Those who see the Arecibo radio telescope for the first time are astounded by the enormousness of the reflecting surface, or radio mirror. The huge "dish" is 305 m (1000 feet) in diameter, 167 feet deep, and covers an area of about twenty acres. The surface is made of almost 40,000 perforated aluminum panels, each measuring about 3 feet by 6 feet, supported by a network of steel cables strung across the underlying karst sinkhole. It is a spherical (not parabolic) reflector .
Suspended 450 feet above the reflector is the 900 ton platform. Similar in design to a bridge, it hangs in midair on eighteen cables, which are strung from three reinforced concrete towers. One is 365 feet high, and the other two are 265 feet high. All three tops are at the same elevation. The combined volume of reinforced concrete in all three towers is 9,100 cubic yards. Each tower is back-guyed to ground anchors with seven 3.25 inch diameter steel bridge cables. Another system of three pairs of cables runs from each corner of the platform to large concrete blocks under the reflector. They are attached to giant jacks which allow adjustment of the height of each corner with millimeter precision.
Just below the triangular frame of the upper platform is a circular track on which the azimuth arm turns. The azimuth arm is a bow shaped structure 328 feet long. The curved part of the arm is another track, on which a carriage house on one side and the gregorian dome (installed in 1996) on the other side can be positioned anywhere up to twenty degrees from the vertical. Inside the gregorian dome two subreflectors (secondary and tertiary) focus radiation to a point in space where a set of horn antennae can be positioned to gather it. Hanging below the carriage house are various linear antennas each tuned to a narrow band of frequencies. The antennas point downward and are designed specially for the Arecibo spherical reflector. By aiming a feed antenna at a certain point on the reflector, radio emissions originating from a very small area of the sky in line with the feed antenna will be focused on the feed antenna.
Attached to the antennas are very sensitive and highly complex radio receivers. These devices operate immersed in a bath of liquid helium, to maintain a very low receiver temperature. At such cold temperatures the electron noise in the receivers is very small, and only the incoming radio signals, which are very weak, are amplified. The Arecibo system operates at frequencies from 50 megahertz (6 m wavelength) up to 10,000 megahertz (3 cm wavelength).
A total of 26 electric motors control the platform. These motors drive the azimuth and the gregorian dome and carriage house to any position with millimeter precision. The tertiary reflector can be moved to improve focusing, receivers are moved into focus on a rotating floor inside the gregorian and the dynamical tie downs activate as needed to maintain platform position. The 1 MW planetary radar transmitter located in a special room inside the dome, directs radar waves to objects in our solar system. Analyzing the echoes provides information about surface properties and object dynamics.
This giant telescope has scrutinized our atmosphere from a few kilometers to a few thousand kilometers where it smoothly connects with interplanetary space. With its radar vision it studies the properties of planets, comets and asteroids. In our Galaxy it detects the faint pulses emitted hundreds of times per second from pulsars. And from the farthest reaches of the Universe quasars and galaxies emit radio waves which arrive at earth 100 million years later as signals so weak that they can only be detected by a giant eye like this one.
The giant size of the reflector is what makes the Arecibo Observatory so special to scientists. It is the largest curved focusing antenna on the planet, which means it is the world's most sensitive radio telescope. Other radio telescopes may require several hours observing a given radio source to collect enough energy for analysis whereas at Arecibo this may require just a few minutes of observation.