The Arecibo Observatory was built on the island of Puerto Rico in the early 1960s. It was owned by the US National Science Foundation. For many years it was the largest radio telescope in the world. Its huge main dish was over 300 meters wide. It was so heavy, it had to be set into a large natural hole in the ground. Arecibo closed in 2020.
Astronomers used Arecibo to carry out lots of studies. These included measuring the speed at which planets rotate and hunting for pulsars. It was also part of the SETI project to search for signals from aliens!
Arecibo could observe for 24 hours a day. Unlike telescopes which collect visible light, radio telescopes can be used during the daytime as well as at night. Arecibo could collect data from the large region of the sky above the dish. The visible objects changed through the day as the Earth rotated.
Some radio telescopes can turn to point at specific parts of the sky. The Arecibo dish was too huge to move. Instead, scientists and engineers designed a detector which could move. 18 cables held the 900 tonne detector platform 150 metres above the dish. Breaks in two of these cables led to the closure of the telescope in 2020.