Why the next step in Chinese astronomy is cold as hell

The Lenghu observation site will be dry, frigid, and excellent for gazing into the cosmos with large telescopes.

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Dark, quiet, and cold as hell. That’s how Chinese astronomer Licai Deng describes the site on the Tibetan Plateau he and his colleague spent three years investigating as a possible location for large telescopes similar to those currently operating from Mauna Kea, in Hawaii, and Cerro Paranal, in Chile.

The site, which sits 4,200 meters above sea level on the Saishiteng Mountain near the town of Lenghu, is “so quiet that sometimes in the night you can hear your own heartbeat,” Deng tells Inverse. Camping there at night, Deng says, is a gamble — the temperatures are so cold that it is difficult, even with the best equipment, to stay warm.

But the same thin air that makes the site inhospitable to camping scientists also makes it ideal for peering into the heavens. That’s the conclusion of Deng’s study of the site, published Wednesday in the journal Nature. It is essentially a scientific stamp of approval for the telescope construction projects already underway at Lenghu.

Once constructed, the observatory complex will constitute the first elite sky-gazing site in the Eastern hemisphere.

What they found — The stargazing from Saishiteng Mountain is obviously good, even to the untrained observer, with stars shining fiercely through the calm, thin air, and the arc of the Milky Way easily visible to the naked eye, according to Deng.

But he and his colleagues needed to quantify just how well the site would stack up against the elite western observatories.

This comes down to four major variables:

  • How dark the sky is
  • How many clear nights the site sees each year
  • The humidity in the air
  • The stability of the air, which depends in part on overnight temperature variation

All of these factors have a bearing on how much quality observation time a site will see and the overall quality of those observations. They found conditions at the site to be very similar to Mauna Kea, although they noted Lenghu’s lower humidity might make the site a better location for infrared telescope observations.

A panoramic view of Saishiteng Mountain, near the town of Lenghu, in China.

Licai Deng

“The site survey is well done, and the current data are reliable,” University of California, Berkeley, astronomer Alex Filippenko tells Inverse. However, he notes that “more testing needs to be continued to make sure that there are no unexpected long-term trends.”

The scientists can control how dark the site is — light pollution is a human-created problem. Deng notes that the local government of Lenghu has enacted a law limiting development around the site to ensure the dark skies are protected going forward.

How much work such a law will actually do is uncertain, given the region’s historical emptiness.

Then and now — University of California, Berkeley Professor of History of Tibet and Inner Asia Stacey Van Vleet tells Inverse that — with the exception of a 30-year period from 1950 to 1980 when the oil industry brought an influx of Chinese workers — the region around Lenghu is sparsely populated.

Lenghu was “the key petroleum town in the province of Qinghai,” she says. “But that oil industry seems to have dried up in the 1980s — the population shrank from possibly around 120,000 to 800 people.”

Prior to the 1950s, groups of nomadic peoples, including Uyghurs and Mongols, lived in the region, according to Van Vleet. The area was nominally under Mongol control following an invasion in the 16th and 17th centuries, however, the more recent history is somewhat obscure, she says.

It’s possible there are people in the region who might oppose the construction of the observatory for cultural or religious reasons just as some Native Hawaiians have objected to the Thirty Meter Telescope on Mauna Kea, Van Vleet says. But if there are protests or ill feelings toward the project, “it would be hard for us to know.”

The summit of Mauna Kea, a dormant volcano on the Big Island of Hawaii, is considered sacred in traditional Hawaiian culture.

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Why it matters — With space telescopes like the Hubble bringing mind-blowing images of deep space and time down to Earth, it may seem strange that astronomers would put so much effort into finding locations for new Earth-bound observatories. But Deng says that there are still some advantages to placing telescopes on the ground.

“Very faint, very far away objects can only be seen with large telescopes,” and such telescopes are too large and complicated to be placed in orbit, he says — at least for now.

NASA’s upcoming James Webb telescope will boast a primary mirror 6.5 meters (just over 21 feet) in diameter, but the Thirty Meter Telescope planned for Mauna Kea is, well, 30 meters (more than 98 feet) in diameter. The Thirty Meter Telescope will render images 12 times sharper than the Hubble with its 2.4 meters (7.87 feet) diameter mirror.

Space telescopes have the advantage of operating in a vacuum without the shimmering veil of an atmosphere to contend with, Deng says. But that’s exactly why sites like Lenghu, as well as adaptive optics, are so important to astronomers.

What’s next — Lenghu, which is in the Qinghai province of China, is very important for Chinese astronomers. Once finished, it will be the Eastern hemisphere’s sole elite observatory site.

“The Lenghu site is being turned into an observatory as we speak,” Deng says. At least three telescopes are already being built at the site, including two one-meter telescopes, a solar infrared telescope called AIMS, and another node in the Stellar Oscillations Network Group (SONG), an international project looking at seismic activity in stars.

“There are several peaks, and survey instruments are currently being installed on them to see which one has the best performance,” Filippenko says of the work being done. “It is also important for the instruments to be cross-calibrated.”

Soon, Deng says Tsinghua University plans to build a 6.5-meter telescope to study dark matter and other phenomena. Originally planned for a location in Chile, it is now set to construct in Lenghu. There is also discussion, he says, of building a large, 12-meter infrared telescope at the site.

Abstract: On Earth’s surface, there are only a handful of high-quality astronomical sites that meet the requirements for very large next-generation facilities. In the context of scientific opportunities in time-domain astronomy, a good site on the Tibetan Plateau will bridge the longitudinal gap between the known best sites1,2 (all in the Western Hemisphere). The Tibetan Plateau is the highest plateau on Earth, with an average elevation of over 4,000 metres, and thus potentially provides very good opportunities for astronomy and particle astrophysics3–5. Here we report the results of three years of monitoring of testing an area at a local summit on Saishiteng Mountain near Lenghu Town in Qinghai Province. The altitudes of the potential locations are between 4,200 and 4,500 metres. An area of over 100,000 square kilometres surrounding Lenghu Town has a lower altitude of below 3,000 metres, with an extremely arid climate and unusually clear local sky (day and night)6. Of the nights at the site, 70 per cent have clear, photometric conditions, with a median seeing of 0.75 arcseconds. The median night temperature variation is only 2.4 degrees Celsius, indicating very stable local surface air. The precipitable water vapour is lower than 2 millimetres for 55 per cent of the night.
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