Five-hundred-meter Aperture Spherical Telescope

A bowl in the karst

The idea of using one of Guizhou's natural karst depressions as the bowl of a very large telescope was proposed in the 1990s. Construction at the Dawodang depression in Pingtang County began in March 2011, and the last panel was installed on 3 July 2016. First light came on 25 September 2016, and the first two new pulsars were announced in August 2017. Formal operations began on 11 January 2020. Nan Rendong was its chief scientist and engineer until his death in 2017.

Terrain does much of the structural work: the ground holds the bowl, and a net of about 6,700 steel cables supports the 4,450 aluminium panels that form the reflector.

A surface that moves

FAST cannot tilt like a conventional dish. Instead, a patch of about 300 metres across is formed into a paraboloid by actuators that pull on the cable net, and the patch moves across the bowl as the telescope tracks a source, while the receiver cabin hangs from cables on six towers. The surface is reshaped continuously, with each of the 2,225 actuators adjusting hundreds of times per observation.

This gives the telescope the sensitivity of a 300 metre dish with the reach of a wide zenith angle.

Pulsars and a quiet zone

By November 2024 FAST had discovered more than a thousand pulsars. It surveys neutral hydrogen in the Milky Way and nearby galaxies, listens for fast radio bursts and searches for signals of extraterrestrial origin. Since 2021 it has been open to proposals from astronomers worldwide. A radio quiet zone around the telescope required relocating residents who lived within five kilometres, which was a cost to local people that the project's supporters and critics both discuss.

A telescope and its community

FAST's location was chosen for its natural bowl and for its radio silence, and its surroundings were reshaped to protect it: villages were relocated, roads restricted and a core zone kept free of transmitters. The telescope's discoveries, such as pulsars in globular clusters and a population of fast radio bursts, show the return on the investment, but the social cost is part of the record. Large instruments in inhabited areas need agreements with the residents, and the quality of those agreements affects how long the instrument can work without friction.

Instruments and technologies

• Active reflector A cable net with 4,450 triangular panels, each moved by actuators to form a 300-metre paraboloid.
• Feed cabin A cabin suspended from six cable towers, with a Stewart platform for fine positioning of the receiver.
• 19-beam receiver A receiver with 19 horns for surveys of the sky at 1.4 GHz.

The people

• Nan Rendong Chief scientist and engineer of FAST; died in 2017.

Timeline

• 2011 Construction begins in March.
• 2016 Last panel installed on 3 July; first light on 25 September.
• 2017 First two pulsar discoveries in August.
• 2020 Formal operations begin on 11 January.
• 2024 Pulsar discoveries pass 1,000 by November.

Key lessons for observation today

• Let terrain be structure The bowl is part of the telescope. Siting on existing geology saves cost and builds in stability.
• Reshape the instrument, do not move it The surface adapts to the target. Adaptive hardware replaces mechanical size.
• Count the cost to the neighbours A quiet zone has a human cost. Spectrum protection must be agreed with the people it affects.

Sources

• Wikipedia, Five-hundred-meter Aperture Spherical Telescope
• NAOC, World's largest radio telescope starts formal operation
• Wikipedia, Nan Rendong