News & Analysis Physics World  October 2021

Tibet seen as ideal stargazing spot

Chinese astronomers think that a location on Saishiteng Mountain in the Tibetan Plateau rivals existing astronomy meccas and could be home to next-generation observatories. Ling Xin reports

Top of the world A mountain near the town of Lenghu in Qinghai Province could have ideal conditions as an astronomy hub. (Courtesy: Licai Deng)

Astronomers in China have found that the Tibetan Plateau offers conditions that make it ideal for future gigantic optical-based telescopes. The researchers report that a mountain near the town of Lenghu in Qinghai Province has observing conditions similar to existing astronomy meccas in the Hawaiian mountains and Chilean deserts (Nature 596 353).

For years, astronomers had hoped to find a good observing site on the Tibetan Plateau – known as “the roof of the world” – due to its high altitude, dryness and minimal light pollution. However, the region’s harsh environment makes it extremely challenging to collect reliable and continuous observation data. Indeed, there was a general belief among astronomers that sandstorms ruled out Lenghu, while other nearby locations suffer from strong winds.

Since March 2018 a team led by Licai Deng from the National Astronomical Observatories of China, Chinese Academy of Sciences, has been monitoring cloudiness, night-sky brightness, air temperature, humidity, wind speed and direction at “Summit C” on Saishiteng Mountain, which lies some 4200 m above sea level. They discovered that about 70% of the nights at the summit were clear enough for observation. As for “seeing” – a key parameter to describe the blurring of stars due to atmospheric turbulences along the light path – the median value was 0.75 arcseconds, or 1/4800 of a degree. The median night temperature variation was 2.4 °C when precipitable water vapour was lower than 2 mm for most of the night.

These parameters match those of Mauna Kea in Hawaii, Cerro Paranal in Chile and La Palma in Spain, where the world’s most cutting-edge telescopes are located. In particular, the temperature fluctuations at the Lenghu site are much lower than the other sites, indicating very stable surface air. In winter, temperatures on Saishiteng Mountain could drop to below –20 °C at night, which is favourable for infrared astronomy. Meanwhile, with little water vapour, Lenghu’s exceptional atmospheric transparency could open a new window on terahertz astronomy to examine the interstellar medium and gain new insights into the origins of stars, galaxies and the universe.

“The [work] provides convincing evidence that the Lenghu site is indeed comparable in quality to the best-established sites such as Chile, Hawaii and the Canary Islands for large telescopes,” wrote astrophysicist Michael Ashley from the University of New South Wales in his reviewer comments for the paper. “The authors should be congratulated on completing this difficult task at such a remote location.” That view is backed up by Paul Hickson from the University of British Columbia in Vancouver, Canada, who is not involved in the research, and calls the study extensive and thorough. “Three years of data is quite good, as it covers all seasons and provides some information on year-to-year variations,” he says.

Home for future telescopes

China’s astronomical aspirations have soared in recent years, but the country only has a couple of 2 m-level telescopes that work in the optical band – while the largest of its kind now under construction is about 40 m in diameter. The lack of an ideal observing site has been a particular bottleneck for advancements in the country but that could now change thanks to the discovery of favourable conditions on Saishiteng Mountain.

Indeed, Chinese astronomers have already proposed a line-up of telescopes for the mountain site. They include a 2.5 m-aperture optical telescope now being built by the University of Science and Technology of China, which will have first light in 2023. It will be joined by a solar telescope and a telescope array called the Near Earth Object Hunter. The list could also include a 12 m-aperture telescope being proposed by the whole Chinese astronomy community to the government.

Deng believes, however, that Lenghu should not be just for China. With the rise of time-domain astronomy, it will be the only site in the eastern hemisphere to benefit the global observation of high-energy phenomena and transient events. Good astronomical sites are always in high demand – in particular given the construction deadlock at Mauna Kea – and Deng hopes that Lenghu could be home to international telescopes in the future.

Ashley, who has worked with Chinese astronomers at Antarctica’s Dome A, adds that the new site not only fills an observational gap in the eastern hemisphere but will also be critical for China’s astronomical ambitions, including the chance to boost international collaboration. “My team would be more than happy to contribute part of a telescope to be put on Saishiteng Mountain on the Tibetan Plateau,” he says.