A dish in a sinkhole
In a deep, circular karst depression in China's Guizhou province sits the Five-hundred-meter Aperture Spherical Telescope (FAST). Its enormous dish, with a diameter of 500 meters and a receiving area equal to 30 football fields, is the largest filled-aperture radio telescope on Earth. Construction began in March 2011, and the telescope saw its "first light" in September 2016, becoming fully operational in January 2020.
The site was chosen specifically for its geology. The Dawodang depression, a natural sinkhole, provided the ideal shape to support the vast structure. The surrounding karst hills also provide a natural shield against radio-frequency interference from human activities. The reflecting surface is not a solid piece but an active surface composed of 4,450 individual triangular aluminum panels, each about 11 meters on a side. Below the panels, a system of 2,225 actuators can adjust the shape of the reflector. The system allows the telescope to deform a 300-meter section of the spherical dish into a precise parabola, effectively aiming it at different points in the sky. Suspended 140 meters above the dish by six cables is a 30-tonne feed cabin, which houses the sensitive receivers that collect the focused radio waves.
Scanning the cosmos
FAST is the most sensitive single-dish radio telescope in the world. Its primary scientific missions include scanning for pulsars, studying neutral hydrogen in distant galaxies, detecting interstellar molecules, and participating in the Search for Extraterrestrial Intelligence (SETI). The telescope operates across a frequency range of 70 MHz to 3.0 GHz.
One of its most significant accomplishments is the discovery of new pulsars—the dense remnants of massive stars. Since commencing operations, FAST has identified over 1,000 new pulsars, more than other telescopes combined during the same period. These discoveries include faint pulsars, millisecond pulsars, and pulsars in binary systems, which provide data for understanding neutron star evolution and detecting gravitational waves. The telescope's sensitivity allows it to detect pulsars with a flux density down to 5 microjanskys, an order of magnitude fainter than what was previously possible. The data generated by FAST's observations is immense, and the discoveries are expected to continue for decades, expanding knowledge of the radio universe.