The largest radio telescope in the world and the one used for the Berkeley SETI search is the Arecibo Radio Telescope, in northwestern Puerto Rico, near the town of Arecibo. The telescope has a fixed dish 305 meters (1000 feet) across that is built into a sinkhole in rugged Karst terrain. To imagine how big this dish is, it would take 10 billion normal bowls of cereal to fill the radio dish to the brim! Still hard to imagine? The large surface of this dish is over 20 acres with about 18 acres or 26 football fields of available aperture! The dish reflects and concentrates the weak celestial signals on the receiving antennas hung 450 feet above. Since the dish is fixed and can't swivel, the receiving antennas are mounted on a bow-shaped track that allows them to "look at" objects as far as 20 degrees from the zenith (directly overhead). This bow-shaped arm is itself mounted on a circular track to allow the antennas to follow an object as it moves across the sky due to the earth's rotation. These two motions give the telescope the ability to scan a good portion of the sky.
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The "Gregorian focus" is inside the dome. The line feed is the spear-like structure to the left of the dome.
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The telescope has two main antenna systems: SETI@home's line feed, and the giant domed "Gregorian" focus. The Gregorian is a new addition to the Arecibo Observatory. Inside the dome are two radio reflectors that will more finely focus the radio signals onto the detectors. Although the dome may look rather small in photographs, it's actually 85 feet in diameter!
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