Arecibo subreflector system. View of the 90-tonne dome which houses the gregorian subreflector system (centre left) of the Arecibo radio telescope at night. This radio telescope is situated in a natural crater in the mountains of Puerto Rico. The subreflector system with two mirrors is suspended 130 metres above the fixed dish. To scan the sky the dome is steered above the 305 metre dish. It 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.