Spat threatens China's plans to build world's largest telescope

China's astronomers are united in wanting a world-class giant optical telescope, one that would serve notice that they are ready to compete on the global stage. But a squabble has opened up over the telescope's design. On one side is an established engineering team, led by a veteran optics expert responsible for the nation's largest existing telescope, that is eager to push ahead with an ambitious design. On the other are astronomers revelling in a grassroots priority-setting exercise—unprecedented for China—who have doubts about the ambitious design and favour something simpler.

Now, a panel of international experts has reviewed the designs and come out squarely in favour of the simpler proposal, according to a copy of the review obtained by Science. But the conclusion has not ended what one Chinese astronomer calls "an epic battle" between the high-ranking engineers accustomed to top-down control over projects and the nascent grassroots movement.

At issue is a project that emerged in 2015, when the Chinese Academy of Sciences (CAS) set up a Center for Astronomical Mega-Science that polled senior astronomers on their priorities. Top was a desire to boost China's participation in the Thirty Meter Telescope (TMT), which is being developed by an international consortium. (Construction on Hawaii's Mauna Kea has been delayed by legal claims raised by Native Hawaiians.) Second was a giant telescope of the country's own.

For now, China's largest optical telescope is the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), a 4-meter survey telescope completed in 2008 in Hebei province near Beijing. China's astronomers rallied around the idea of leapfrogging to a 12-meter telescope that, if completed quickly before other giants like the TMT, would for some years be the largest telescope on Earth. In early 2016, the National Development and Reform Commission (NDRC), responsible for funding large domestic projects, gave the mega science centre approval to develop plans for what is now being called the Large Optical/Infrared Telescope (LOT), to be sited in western China. To secure NDRC funding for construction—an estimated 1.5 billion renminbi ($220 million)—the plans must be approved by the end of 2018.

Xiangqun Cui was ready. An optics specialist, Cui heads a group at CAS's Nanjing Institute of Astronomical Optics & Technology (NIAOT) that had developed LAMOST and was already working on a 12-meter telescope design.

In most large telescopes, a large primary mirror captures light and reflects it off one or two secondary mirrors to the telescope's instruments. The daring NIAOT design calls for four mirrors—one primary and three secondary. The fourth mirror allows for exquisite control of the streams of photons so that they fall almost perpendicular to the instrument's focal plane, ensuring "very good image quality," Cui says. She adds that, because the TMT and other telescopes would eventually surpass the LOT's sensitivity, the NIAOT design needed to provide a wide field of view that would enable the telescope to act as a spotter for the bigger scopes. "This is a new century, we need new optical systems," Cui says.

In an unusual step, the mega science centre set up meetings, working groups, and a science advisory committee to solicit input from the wider astronomical community—"a first for Chinese astronomy," says Johannes Andersen, an astronomer at the University of Copenhagen. Astronomers who took part expressed concerns with the NIAOT design. "I found many scientific and engineering issues," says Donglin Ma, an optics scientist at Huazhong University of Science and Technology (HUST), in Wuhan, China.

One concern centres on the four-mirror design. Many astronomers fear the additional mirror will degrade sensitivity, or the ability to see faint objects, because photons are lost with each reflection. Cui counters that a new mirror coating developed in the United States promises 98% reflectivity. "There will be no problem" with the additional mirror, she says.

A second point of contention is how quickly the scope can shift from a wide-field survey mode to one that would focus on transient phenomena, such as gamma ray bursts and supernovae. With the complexity of the NIAOT design, astronomers worry the shift would be slow.

Finally, the astronomers want proven technology that will work reliably from the start. They note that LAMOST has fallen short of its primary goal: observing faint galaxies beyond the Milky Way. Cui says the issue is not with the telescope, but with increasing dust and humidity at the site, which now gets only 120 clear nights a year, down from more than 200 when LAMOST was being planned.

After reviewing the NIAOT design, Ma formed a group that began developing a rival design with just two secondary mirrors. The HUST team has received advice from outsiders like Jerry Nelson of the Lick Observatory at the University of California, Santa Cruz, an applied physicist who led the design of the 10-meter Keck telescopes in Hawaii and was the TMT project scientist. "We will be involved as consultants for the telescope and instruments to the extent we are asked," says Nelson, who passed away last week. But Cui, a senior scientist and CAS academician, refused to back down.

To resolve the impasse, the mega science centre had an international panel weigh the two alternatives. The nine-member panel, led by Andersen, met in Beijing on 19 and 20 April. Their report, which has been circulated among key personnel but not publicly released, firmly sides with advocates of a simpler design. It calls the mirror coating proposed by the NIAOT team "not yet proven technology" and says that atmospheric turbulence would prevent the image quality in the NIAOT design from living up to hopes. It also says that the telescope would have a hard time switching quickly between surveys and targeting transient objects.

The panel report concludes that the NIAOT optical system "cannot compete" with more standard designs like the HUST approach, "in terms of meeting scientific objectives, providing operational flexibility and keeping within a limited budget." With the panel's recommendation in hand, the mega science centre board decided on 19 May to proceed with the HUST design.

Cui is now reportedly lobbying CAS for a second review. But some astronomers are confident CAS will leave the matter in the hands of the mega science team. "We think the debate is over," says Suijian Xue, a vice director of CAS's National Astronomical Observatories in Beijing. He says that getting everyone to work on optimizing the three-mirror system is "the only way to unite the entire community." He also hopes that the bottom-up process the mega science centre followed will set a precedent that will help future science projects avoid clashes.