to 2.5 microns) many absorption features caused by water vapour and carbon dioxide exist. The near-IR transmission spectrum is shown in the first figure below whilst the emission spectrum is shown in the last figure. Both figures are shown with common near-IR filter profiles for reference.