Site Characterization

Extreme desert conditions at the South Pole make it an excellent site for radio astronomy, apart from the obvious advantage of six months without sun. Due to the flattening of the atmosphere at the poles (the tropopause occurs at approximately 7 km above the South Pole, compared to 11 km at more equatorial latitudes) and the physical elevation of 2.8 km, the site is above much of the atmospheric water vapor. Moreover, with an average year-round temperature of $-47\deg~{\rm C}$, the residual atmosphere is quite cold.

From load calibrations performed during the austral summer, we determined the mean zenith opacity over the DASI bandpass to be $\tau\sim 0.02$. The opacity shows little day-to-day variation, as determined from skydips performed twice daily throughout the year. At typical ambient temperatures during the winter ( $-60\deg~{\rm C}$), these results suggest that over much of the DASI bandpass, the atmosphere contributes little more to our system temperature than the CMBR.

The Python experiment, which operated at the South Pole during the 1996-1997 austral summer, found that the atmosphere could rarely be seen in the data; the experiment was noise limited $75\%$ of the time (Lay & Halverson 2000). DASI data suggest that during the austral winter, the efficiency is closer to 90%.

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