The Near-infrared Sky Emission at the South Pole in Winter

The Antarctic plateau provides superb sites for infrared astronomy, a result of the combination of low temperatures, low levels of precipitable water vapor, high altitude, and atmospheric stability. We have undertaken measurements of the sky background from 1 to 5 km at the South Pole, using a single channel InSb spectrometer, the Infrared Photometer Spectrometer (IRPS), during the winter (dark) period of 1995. The IRPS records the DC level of the sky Ñux through a 4¡ beam and a variety of broadband and narrowband (1%) Ðlters. It can be scanned in elevation from horizon to horizon through the zenith. We Ðnd a 20È100 times reduction in the background of thermal emission compared to that from mid-latitude sites such as Siding Spring and Mauna Kea, with typical background levels of 80È200 kJy arcsec~2 at 2.43 km, 100È300 mJy arcsec~2 at 3.6 km and D0.5 Jy arcsec~2 at 4.8 km. Airglow emission contributes signiÐcantly to the sky Ñux shortward of D2.4 km, which is why the (2.27È Kdark 2.45 km) band emission does not drop to the 10È20 kJy arcsec~2 levels originally predicted. The darkest window for IR observations from the South Pole is from 2.35 to 2.45 km, where the Ñuxes from the atmosphere may drop to as low as D50 kJy arcsec~2 at times. Airglow dominates the emission at J (1.25 km) and H (1.65 km), but the Ñux levels of 300È600 kJy arcsec~2 and 800È2000 kJy arcsec~2, respectively, are also one-third to one-half those at temperate sites. We Ðnd no evidence for any signiÐcant contribution from auroral emission to the band. During twilight, when the Sun is \10¡ below Kdark the horizon, scattered sunlight contributes to the sky background with a Rayleigh-type spectrum. Scattered moonlight is also evident in the sky emission at the J band when the Moon is up. Subject headings : atmospheric e†ects È infrared : general È instrumentation : photometers È radiation mechanisms : thermal È radiative transfer