Arecibo radio observatory. Aerial view of the Arecibo radio telescope, with its gregorian subreflector system (centre). This radio telescope is situated in a natural crater in the mountains of Puerto Rico. The subreflector system has two mirrors and is housed in a 90-tonne dome receiver seen suspended 130 metres above the fixed dish. To scan the sky the dome is steered above the 305 metre dish. The Arecibo radio telescope is used by the SETI (Search for Extraterrestrial Intelligence) institute amongst others. SETI is an organisation dedicated to investigating evidence of intelligent civilisations elsewhere in the universe.