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Arecibo Observatory (NAIC, National Astronomy and Ionosphere Center) is a radio-astronomy observatory in Puerto Rico, which hosted a 305 m aperture radio telescope now unusable after a collapse. Since the collapse, the observatory continues to operate its other much smaller radio telescopes. Replacement telescopes for the collapsed telescope have been proposed, but the NSF currently declines to fund one and so far, financial commitments are a small fraction of what would be required.
The defunct telescope (Arecibo Telescope, commonly what is meant by Arecibo Observatory) is being demolished. It had a non-movable antenna facing straight up, installed in a similar-sized depression in the ground. It was completed in 1963 and used until 2020 when portions of it collapsed. For nearly all its life, it was the largest existing single-aperture telescope. It used its full 305 m when observing vertically, and could be aimed to roughly 20° from the zenith, but with some reduction in its effective aperture. I figure at best, that supported observation from just south of the celestial equator to roughly straight above Washington DC, about a third of the sky.
During its lifetime, the telescope received a number of upgrades. ALFA (The Arecibo L-band Feed Array), one of its receivers, detected radio at 1225 to 1525 MHz (nominally 1.4 GHz or 21-cm line), useful for HI observations. An ongoing pulsar search using ALFA was termed PALFA.