Site-testing results from Dome A, Antarctica

Dome A, the highest point of Antarctic plateau, has long been considered as a promising site for astronomy. Since it was first reached in 2005, the site-testing campaigns were initiated. The previous results prove that Dome A has dark and clear sky, the coldest air temperature (i.e., low thermal IR background), low wind speed and very low precipitable water vapour (i.e., high atmospheric transmission for terahertz and far-infrared astronomy). Moreover, the latest seeing measurements show that, at a height of just 8 m, the free-atmosphere seeing was achieved for 31% of the time, with a median value of 0.31 arcsec and a best value of 0.13 arcsec. And the fraction will be increased to one half at a height of 14 m. Compared with seeing values around 0.6 - 0.8 arcsec in Hawaii and Chile, the superb seeing at Dome A will result in both higher angular resolution and deeper limiting magnitude. According to these advantages, Dome A is arguably the best site on Earth for ground-based optical, infrared, and terahertz astronomy.