Optical turbulence at Ali, China -- Results from the first year of lunar scintillometer observations

TL;DR

This study reports initial atmospheric turbulence measurements at Ali, China, indicating exceptionally weak ground layer turbulence and excellent median seeing conditions, highlighting its potential as a premier astronomical site.

Contribution

First-year lunar scintillometer observations at Ali provide new detailed turbulence profiles, demonstrating its suitability for high-quality ground-based astronomy.

Findings

Median seeing of 0.34 arcsec from turbulence 11-500 m above ground

25% of the time, seeing is better than 0.26 arcsec

Half of the ground layer turbulence is below 62 m

Abstract

The location of an astronomical observatory is a key factor that affects its scientific productivity. The best astronomical sites are generally those found at high altitudes. Several such sites in western China and the Tibetan plateau are presently under development for astronomy. One of these is Ali, which at over 5000 m is one of the highest astronomical sites in the world. In order to further investigate the astronomical potential of Ali, we have installed a lunar scintillometer, for the primary purpose of profiling atmospheric turbulence. This paper describes the instrument and technique, and reports results from the first year of observations. We find that ground layer (GL) turbulence at Ali is remarkably weak and relatively thin. The median seeing, from turbulence in the range 11- 500 m above ground is 0.34 arcsec, with seeing better than 0.26 arcsec occurring 25 per cent of the time. Under median conditions, half of the GL turbulence lies below a height of 62 m. These initial results, and the high altitude and relatively low temperatures, suggest that Ali could prove to be an outstanding site for ground-based astronomy.