The big dish is so huge (1000 ft. across - compare it to the buildings in the lower right corner of the photo!) that it can't possibly be aimed - it can only point straight up. By making the dish spherical instead of parabolic, the need for aiming the dish is removed. Instead, an antenna at the focus can be aimed at different parts of the dish to choose a viewing direction. Of course, the dish won't focus perfectly like a paraboloid, but if we characterize the circular profile of the dish as the circle of curvature of a parabola at the vertex (see above figure), we see that it nearly coincides with the parabola in the neighborhood of the vertex. Thus it focuses almost as well. In fact, the "official" focal length of a spherical reflector (for parallel rays) is defined to be the focal length of the parabola I just mentioned. For a parabola, "focal length" means the distance from the vertex to the focus. If the radius of the sphere is r, then the focal length p is given by

If you're taking Math 261, you can easily prove this yourself (Hint: Start with a parabola y = x²/(4p) and show that the radius of curvature r at the vertex is equal to twice the focal length p). Or, if you like, look here for an easy proof. According to information on the spherical dish provided by NAIC, the radius of curvature r is about 870 ft., so the focal length p is approximately 870/2 = 435 ft. Thus the huge, 600 ton antenna platform (the white triangular structure in the photo) is suspended more than 400 ft. above the bottom of the dish!

LITTLE GREEN MEN

"Little green men" is a phrase people commonly use when referring jokingly to intelligent beings from outer space. However, there are many scientists who don't think it's a joke (although such beings are unlikely to be men, green or otherwise). In addition to traditional radio astronomy, scientists have used the big dish at Arecibo to try to detect alien civilizations. The University of California at Berkeley program SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) is an ongoing scientific research effort aimed at detecting radio signals from extraterrestrial civilizations. The project is the world's only ``piggyback" SETI (search for extraterrestrial intelligence) system, operating alongside simultaneously conducted conventional radio astronomy observations.

In fact, the Arecibo observatory is so famous for the SETI research done there that it has developed a bit of a mystique. The hit TV program The X-Files had an episode entitled ``Little Green Men" (second season, episode one) which supposedly took place there.

Agents Scully and Mulder face the unknown.	Mulder squares of with aliens at Arecibo.

One of the first radio indications of little green men was detected at Cambridge University in 1967 by then graduate student Jocelyn Bell, now Jocelyn Bell-Burnell. Ultimately, Bell and her thesis adviser, Antony Hewish, found four sources of regularly-pulsed radio signals, called LGM-1, LGM-2, LGM-3, and LGM-4. "LGM," of course, stands for "little green men." The scientists eventually eliminated the possibility that the signals came from intelligent beings on a planet somewhere. They had, however, made one of the most important discoveries in astrophysics, namely, pulsars - rapidly spinning remnants of gigantic star-explosions called supernovas. Hewish was awarded the Nobel Prize for this discovery, and his former student is known as the woman who discovered pulsars.

But what about extraterrestrial intelligence? There's no radio evidence yet, but there's hope. For instance, we assume that aliens will need planets to live on, and extrasolar planets are now being found rather often, including "pulsar planets" recently discovered using the big dish at Arecibo. Also promising is the recent evidence for ancient life on Mars. Even though the jury is still out on extraterrestrial intelligence, I'm sure that radio astronomers, whatever their research interests, would at least agree with X-Files agents Scully and Mulder that