Mid‐Infrared Sky Brightness Site Testing at the South Pole

Abstract

ABSTRACTDuring the austral summer of 1996, the mid‐infrared imaging polarimeter NIMPOL was operated at the Amundsen‐Scott South Pole Station, to obtain quantitative measurements of the 10 and 20 μm sky brightness and stability. These observations were conducted as part of the Joint Australian Centre for Astrophysical Research in Antarctica (JACARA) site testing program on the Antarctic Plateau. The results of this site testing program are presented. The observations show that the mid‐infrared sky brightness at the South Pole Station is much less than comparison sky brightness observations made at the Canberra base of the instrument. This reduction in sky brightness is attributed largely to the low emissivity of the atmosphere (because of its dryness and lack of aerosols), and the effect of the reduced atmospheric temperature (there is an expected decrease by a factor of 2.5 from the temperature difference between the two sites alone). The measured 11 μm sky emissivity at the South Pole is also lower than previous measurements of the sky emissivity at Mauna Kea Observatory in Hawaii. The sky brightness was also found to be more stable than at the warmer, mid‐latitude site (Canberra), and it is expected that “stare” mode operation of the instrument for astronomical observations would be quite feasible under these conditions. Measurements were also made during a period of “ice haze,” and the suspended ice crystals were observed to increase the sky background by 16% compared with clear weather and to add low‐frequency brightness variations. However, this variability could be removed by slow (2 Hz) chopping, allowing high‐sensitivity observing to continue through the “ice haze.”