The main antenna dish of the Arecibo Observatory in Puerto Rico. The large antenna reflector (305m in diameter) was the world's largest radio telescope for many years and is suspended in a natural hollow in the limestone scenery. The radio telescope's main feed can be seen above the antenna. The radome houses the Gregorian sub-reflector, which corrects for the non-parabolic shape of the main reflector. Surrounding the antenna is a ground screen designed to screen out terrestrial radio interference that would otherwise disrupt the operation of the telescope. Arecibo is also known for its use in the search for extraterrestrial intelligence (SETI).