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Peak Stargazing

High on the Qinghai-Tibet Plateau, Chinese scientists have found one of the best sites on Earth for the world’s next generation of large astronomical telescopes

By Peng Danni Updated Jan.1

Deng Licai hikes on Mount Saishiteng, Lenghu, Qinghai Province, 2018

In May 2018, before the helicopter lifted Deng Licai, a researcher at the National Astronomical Observatories, and his colleagues onto Mount Saishiteng, no human had set foot on its 4,200-meter summit. For the past few years, Chinese scientists have eyed this remote and sparsely populated area in northwestern China as a potential site for a cluster of astronomical telescopes. Due to its unique geographic location and natural conditions, Mount Saishiteng, near Lenghu Town on the Qinghai-Tibet Plateau, is ideal for a world-class observatory.  

An article based on three years of research by Chinese astronomers titled “Lenghu on the Tibetan Plateau as an astronomical observing site” was published in Nature magazine in mid-August, comparing Lenghu to existing world-class observatory sites in Chile, Hawaii and the Canary Islands. The study, conducted by the National Astronomical Observatories of China, China West Normal University, the Institute of Geology and Geophysics and Qinghai Observing Station affiliated to the Purple Mountain Observatory in Nanjing, Jiangsu Province, found the area was by far the best on the Tibetan Plateau.  

There are only a handful of astronomical sites on the Earth’s surface that meet the strict requirements for very large next-generation facilities: the high-altitude Atacama Desert of northern Chile, Mauna Kea volcano on Hawaii (Big) Island, and Antarctic inland ice domes, although none have been built there. Chile houses 68 percent of the world’s ground-based optical/infrared and highfrequency radio astronomical observation devices. Previously, China, even the entire Eastern Hemisphere, had not found sites comparable to those in the Western Hemisphere, which has restricted the development of optical astronomy.  

Finding the Lenghu site was not a planned outcome. In 2009, the National Astronomical Observatories of China Stellar Observations Network Group (SONG) was ordered to build a telescope. Deng Licai and his team started looking for a site, taking five years to choose the location for the Purple Mountain Observatory’s observation station in Delingha in Haixi Mongol and Tibetan Autonomous Prefecture, Qinghai Province. By 2013, a one-meter telescope was in operation. But by 2017, as local urbanization sped up, light pollution had increased 1,000-fold compared to 2013, affecting night clarity and observations.  

That same year, officials from Lenghu, which means “cold lake,” also in Haixi Prefecture, approached Deng to talk about the high-quality dark sky environment of Lenghu and the nearby Yadan National Geological Park, known for its wind-eroded rocks. The officials wanted to push the idea of Lenghu becoming an observatory site.  

Astronomers categorize observation devices built on the ground based on wave band, radio and optical telescopes. Optical telescopes work in the visible light band – the larger the caliber, the better. Radio telescopes work at the long end of the electromagnetic spectrum, so they are not restricted by clouds or night.  

The Five-hundred-meter Aperture Spherical Radio Telescope (FAST), which took five years to construct and opened in 2016 in Pingtang County, Guizhou Province, is the world’s largest single-dish radio telescope, with a receiving area equivalent to 30 football fields. Chang Jin, head of the National Astronomical Observatories, told NewsChina there is still a sizable gap between China and the world. There are already 10 optical telescopes of 10 meters elsewhere, while China’s largest general optical telescope is only 2.4 meters. The Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST), or Guo Shoujing Telescope in Xinglong County, Chengde in Hebei Province, though four to six meters in caliber, is a spectroscopic telescope, which uses radio waves, ultraviolet light, X-rays, infrared and visible light, but it cannot make imaging observations. Although funding for large devices is possible in the future, Chang pointed out without a proper site, telescopes will not achieve their designed effect.  

Deng, also lead author of the Nature article, told NewsChina that Europe attaches great importance to the development of astronomy, and in the 1960s, 16 countries came together to form the European Southern Sky Observatory (ESO) with observatories in Chile. In the 1980s, they started searching for observatory sites for the next generation, or 8-meter telescopes. 
Both China’s first modern observatories, the Purple Mountain Observatory in Nanjing, capital of Jiangsu Province, and Xinglong Station in Hebei Province which is near the cities of Tianjin and Beijing, are now obscured by light and air pollution as they are close to major urban areas. This renders them unsuitable for new-generation telescopes with higher caliber and sensitivity requirements.  

At the turn of the century, Chinese astronomers began looking for suitable observatory sites in high-altitude locations like the Qinghai-Tibet Plateau. In 2003, the National Astronomical Observatories launched the strategic astronomical site selection project in western China. In 2016, the National 12-meter Lunar-based Optical Telescope (LOT) project was approved and site selection began. There were three candidates, Ali in Tibet Autonomous Region and Daocheng in western Sichuan Province, both on the Qinghai-Tibet Plateau, and Muztagh Ata in Xinjiang Uygur Autonomous Region, on the Pamir Plateau. But none was picked.  

According to Deng, Lenghu has more geographical advantages than the others. On the northwestern edge of the Qinghai-Tibet Plateau, it is at the junction of Gansu, Qinghai and Xinjiang, thus it is not too remote. “Land transport from the site to the local support base, Lenghu Town, and then to the developed areas of China by road and railway networks is convenient,” Deng said.  

According to the Nature article, the nearest international airport, highway access and cargo railway stations of the tourist city of Dunhuang are within 250 kilometers. Lenghu Town sits only 2,700 meters above sea level and is 80 kilometers from the site, which provides comfortable conditions for a support base. This infrastructure enables good logistics for future activity. Climate records at three local weather stations for the past 30 years show average annual precipitation of around 18 millimeters, with over 3,500 hours of annual sunshine.  

Deng said that in the late 1980s and the early 1990s, China conducted a general survey of resources on the Qinghai-Tibet Plateau, including evaluating whether it could build an observatory. Lenghu was excluded because of extensive areas of wind erosion and closeness to the Taklimakan Desert. Yet, Deng’s team found that the vertical distance between the Lenghu site and the windy Qaidam Basin was over 1,000 meters, making it difficult for dust from the desert basin to reach the top of the mountain. Even if there were a sandstorm at the foot of the mountain, the air at the summit would remain clear.  

Lenghu used to be an important oil producer, and at its most prosperous had a population of 100,000. As the oil dried up in the 1980s, people left and only some 100 people live in the town today. In 2017, an official from Lenghu promised Deng that the government would designate the entire area as a dark sky reserve. He vowed Lenghu would regulate the use of light for local enterprises and municipal and household lighting to guarantee the long-term night-sky protection for the telescope. With potential threats to astronomical observations addressed, Deng and his team started searching for mountain peaks at an altitude of 4,200 to 4,500 meters. Finally, they chose Saishiteng summit at 4,200 meters.  

The Milky Way as seen from a site on Mount Saishiteng, August 12

Clear Skies 
Deng’s team began construction in January 2018. As road construction on Saishiteng is difficult and slow-going, Haixi government provided a helicopter to take researchers to the site and transport materials and equipment. By October 2018, all the equipment was at the summit and data collection work started. But the gravel road from Lenghu to the site was not finished until September 2019.  

“Seeing,” a term used by astronomers to describe atmospheric conditions, or the blurring of stars due to atmospheric turbulence along the light path, is one of the key metrics for the quality of observations at an observatory site. The lower the level of seeing, the better the observation quality. According to data, median seeing at the Lenghu site is better than others on the Qinghai-Tibet Plateau.  

Compared to the world’s top three observation sites – Mauna Kea in Hawaii, Cerro Paranal in Chile and La Palma in Spain, the median value at Lenghu is the same as that at Mauna Kea (0.75 arc-seconds) and is better than those at Cerro Paranal and La Palma.  

Also, median night temperature variation is only 2.4 C, indicating very stable local surface air. “Taking into consideration all the site monitoring data, this is a world-class astronomical observatory site,” Deng said.  

As the superiority of the site became clear, astronomy projects lined up for a spot, including the 2.5 meter diameter optical Wide Field Survey Telescope (WFST) of the University of Science and Technology of China (USTC), the one meter SONG telescope and one-meter solar mid-infrared telescope, the 50 cm Binocular Network (50BiN) of China West Normal University, and the 1.8 m and 80 cm Planetary Observation Telescope of the Institute of Geology and Geophysics of the Chinese Academy of Sciences.  

In May 2021, Qinghai Province and Tsinghua University signed an agreement to build the Multiplexed Survey Telescope (MUST) in Lenghu within seven years. On June 26, construction started on the Multi-Applied Sky Survey Telescope Array (MASTA) of the Purple Mountain Observatory for the search and research of near-Earth objects. The deputy director of the observatory said Lenghu will serve as the most important base for the observatory.  

Thirty telescopes will be sited at Lenghu. By the end of next year when these planned telescopes are completed, the site will surpass Xinglong National Observatory as the largest in China, Deng said.  

Gou Lijun, a researcher at the National Observatory of the Chinese Academy of Sciences, told NewsChina that equipment is the most critical element in the advancement of astronomy. Over the past 20 years, as the economy develops, China has invested more in astronomical observation equipment. But where to place these telescopes has been a heated topic. The US, Japan and some European countries have installed telescopes at Chilean observatories. For China, it would be very expensive to adopt the same approach.  

“When China finds an ideal site, it’ll quickly attract a number of large telescope construction projects,” Guo said.