Click each image for a larger, readable version.
In these plots, we present a comparison of the measured sky brightness at zenith between the South Pole and Siding Spring (Australia) from 2-4.3 µm. In the first figure (2-3 µm), the South Pole spectra was collected on 31 May 1994 when the ambient temperature was -62 °C, while in the second figure (3-5 µm) the South Pole spectra was collected on 2 June 1994 when the ambient temperature was -66 C. In both plots, the Siding Spring data was collected on 9 December 1993 at +10 C. The units on the vertical axis of the left-hand plot are micro-Jy arcsec. Note the large dip in the South Pole sky brightness between 2.25-2.5 µm compared to the temperate latitude site. The South Pole sky is 20-100 times darker at these wavelengths than any other sight on earth which is accessible for large telescopes. CARA specifically designed the so-called K_dark (2.29-2.43 µm) filter to exploit this atmospheric window. The units on the vertical axis of the right-hand plot are milli-Jy arcsec. Note that between 2.9-4 µm, the South Pole sky is more than 10 times darker than at a temperate latitude site. For more information, please see Ashley et al. 1996.
For more information, see the site testing section of the Stark et al. decadal report and this site.