Near IR
The natural near-infrared (NIR) sky background is dominated by the emission lines produced by the OH radical. The OH is generated in reactions between H and O_3 in the high atmosphere at an altitude of about 87 Km. The NIR atmospheric windows centered at 1.25, 1.65 and 2.2 mm, corresponding to the astronomical broad band filters J, H and K, are particularly rich of OH emission lines. Their effect on the astronomical observations has been reviewed by Oliva & Origlia (1992) and Rousselot et al. (2000). At wavelengths greater than 2 mm the effect of the thermal radiation starts to be important, this is mostly due to the telescope and its surroundings rather than to the sky.
We have measured the sky brightness at the zenith in the J, H and Ks filter at the ESO La Silla Observatory during bright (full moon) and dark (new moon) time. All measures have been obtained at the ESO 3.5m NTT with the NIR camera-spectrometer SOFI. The values obtained are listed in Table 1.
These measures compare well with the average values obtained at other astronomical sites worldwide recognized amongst the best, see Table 2, and make of La Silla a site with excellent background conditions.