Arecibo radio observatory. Illustration of the Arecibo radio telescope, situated in a natural crater in the mountains of Puerto Rico. With a diameter of about 305 metres Arecibo's telescope has the largest fixed, single dish devoted to radio observations. The dish reflects radio waves up to a receiving antenna (upper centre) which is situated 130m above the dish and sustained by three pylons. The receiver can be steered over the surface of the dish in order to scan different regions of the sky. Arecibo is one of the radio antennae used by NASA to scan the sky for signals from alien civilisations.