Arecibo Observatory - Design and Architecture

Design and Architecture

The Arecibo telescope was built between the summer of 1960 and November 1963, by William E. Gordon of Cornell University, who intended to use it to study Earth's ionosphere. Originally, a fixed parabolic reflector was envisioned, pointing in a fixed direction with a 150 m (500 ft) tower to hold equipment at the focus. This design would have limited its use in other areas of research, such as planetary science and radio astronomy, which require the ability to point at different positions in the sky and to track those positions for an extended period as Earth rotates. Ward Low of the Advanced Research Projects Agency (ARPA) pointed out this flaw, and put Gordon in touch with the Air Force Cambridge Research Laboratory (AFCRL) in Boston, Massachusetts, where one group headed by Phil Blacksmith was working on spherical reflectors and another group was studying the propagation of radio waves in and through the upper atmosphere. Cornell University proposed the project to ARPA in the summer of 1958 and a contract was signed between the AFCRL and the University in November 1959. Cornell University and Sears published a request for proposals (RFP) asking for a design to support a feed moving along a spherical surface 435 feet (133 m) above the stationary reflector. The RFP suggested a tripod or a tower in the center to support the feed. At Cornell University on the day the project for the design and construction of the antenna was announced, Gordon had also envisioned a 435 ft (133 m) tower located in the center of the 1,000 ft (300 m) reflector for the feed's support.

George Doundoulakis, who directed research at General Bronze Corporation in Garden City, New York, along with Sears, who directed Internal Design at Digital B & E Corporation, New York, received the RFP from Cornell University for the antenna design, and studied the idea of suspending the feed with his brother, Helias Doundoulakis, a civil engineer. George Doundoulakis identified the problem that a tower or tripod would have presented around the center, the most important area of the reflector, and devised a more efficient, cost-effective approach by suspending the feed. He presented his proposal to Cornell, by using a doughnut truss suspended by four cables from four towers above the reflector, and providing along its edge a rail track for the azimuthal positioning of the feed. A second truss, in the form of an arc, or arch, was to be suspended below, which would rotate on the rails through 360 degrees. The arc also provided rails onto which the unit supporting the feed would move to provide for the elevational positioning of the feed. A counter-weight would move symmetrically opposite to the feed for stability, and the entire feed could be lowered and raised if a hurricane were present. Helias Doundoulakis ultimately designed the cable suspension system which was adopted in the final construction. Although the present configuration is substantially the same as the original drawings by George and Helias (with the exception of the suspension of the feed positioning assembly by three towers rather than the four towers in the original proposal), the U.S. Patent office granted Helias a patent for the brothers' innovative idea. William J. Casey, later to be the director of the Central Intelligence Agency under President Ronald Reagan, was also an assignee on the patent.

Construction began in the summer of 1960, with the official opening on November 1, 1963. As the primary dish is spherical, its focus is along a line rather than at a single point (as would be the case for a parabolic reflector); therefore, complicated line feeds had to be used to carry out observations. Each line feed covered a narrow frequency band (2–5% of the center frequency of the band) and a limited number of line feeds could be used at any one time, limiting the flexibility of the telescope.

The telescope has been upgraded several times. Initially, when the maximum expected operating frequency was about 500 MHz, the surface consisted of half-inch galvanized wire mesh laid directly on the support cables. In 1974, a high-precision surface consisting of thousands of individually adjustable aluminum panels replaced the old wire mesh, and the highest usable frequency was raised to about 5,000 MHz. A Gregorian reflector system was installed in 1997, incorporating secondary and tertiary reflectors to focus radio waves at a single point. This allowed the installation of a suite of receivers, covering the whole 1–10 GHz range, that could be easily moved onto the focal point, giving Arecibo a new flexibility. At the same time, a ground screen was installed around the perimeter to block the ground's thermal radiation from reaching the feed antennas, and a more powerful 2,400 MHz transmitter was installed.