Aerial view of the Arecibo radio telescope which is 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 is steered over the surface of the dish in order to scan different regions of the sky. However, only a small area of the sky may be scanned as it transits above the telescope. Nowadays smaller, fully steerable radio dishes are preferred to large, fixed dishes.