Mid-infrared observing conditions at the South Pole

Measurements of the mid-infrared sky brightness at the South Pole throughou t the winter of 1998 show that the sky background is extremely low and stab le. For 50% of the time, the flux in the 8.78 to 9.09 mu m window is below 50 Jy per square arcsecond. Typical background levels in this window during clear conditions are of the order of 20 Jy per square arcsecond. This is a lmost an order of magnitude better than any other site on earth. The lower limit to the sky background across most of the N window appears to be set b y the aerosol content of the sky rather than by residual water vapor. These data were acquired remotely using an automated instrument housed in the AA STO (Automated Astrophysical Site-Testing Observatory).