Arecibo

The Arecibo Observatory is the largest single-aperture telescope in the world. Completed in 1963 in Puerto Rico, its spot was chosen due to a very conveniently placed karst sinkhole. Its dish is 1000 feet (305 m) in diameter, and is spherical rather than parabolic. The advantage of having a spherical dish as opposed to a parabolic one is due to the fact that the error of a spherical dish is the same in every direction. A parabolic reflector can only see in one direction very well, vastly limiting its range. A spherical reflector also permits the telescope to employ three areas of research: radio astronomy, aeronomy, and radar astronomy (within the solar system).

Its design originally included a tower smack dab in the center of the dish, but was later scrapped for a suspended feed for obvious reasons. This also allowed for greater mobility, and protection from harsh weather. The idea came from Helias Doundoulakis, who was granted a U.S. Patent for the idea. When it was completed, Arecibo's surface was covered in wire mesh, and had a maximum operation frequency of about 500 MHz. It was updated in 1974, replacing the wire mesh with individually adjustable aluminum panels, increasing the maximum operation frequency by ten fold. Upgraded again in 1997, the aluminum panels were replaced with a Gregorian reflector (using concave parabolic and elliptical mirrors) system, and now the maximum operating frequency can reach 10 GHz. A new transmitter was also installed.

We have Arecibo to thank for many new astronomical discoveries over the past 50 years. Less than a year after it was completed, it was determined the rotation rate of Mercury was 59 days, not 88 days as it was thought to be. It helped provide evidence for neutron stars. In 1968, the Crab Pulsar was found to have a periodicity of 33 milliseconds. The first binary pulsar was discovered in 1974 using Arecibo. In 1982 the first millisecond pulsar PSR B1937+21 was discovered, which spins at a rate of 642 times per second. Arecibo was also the first telescope to directly image an asteroid. It also helped to discover the first exoplanets! Another interesting tidbit is that Arecibo was used to transmit a binary radio message directed at M13, in an attempt to communicate with aliens.

Another exciting thing about Arecibo is the opportunity for undergrad students to participate in research at the observatory during the summer. http://www.naic.edu/reu_program.html Students spend 10 weeks during the summer months working with astronomers, and even get to conduct their own independent observational experiments. Of course, spending a summer in Puerto Rico is also nice.