The Arecibo Observatory is the largest single-dish radio telescope in the world, at around 305 metres in diameter, and is located on the island of Puerto Rico in the Carribbean sea. It was built in the early 1960's and has conducted a whole range of astronomical observations, such as measuring the rotation rates of planets, discovering pulsars (rapidly rotating neutron stars), and even searching for signs of extra-terrestrial life as part of the SETI programme.

Because of its size, the dish at Arecibo works in a slightly different way to normal radio telescopes, in that it is fixed within the ground and doesn't move about. It is able to see regions of the sky above it by having a moveable detector suspended 150 metres above the base of the dish. This 900 tonne platform is held aloft by eighteen cables and can be moved around by electric motors with millimetre precision.

In this way, the dish can receive signals from nearly a quarter of the visible sky, and not just straight up. Also, because the Earth rotates every 24 hours, the telescope can actually see a fairly large region of the sky throughout the day and night. Remember, radio telescopes can work during the daytime because they do not observe in normal light.