Decoding China's new 'giant eye' — telescope JUST

XINING -- China's astronomical research is poised for a transformative leap with the construction of the Jiao-tong University Spectroscopic Telescope (JUST), the nation's first large, general-purpose spectroscopic telescope, commencing on Wednesday.

The 4.4-meter-aperture telescope, led by Shanghai Jiao Tong University (SJTU) and situated at the Lenghu astronomical observatory in Qinghai province, Northwest China, is expected to advance time-domain astronomy while strengthening China's independent observational capabilities.

JUST, with its 4.4-meter primary mirror composed of 18 lightweight hexagonal segments, will feature three cutting-edge instruments: a multi-object fiber spectrograph to explore the "dark universe," an integral field spectrograph to track cosmic transients, and a high-resolution spectrograph to detect exoplanets.

Its design prioritizes rapid response, high light-gathering power, and precision-qualities that will empower China's astronomical research and the global astronomical exploration as well.

WHY LENGHU?

For decades, Chinese astronomers have relied on international partnerships or costly access to foreign telescopes for advanced observations.

"Purchasing telescope time overseas is costly and carries the risk of delays, which can disrupt research progress," said Yang Xiaohu, deputy director of the Tsung-Dao Lee Institute of the SJTU and lead scientist of the JUST project.

"Owners of foreign telescopes prioritize their own teams, leaving us with less favorable observation slots. If bad weather occurs during our allocated time, the data becomes unusable."

This reliance has consistently emphasized the need for a domestically developed facility.

The choice of Lenghu town, where the observatory is built in the Serteng Mountain at 4,320 meters above sea level, was no accident. This remote region in Qinghai's Mangya city boasts atmospheric stability, minimal light pollution, and dryness comparable to world-class sites like Hawaii's Mauna Kea or Chile's Atacama Desert.

It's a sanctuary for stargazers.

"Lenghu's exceptional seeing conditions, like low atmospheric turbulence and water vapor, make it a goldmine for astronomy," Yang explained.

After rigorous site surveys, the SJTU selected Lenghu in 2022 as JUST's permanent home.

SCIENTIFIC MISSIONS

JUST's scientific missions span three key frontiers. First, its high-density fiber array — 2,000 fibers optimized for high-density galaxy clusters — will map the universe's large-scale structure.

While the US-based Dark Energy Spectroscopic Instrument (DESI) deploys 5,000 fibers, JUST's higher fiber density will achieve unprecedented sampling rate in galaxy clusters, where current surveys capture only 10-20 percent of targets.

Second, JUST's agility will make it indispensable for studying transient phenomena like supernovae — cataclysmic stellar explosions that forge heavy elements essential for life.

"Understanding supernovae provides insights into the specific processes of stellar evolution," Yang said.

He explained that supernovae represent a dynamic explosive event. Capturing this event and confirming it through spectroscopy allows for the analysis of elemental composition, velocities, and classifications, which is crucial for studying the chemical evolution of the universe, the life cycles of stars, and the origins of heavy elements.

Additionally, supernovae function as standard candles, which are astronomical objects having a known luminosity due to a characteristic quality possessed by that class of objects, making them vital for probing the universe's expansion.

While China's existing facilities, such as the Wide Field Survey Telescope (WFST) detect dozens of supernova candidates nightly, follow-up spectroscopy remains a bottleneck.

JUST will fill this gap, providing timely spectroscopic confirmation to guide deeper analysis, said Yang.

Third, the telescope's high-resolution spectrograph will scrutinize nearby stars for exoplanets, aiding the search for Earth-like worlds like a "second solar system," he added.

A DECADE OF INNOVATION

Scheduled for completion in 2032 after two phases, JUST's decade-long timeline reflects its technical ambition. Phase I from 2023 to 2027 focuses on telescope assembly and instrumentation, with a precursor telescope achieving "first light," or the first time a telescope lens is exposed to the night sky for viewing, by 2027.

Phase II will upgrade software and hardware to maximize data output from 2028 to 2032.

The challenges are immense. The primary mirror's active optics system must correct distortions caused by gravity and temperature shifts in real time.

Meanwhile, the fiber positioning system demands micron-level precision. "Aligning 2,000 fibers is like using needles to hit targets one-tenth the diameter of a human hair simultaneously," Yang said.

Software poses another hurdle. JUST's control system must dynamically reroute fibers to prioritize urgent targets, such as a newly spotted supernova, without disrupting scheduled surveys.

"We aim to reprogram 2,000 fibers within two minutes," Yang emphasized. "Every second lost impacts efficiency."

Once operational, JUST is expected to generate over 1 petabyte (PB) of data annually, equivalent to 200,000 hours of HD cosmic footage, said Yang.

For Yang and his team, JUST is more than a telescope.

"It's a gateway to answering fundamental questions: Where did we come from? Are we alone in the Universe?" he said.