South Pole observations of the near-infrared sky brightness

To observe the faintest objects in the Universe astronomers require the darkest skies. In the infrared, sensitivities are limited by the thermal emission from the atmosphere and the telescope. By placing a telescope in Antarctica, and exploiting the reduced thermal emission and the natural absence of strong airglow emission between 2.3 and 2.5 mu m, we can minimize the sky brightness. In this paper, and in an accompanying paper by Nguyen et al. (1996, PASP, 108, 718), we provide the first ground-based measurements of this ''cosmological window.'' At 2.4 mu m the sky flux can be as low as 50 mu Jy per square arcsecond, up to two orders of magnitude lower than the corresponding flux at temperate observatories. We also show that substantial reductions in the background can be achieved throughout the 2.9-4.1 mu m region.