Measurement of optical seeing on the high antarctic plateau

Abstract

Abstract.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 ∼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.

Dome C site testing: surface layer, free atmosphere seeing, and isoplanatic angle statistics

“…We found a median value of 3.9 , which is slightly lower than the 5.7 found by Lawrence et al (2004) with MASS but still very competitive compared to more classical sites (Table 1 of Lawrence et al 2004). Our value is very close to the South Pole isoplanatic angle of 3.2 as estimated by Marks et al (1999) from in situ radio soundings. This is not surprising; the isoplanatic angle expressed as a weighted integral over the vertical distribution of turbulence is…”

“…Both sites of Dome C and South Pole actually exhibit strong winds in winter above 10 km (Marks et al 1999;Trinquet et al 2008a). Figure 19 shows the dependence of θ 0 on the period of the year.…”

“…It has been measured at about 1.7 arcsec in the visible at the South Pole (Marks et al 1999;Travouillon et al 2003). However, microthermal radio soundings by balloonborne sensors estimated a seeing that would be around 0.3 arcsec above a turbulent surface layer (SL), measured to be of 200 m thickness at the South Pole.…”

On the atmosphere for astronomers above Dome C, Antarctica

“…If this layer appears to be an excessively difficult obstacle at the South Pole site, with a thickness of more than 200 m (Marks et al 1996(Marks et al , 1999, its much reduced thickness at Dome C (around 30 m, as it appears now both from models and from measurements) makes it a real difficulty, but not an insurmountable problem. Different strategies must be studied to make it possible to exploit the unique seeing encountered up there, either to set up the most demanding instruments at 30 m, or to use highly performing adaptive optics that must solve an unusual but probably not so difficult problem with a unique turbulent layer located extremely close , LeLouarn & Hubin 2006.…”

“…In 1991-92 Gillingham suggested that exceptionally good seeing could be expected above the high part of the Antarctic Polar Plateau. A preliminary site testing campaign was done at Dome C in 1996 (Marks et al 1999).…”