China Now Has the World’s Biggest Radio Dish

The 500-meter telescope will be dedicated to astronomical research and SETI.

Surrounded by limestone hills and hours from the closest highway, FAST took more than seven years to build, through mostly manual labor; the narrow, treacherous roads to the site kept heavy equipment from reaching it. (© Naoc)
smithsonian.com

The last bolt was secured on the Five-hundred-meter Aperture Spherical Telescope, or FAST, in July, officially making it the largest single-dish radio telescope in the world. Nestled in a natural karst depression in the southern Guizhou province, this vast bowl of perforated aluminum began observations in September. It is China’s attempt to race to the cutting edge of one field of astronomy.

“FAST has a truly stunning amount of collecting area,” says Tony Beasley, the director of the National Radio Astronomy Observatory, which operates telescope arrays in Chile and New Mexico. That raw sensitivity will make it a powerhouse for finding certain celestial objects, such as distant galaxies and pulsars—rapidly spinning stars that emit radio bursts at regular intervals. 

“At the moment, there are about 2,000 pulsars we know about,” Beasley says. “By the time FAST has been observing for 10 or 15 years, we’re going to have 20,000 pulsars.” Astronomers could use them to detect gravitational waves by observing large sets of pulsars, all the same distance away, and measuring the times the radio signals take to arrive. Delays could indicate a wave-related distortion in space-time.

FAST will have many of the same capabilities as the Arecibo Observatory, says Chris Salter, a senior research associate at the Puerto Rican facility, but because the Chinese telescope will view a different part of the sky, the observations of the two instruments will be complementary rather than competitive. “As we lose the source at Arecibo,” Salter says, “it could then be tracked by FAST, giving you much better coverage than you would get with just one telescope.”

FAST has also announced plans to make observations along with the Green Bank telescope in West Virginia and Parkes Observatory in Australia for the Breakthrough Initiative, a coordinated search for evidence of intelligent life in the universe (“Listen to the Nearest Million Stars,” June/July 2016). 

FAST is a welcome contributor to radio astronomy, but its enormous size is a bit misleading when it comes to capability. It’s a fixed telescope, so the dish can move to different targets in the sky only as Earth rotates. It can, however,  expand its view by using actuators beneath the dish to move individual panels into a parabolic shape that focuses the incoming radio waves onto a receiver suspended overhead. This “parabolizing,” though an engineering marvel, can be done with only part of the dish, giving it an effective aperture of about 300 meters—the same as Arecibo.

And FAST shares a weakness with Arecibo: Large, single-dish telescopes aren’t ideal for imaging at the highest resolutions. For that, most radio observatories today rely on interferometry, which combines the input from many small telescopes spread over a distance. Even at its full aperture, FAST is still relatively small compared to the Very Large Array in New Mexico, which uses 27 dishes to get an aperture of 22 miles. However, large dishes are essential for very-long baseline interferometry, which connects observatories around the world to create effective apertures several thousands of miles wide.

FAST is undoubtedly a scientific spectacle, and it will certainly discover a trove of objects in the sky, but it’s not exactly the leap ahead for radio astronomy that the colossal size of the structure might indicate.

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About Dennis Hollier

Dennis Hollier is a science writer and incrementalist, trying to make sense of the world one story at a time.

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