Arecibo radio observatory. View of the underside of the dish antenna of the Arecibo radio telescope, the world's largest. Built into a volcanic crater in Puerto Rico, Arecibo's dish is made of 38,800 aluminium panels which reflect radio waves up to a receiving antenna suspended over it (at upper centre). The dish is 305 metres across and has an area of about 70,000 square metres. The dish is so large it cannot be steered, and instead the antenna is moved over it to scan the sky. Arecibo can send and receive signals to examine planets and asteroids and also analyse Earth's upper atmosphere. Photographed after Arecibo's upgrade in 1997.