Measurements of optical seeing on the high Antarctic plateau

Results from the 1995 season of site-testing experiments at the South Pole are presented, in which the seeing was measured using balloon-borne microthermal probes. Our analysis shows a marked division of the atmosphere into two characteristic regions: (i) a highly turbulent boundary layer (0 - 220 m) associated with a strong temperature inversion and wind shear, and (ii) a very stable free atmosphere. The mean seeing, averaged over 15 balloon flights, was measured to be 1.86 ", of which the free atmosphere component was only 0.37 ". The seeing from similar to 200 m upward is superior to the leading mid-latitude sites (e.g. Fuchs 1995; Roddier et al. 1990) by almost a factor of two. The results are in good agreement with optical seeing data obtained by a differential image motion monitor on three of the five occasions when the two measurements were performed simultaneously. The boundary layer winds are of katabatic origin, and so we may consider the possibility of exceptional seeing conditions from surface level at other locations on the plateau such as Domes A and C, where there is little or no katabatic wind. In addition, the proximity of the optical turbulence to the focus of a telescope situated at ground level is a highly favourable situation for the use of adaptive optics, since the wavefront spatial coherence scale is related to the altitude of the turbulent layers producing the image distortion. Some comparisons are made between the relevant adaptive optics parameters measured at the South Pole and Cerro Paranal, one of the best mid-latitude sites.