The main feed of the primary dish of the Arecibo Observatory in Puerto Rico, viewed through the ground screen that surrounds the antenna. The ground screen is designed to screen out terrestrial radio interference that would otherwise disrupt the operation of the radio telescope. The radome suspended from the beam steering mechanism houses the Gregorian sub-reflector, which corrects for the non-parabolic shape of the main reflector. Apart from being the world's largest radio telescope for many years (305m in diameter), Arecibo is also known for its use in the search for extraterrestrial intelligence (SETI).