A team of Chinese scientists has identified a location on the Tibetan Plateau that could host some of the world’s most powerful telescopes. The high-elevation plateau has been considered for observatory sites before, but the paper published today in Nature pinpoints a site that holds particular promise. A multi-year assessment suggest the site meets all the key parameters for a world-class observatory, including dark skies, high elevation, low humidity and more.
Chinese astronomers are eager to get a powerful observatory in their country, “but first of all, we need to find a place to put the telescope” says Licai Deng, an astronomer at the Chinese Academy of Sciences and lead author of the study. Though China has a number of smaller observatories across the country, Deng says they’ve only recently been able to fund large telescope projects.
When Deng and his team began searching for an observatory site, they looked for a specific combination of factors. Unlike backyard telescopes amateur astronomers use to spot stars and planets on a cloudless night, high-sensitive, massive optical and infrared telescopes require extreme conditions. The telescope China hopes to build, says Deng, could have an aperture—the diameter of the telescope’s light-gathering lens or mirror—of 100 feet and be the world’s largest, at least for a short time. Europe’s Extreme Large Telescope, which is slated to be complete in 2027, has a primary mirror of 138 feet and is the largest telescope ever planned.
When selecting a location for an observatory, “we tend to look for the not any one individual trait, but the best, optimal combination of multiple traits,” says John O’Meara, chief scientist at the W.M. Keck Observatory in Mauna Kea, Hawai’i, who wasn’t involved in the work. Observatories need be remote enough to avoid the polluting light of nearby cities, but still possible for scientists to access. Telescopes should be built at high elevations to avoid blurring caused by the atmosphere, but not so high that humans can’t visit. Too much wind, dust or water vapor can impede a telescope’s accuracy.
As the region with highest elevation of any on Earth, the Tibetan Plateau has long been of interest to both Chinese and international astronomers. To narrow down their search on the Plateau, Deng and his team first looked at weather information and satellite data to find places with clear, dark skies. They were excited when they saw that the city Lenghu, which hadn’t been considered until then, had 3,500 annual hours of sunshine a year and barely a drizzle of rain. They then identified an ideal observatory site on the nearby Saishiteng Mountain, which sits 5,000 feet above the city.
In 2018, the team from the Chinese Academy of Science set up equipment on the mountain to monitor conditions that could interfere with a clear view of the sky: dust, cloud cover, turbulence, water vapor and more. After three years of data collection and analysis, the team agreed that the location had conditions on par with some of the best observatory sites in the world.
Gary Hill, the chief astronomer at the University of Texas’ McDonald Observatory who was not involved in the study, was particularly impressed by the site’s darkness. “If you go through the expense of building a telescope of that scale, you want to be able to have very good sensitivity to extremely faint objects,” says Hill. “Many sites that started off very dark increasingly have trouble with encroaching civilization and the stray light pollution that brings.” Since the mountain is far from major cities the area is likely to stay dark for a while. Overall, Hill thinks “it’s an extremely promising site.”
Because the site is so far inland, it avoids some—but not all—of the moisture that complicates the use of infrared telescopes. Water vapor absorbs infrared radiation which is “a real killer” for certain telescopes says Scott Paine an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who was not involved in the research. For most of the year, he suspects water vapor wouldn’t be an issue but suspects it could post a challenge during wetter summer months.
Because all high-quality astronomical observatories are located in the Western Hemisphere, an observatory on the Tibetan Plateau could bridge a gap. If astronomers want to continually monitor an event happening over the course of a day, for example, they have to collaborate across observatories, like runners passing a baton. “Say there's a particularly unusual event, like two black holes merge or two neutron stars merge, where every data point counts as the Earth rotates,” says Patrick McCarthy, director of the NOIRLab, who was not involved in the work. “Right now, there are just gaps in time. There are parts of the earth that aren't covered. By putting an observatory in this longitude range in the middle of continental Asia, that gives us that extra time coverage that we don't have now.”
The world’s top observatories in Chile, Hawai’i and the Canary Islands stand to benefit from the uninterrupted monitoring that a site in the Eastern Hemisphere provides. “The international astronomical community is very collaborative because we're looking at the same sky,” says Deng. Though conduction and observations are being led by Chinese scientists, Deng says he’s looking forward to eventually opening the site at Lenghu to international astronomers.
Soon after the team completed their analysis, they broke ground on the observatory. “Construction is going on as we speak,” says Deng. The site already has a small working telescope and will soon host a 21-foot telescope, which will surpass the county’s 16-foot LAMOST telescope to become China’s biggest. The growing observatory is on track to become the next hub of Chinese astronomy, says Deng. The observations that they record over the next few years will help determine if the site is as promising as the initial assessment suggests. If so, Deng is hopeful that the site at Lenghu could be home to the next-generation giant Chinese telescope.