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', which can be reached by this hanging stairway. The control center in on the left. The observatory is the primary location of the National Astronomy and Ionosphere Center, NAIC.