Arecibo Observatory
Published on Jul 25, 2007 at 11:59 pm.
2 Comments.
Filed under astronomy, observatories, radio astronomy, wonders.
The National Astronomy and Ionosphere Center, aka the Arecibo Observatory, is the site of the largest single dish radio telescope (or any other telescope, for that matter) on Earth, and I wrote about it once before. The facility is located just outside of the town of Arecibo, in Puerto Rico. The telescope’s dish is 1000 feet (305 meters) in diameter. 
The receiver is suspended 450 feet (150 meters) above the dish. The dish itself is a spherical reflector, rather than a parabolic reflector. A parabolic reflector at this site would only be able to focus directly upwards. However a spherical reflector does not focus all of the incoming radiation to the same point like a parabolic reflector. So, the receivers only pick up signals from a portion of the dish at a time. But, this gives the telescope some flexibility in where to point. The receivers and secondary reflectors can move somewhat on a track at the bottom of the platform suspended above the dish. Thus, they can pick up signals reflecting off of different portions of the dish by simply moving a little ways. This permits the telescope to pick up signals up to 40 degrees off of the zenith, greatly expanding its range of operations.
But, you might wonder what keeps the reflector from filling up with water? After all, the thing is built in Puerto Rico, where it rains a lot. Well, that is simple. The panels making up the reflector are not solid. Rather they are perforated, so rain runs right through. But, sunlight also shines through, permitting vegetation to grow under the dish.
The facility, though famous as a radio telescope, is not solely used for radio astronomy, nor was that the original idea for the site. Rather, scientists at Cornell University wanted an instrument to study the Earth’s upper atmosphere. That can be done by observing scattered radar transmissions. So, the facility was first envisioned to transmit radar signals and to observe the scattering in the atmosphere. But, the original design would have allowed for only that use. The design finally adopted allows for other uses. The powerful radar transmitter has been used to beam radar waves across the Solar System to study asteroids and even other planets. But, the dish can be used without the radar transmitter as simply a giant reflector to monitor radio signals coming from space. It is in this mode as a radio telescope that the facility is most famous.
The Arecibo Observatory has been the site of several significant discoveries in astronomy. For example, it was at Arecibo that astronomers first measured the rotational rate of the planet Mercury, as well as the discovery of ice in Mercury’s polar craters, and the first radar mapping of the planet Venus. The first extrasolar planets were found at Arecibo (planets orbiting the pulsar B1257+12). Arecibo has been used to make detailed studies of the distribution of galaxies in the universe, and it has mapped asteroids passing near the Earth. Arecibo has been instrumental in understanding pulsars. And, it has, of course, been very important in understanding Earth’s atmosphere, particularly the ionosphere.
So, both for its impressive size and its impressive legacy of discoveries, I pick the Arecibo Observatory to be on my list of wonders of astronomy.
-Astroprof
Photos courtesy of the NAIC - Arecibo Observatory, a facility of the NSF
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