Astronomical observatory near Arecibo, P.R., site of the world's largest single-unit radio telescope (as opposed to multiple telescope interferometers such as the
The telescope dish, 1,000 ft (300 m) across, is built into a valley; celestial sources are tracked across the sky by moving secondary structures suspended about 500 ft (150 m) above the dish. The observatory has produced detailed radar maps of the surface of Venus and near-Earth asteroids (see Earth-crossing asteroid ), made detailed studies of Earth's ionosphere , and made major contributions to studies of pulsar s and hydrogen gas in {{link=galaxy">galaxies .