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UPDATED: July 13, 2009 NO. 28 JULY 16, 2009
Sky Watcher
China's largest optical telescope will soon come online to benefit the country and the world
By TANG YUANKAI
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Since the 1950s, scientists around the world have been looking for a way to build a telescope that can blend the two features.

LAMOST, with its large aperture and wide field-of-view, is being touted as the answer to the puzzle. "It's just a matter of money to build an even larger telescope of this kind, " Zhao said.

New horizons

It took 16 years to complete the LAMOST project.

Predicting the coming needs of international astronomy for large-scale spectra observation in the early 1990s, researchers at the Chinese Academy of Sciences including Wang Shouguan and Su Dingqiang proposed to develop the dual function telescope that would eventually become LAMOST. Experts like Cui Xiangqun and Chu Yaoquan detailed the plan after further research and analysis.

The plan originally called for a 40-meter tube that would create a dynamic high-accuracy mirror, which made a large-aperture and wide-field telescope possible.

In 1994, Cui and Chu presented the LAMOST concept during an international astronomical conference at the University of Cambridge. "That's the meeting where LAMOST began evoking strong international reactions," Cui recalled. He said LAMOST was praised by conference participants though there was also dispute and even discouragement. But the Chinese scientists made up their minds to make their vision a reality.

The LAMOST optical system consists of a spherical primary mirror at its southern end and a reflecting corrector with a focal length of 20 meters.

For eliminating the spherical aberration of the primary mirror, they needed to change the shape of the corrector mirror during tracking. Combining thin and segmented mirrors, the required shape of corrector mirror is formed of 24 thin plane segments. The main innovation of the project is the advanced active optic techniques that can focus 37 spherical segments.

The project also tested the limits of creating astronomical mirrors. The gigantic main bodies of the two mirrors mean gravity can change their shapes, affecting the imaging quality. It is common practice to splice small and thin mirrors to make a single large unit, and China leads the world in this technique. In LAMOST, scientists used the two large mirrors for the first time in a single optical system.

Both the primary mirror and the focal plane are fixed to the ground, the reflecting corrector at the northern end of the instrument. The light collected is reflected from the corrector to the primary mirror, and then reflected by it to form an image of the observed sky on the focal plane, which transfers the information to spectroscopic instruments and cameras.

As the primary mirror and the focal plane are fixed to the ground, LAMOST can only observe celestial objects when they pass the highest point in the sky. The aim of the instrument takes advantage of the time when light is least affected by the Earth's atmosphere and is the best time for observation.

Costing 235 million yuan ($34.5 million), the telescope is located 170 km northeast of Beijing at the Xinglong Observing Station of the Chinese Academy of Sciences' National Astronomical Observatories.

Fiber eyes

The telescope's focal plane has been wired with 4,000 fiber optic probes, which are all connected to dozens of spectroscopic instruments that can obtain the spectra of 4,000 celestial objects at a time.

"The advanced optic nerves of LAMOST make it the telescope that has the highest spectra-obtaining efficiency," said Zhao. Similar telescopes of the same kind around the world, such as the Anglo-Australian Telescope, can obtain 400 spectra at a time, while the U.S. Sloan Digital Sky Survey can measure 640 at a time.

LAMOST's parallel controllable fiber optic positioning system is one of the innovative technologies that took Chinese scientists four years to complete, according to Cui.

The focal plane is divided into 4,000 fiber-positioning units, each having an automatic arm with two stepping motors that allow it to act like a human's shoulder and elbow joints. Each arm is linked to a fiber. When the computer gives the order, the 4,000 arms position the fibers in their designated orientations.

In 2010 LAMOST will be undergoing testing and will be officially put to work in 2011. The telescope will complete sky surveys and astronomical catalog observations to share information on large-scale scientific projects.

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