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.