The Near‐Infrared Sky Emission at the South Pole in Winter

Abstract

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 μm 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 flux through a 4° beam and a variety of broadband and narrowband (1%) filters. It can be scanned in elevation from horizon to horizon through the zenith. We find a 20-100 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 80-200 μJy arcsec-2 at 2.43 μm, 100-300 mJy arcsec-2 at 3.6 μm and ~0.5 Jy arcsec-2 at 4.8 μm. Airglow emission contributes significantly to the sky flux shortward of ~2.4 μm, which is why the Kdark (2.27-2.45 μm) band emission does not drop to the 10-20 μJy arcsec-2 levels originally predicted. The darkest window for IR observations from the South Pole is from 2.35 to 2.45 μm, where the fluxes from the atmosphere may drop to as low as ~50 μJy arcsec-2 at times. Airglow dominates the emission at J (1.25 μm) and H (1.65 μm), but the flux levels of 300-600 μJy arcsec-2 and 800-2000 μJy arcsec-2, respectively, are also one-third to one-half those at temperate sites. We find no evidence for any significant contribution from auroral emission to the Kdark band. During twilight, when the Sun is <10° below 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.