The Arecibo Observatory is a radio telescope just over 1,000 feet (305 m) across, the largest single radio telescope. The main metal collecting dish sits fixed a hemispherical karst sinkhole. Because it is fixed, a spherical reflector hangs 500 feet (150 m) over the dish on a 900 ton platform, known as 'the feed'. The observatory is the primary location of the National Astronomy and Ionosphere Center, NAIC.