SOUTH-POLE OBSERVATIONS OF THE NEAR-INFRARED SKY BRIGHTNESS

To observe the faintest objects in the Universe astronomers require th e darkest skies. In the infrared, sensitivities are limited by the the rmal emission from the atmosphere and the telescope. By placing a tele scope in Antarctica, and exploiting the reduced thermal emission and t he 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 accompa nying paper by Nguyen et al. (1996, PASP, 108, 718), we provide the fi rst 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 t o two orders of magnitude lower than the corresponding flux at tempera te observatories. We also show that substantial reductions in the back ground can be achieved throughout the 2.9-4.1 mu m region.