Arecibo Observatory (located near the town of Arecibo in Puerto Rico) is home to the world's largest radio telescope. The telescope consists of a 305-meter spherical dish which collects incoming radio waves and focuses them onto a group of receivers housed in a dome suspended from a platform some 400 feet from the bottom of the center of the dish. Three towers placed equidistantly around the dish provide support for the dome, which is suspended by an arrangement of cables.

The main reflector dish (the 305-m one) is actually made up of several thousand individual panels. When it comes time to check the alignment of the dish and correct it for shape, workers go out under the dish and manually adjust several thousand screws that are responsible for the positioning of each separate panel.

The surface of the reflector is constructed from a fine mesh, which is made possible by the extremely long wavelengths of the radio waves being collected. The mesh allows sunlight to pass directly through the dish, and produces a thriving ecosystem with surprisingly lush vegetation. (Supposedly it also has the perfect conditions for growing pot, but what with it being a nationally funded research facility, let's forget that for the moment.) However, contrary to the view painted in the movie Golden Eye -- parts of which were filmed at Arecibo -- the mesh does not hold water. (People have been known to call and ask what days the dish will be filled with water so they know not to come for the tour that day.)

On the observatory grounds are a number of housing units where visiting scientists can stay for the length of their observing run, a full-service cafeteria serving authentic (if rather greasy) Puerto Rican cuisine, and two office buildings. There is also a LIDAR facility, which uses a technique similar to radar to probe the ionosphere, only instead of radio waves, it makes use of light waves in the form of several high power lasers. There is also a trailer that is home to the SETI Pegasus Project which gathers data from the telescope for use with its SETI@home efforts.

The Arecibo Observatory Visitor's Center has a number of educational displays and an observation deck with a great view of the dish and dome. The Visitor's Center also shows a short documentary film on the day-to-day activities of the observatory on a regular basis throughout the day.

In the movie GoldenEye, Commander James Bond takes a sharp jab to the chin and tumbles backwards off a steel catwalk.  He catches himself in mid drop and clings perilously to the bottom of the line feed receiver suspended high above the aluminum dish of the Arecibo radio telescope.  The evil traitor Alec looms above leering with his boot resting on Bond's fingers.  Alec snarls wickedly then raises his boot and stomps...

Set on a 118 acre site in the limestone hills, of northwestern Puerto Rico, the Arecibo Observatory is the home of the world's largest and most powerful radio telescope.  The aluminum clad antenna dish is 305 meters in diameter and 51 meters deep.  It covers 8 hectares, about 26 football fields, with it's 38778 aluminum panels supported on a network of cables over a huge natural sinkhole.  The 630 metric ton triangular receiver platform is suspended 137 meters above the dish on 18 cables strung from three concrete towers that hold the receivers in position to within a fraction of a centimeter.  The receiver frame includes the 93 meter long bow-shaped azimuth arm on which the receivers are mounted and a 42 meter circular track that can rotate the entire azimuth arm.  This allows the receivers to focus as much as 20 degrees on either side of the center of the receiver dish.

The idea for the Arecibo Observatory originated in the late 1950's when William E. Gordon, a professor at Cornell University proposed the construction of the Arecibo telescope for the purpose of studying the properties of the Earth's upper atmosphere.  He calculated that an antenna roughly 300 meters in diameter would be necessary for his studies and hit on the idea of utilizing a natural depression in the Earth's surface as an economical platform for the telescope.  The karst geology around Arecibo in northwestern Puerto Rico proved ideal for the project and construction of the telescope began in 1960.  The telescope was completed in 1963 and Gordon's original project was completed successfully soon thereafter. 

The Observatory is operated by a staff of 140 at the site and another 15 at the National Astronomy and Ionosphere Center1 (NAIC) headquarters at Cornell University in Ithaca, New York.  It originally cost $9.3 million dollars to construct in 1963 and another $31 million has been invested in upgrades of the years.  The most recent upgrade was the Gregorian Dome that was installed in 1996. Arecibo's annual operating budget is about $10 million. 

How it works

Radio astronomy is the study of space through the use of radio waves outside the visible spectrum.  Radio waves discovered in 1888 and  by 1901 were used by Guglielmo Marconi to transmit the first transatlantic radio message.  Following this breakthrough, a myriad of practical uses for radio signals quickly followed.  In 1931, Karl Jansky a scientist at Bell Labs detected radio waves from space using a rotating antenna that he dubbed the "merry-go-round."   In 1965, Arno Penzias and Robert Wilson were awarded the Nobel Prize for physics in 1978 for their discoveries of radio signals from the beginning of the universe, the Big Bang.

The receivers at Arecibo Observatory process signals in the frequency range between 50 and 10,000 megahertz.  This corresponds to wavelengths between 6 meters and 3 centimeters.  The telescope and observe signals from as close as three kilometers above the Earth's surface to the most distant parts of the universe, 10 billion light years away.

The primary difficulty with radio astronomy is the weakness of the signals received from space.  To put this in perspective, consider that the combined strength of all the radio signals ever received by all the radio telescopes on Earth is roughly equivalent to the total force of a few raindrops striking the surface of a pond. Visitors to the Angel Ramos visitor center are asked to turn off their cell phones with the admonition that the signal from each cell phone is billions of times stronger than those being received by the telescope. 

Sphere versus Parabola.  

Most radio telescopes use a parabolic antenna to focus the energy received to a single point. The antenna is then pointed at the object of interest using a flexible mount. In the case of Arecibo, this approach would have severely limited the telescope's usefulness because, with the antenna fixed in the ground, the telescope would only be able to look straight up. The ingenious solution that Gordon proposed to solve this problem was to use a spherical reflector that is essentially looking equally in all directions at once, and a movable collector that would allow different parts of the visible sky to be observed.  Arecibo's moveable receiver poised above the antenna dish, combined with the rotation of the Earth, and the 600+ million mile journey that our Earth makes around the Sun each year provides and almost endless series of targets to observe.  An inherent advantage to radio telescopes is that they can operate day and night, and are not affected by cloud cover or other atmospheric conditions, however the constant increase in radio devices creates a similar radio wave pollution that clutters the "view" from Arecibo. 

The downside of this flexibility is that the much weaker reflected signals must be concentrated after being received.  Two methods are currently in use for this purpose, the 29 meter long Line Feed antenna, from which James Bond dangled in the movie Golden Eye, and the Gregorian feed system which uses two smaller dish reflectors to focus the received energy. The Gregorian Dome at Arecibo was named after James Gregory, one of the foremost mathematicians of the 17th century.  Gregory devised the first practical reflecting telescope which uses a parabolic primary mirror to collect light and a smaller secondary mirror to focus it for viewing.  This is the same basic method used to focus the weak radio signals received at Arecibo.

Areas of study

Like most expensive scientific research platforms, every available minute at the Arecibo Observatory is carefully scheduled to accommodate the many competing requests for telescope time. About 80 percent of the operating time at Arecibo observatory is allocated to radio astronomy. Radar astronomy represents 5 percent, and atmospheric studies take up the remaining 15 percent of the time. The search for extra terrestrial radio signals piggybacks on other studies and is in progress almost continuously. Some additional information on these areas of study is provided below.

Radio Astronomy

Most of the observing time at Arecibo is devoted to the reception, detection, processing and recording of electromagnetic radio wave signals from space.  Radio astronomy, is a very powerful and versatile technique that has contributed significantly to our knowledge of the universe. Some of the areas of study addressed include:

Pulsars. Pulsars are rapidly rotating neutron stars that emit strong pulsating beams of radio waves that are as visible to a radio telescope as a lighthouse beacon on a dark night. The first pulsar in a binary star was discovered at Arecibo by Russell Hulse and Joseph Taylor in 1974. In 1991, astronomers at Arecibo discovered the first planetary system outside our solar system. The rotating pulsar B 1257+12 was found to have three planets in orbit around it. 

Quasars. Quasars are thought to be the central regions of young galaxies. They appear as relatively small objects that are releasing enormous amounts of energy, up to 100 times the energy of an entire galaxy.  The quasars observed so far are at enormous distances from Earth and appear to be receding at speeds up to 95 percent of the speed of light.  The radio waves received at Arecibo from quasars may have been en route for over ten billion years.

Galactic studies. Radio telescopes like Arecibo are important tools for studying the creation and lifecycle of the huge clusters of stars known as galaxies.  In the late 1970's, Arecibo contributed to the discovery that in addition to the millions of stars we can see, galaxies contain another component called, "dark matter," that astronomers now believe comprises the bulk of all matter in the universe.  Dark matter cannot be observed directly but its presence and characteristics and be inferred by the effects it has on observable stars and gas.  

Atmospheric studies  

Atmospheric scientists focus much of their attention on the layer of the Earth's atmosphere 50 kilometers above the surface called the ionosphere.  The ionosphere contains mostly neutral gases and ionized oxygen, hydrogen and helium.  These electrically charged particles are affected by the Earth's electrical and magnetic fields as well as geomagnetic events on the sun and metallic particles from falling meteors. The influence of events in the ionosphere on the lower atmosphere and the Earth's climatology are still not entirely understood. 

Radar astronomy

Radar astronomy, also a focus of study at the Arecibo Observatory, measures reflected radio signals to detect the size and motion of planets, comets and asteroids.  Radar signals are transmitted from the observatory into space, then the small fraction of reflected energy is measured by the telescope. Radar astronomy is a very flexible tool that is used for a wide variety of investigations. Recent upgrades at Arecibo allow the telescope to transmit up to one million watts of energy into space for the study of planets, asteroids and other objects in our solar system.  By bouncing radar pulses off these objects, we can create very detailed surface maps, like those used to help NASA determine where on Mars to land the Viking mission, obtain information about the movements of near space objects like meteors and even determine the chemical composition of distant planets.  Radar waves penetrate cloud cover and have made it possible to study the surface of planets with cloud cover such as Venus and Jupiter.  In 1964, Arecibo's radar capabilities allowed astronomers to measure the rotation of Mercury for the first time.

Search for extra terrestial Intelligence

The Arecibo telescope provides many megabytes of data every day that is used in the search for other life in the universe. There are Search for Extra Terrestrial Intelligence (SETI) projects active at the privately funded SETI-Institute/Project Phoenix2 in California,  as well as at Harvard and Berkley universities. The SETI@Home project managed by the Space Sciences Laboratory at the University of California, Berkeley makes use of hundreds of thousands of volunteers to perform the massive amount of computations necessary to scan the universe for signals.  To reduce the cost of the project, the SETI@Home antenna is piggybacked on the main azimuth arm at Arecibo so that it collects information from whatever part of the sky is being observed. 

In the movie Contact, Jodi Foster plays the SETI astronomer Ellie Arroway. Ellie smiles broadly as she looks across the gaping spherical valley sheathed in shiny aluminum panels below her.  Above, the graceful azimuth arm of the huge Arecibo radio telescope slowly and silently glides into the next viewing position above her.  Somewhere out there a signal from another world is coming her way.

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1 National Astronomy and Ionosphere Center (NAIC): http://www.naic.edu/
2 Arecibo journal by a SETI@Arecibo scientist: http://www.space.com/searchforlife/phoenix_diary_five_010319.html

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