The Best of Creative Computing Volume 2 (published 1977)

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An Ear On The Universe (Arecibo Observatory in Puerto Rico, largest radiotelescope on Earth, SETI)
by John Lees

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An Ear n The Universe
by John Lees
University of Missouri-Rolla
Nestled in a natural limestone sinkhole in the jungle
covered mountains of northern Puerto Rico is the largest
radio telescope on Earth. Its reflector is a spherical bowl
1,000 feet in diameter - a surface area of 20 acres. The
telescope is steered by repositioning the 600 tons of
receiving and transmitting equipment, which is supported
50 stories in the air on cables anchored by three massive
reinforced concrete towers. Each of the towers is guyed to
ground anchors with five 3.25 inch steel bridge cables.

When the Arecibo reflector is used as a 2380 MHz S-band
radar transmitter its effective power output is 100 trillion
watts. When used as a receiver for the radar echoes its
sensitivity is one-100 million trillionth ot a watt; a span of
34 orders of magnitude.

The Arecibo Observatory in Puerto Rico is part of the
National Astronomy and Ionosphere Center operated by
Cornell University. Constructed in 1963 at a cost of $9
million, the telescope's reflector originally consisted of a
bowl of wire mesh supported by cables slung from the
edge of the reflector bowl. It was not thought that the
receiver-transmitter platform would be stable enough in
normal winds and temperature changes to make it
worthwhile to have an extremely accurate reflector bowl,
since the aerial platform was expected to sway as much as
one and one-half inches.

When in 1966 Puerto Rico lay in the path of Hurricane
Inez these original expectations proved to be far too
conservative. In the sixty-two mile-per-hour winds of the
hurricane the platform was observed to sway less than
half an inch. This meant that, under normal weather
conditions, the platform could be expected to sway less
than three-tenths of an inch. Because of these findings it
was decided to upgrade the accuracy of the reflector
curvature to match the stability of the receiver-transmitter

With the backing of the National Science Foundation
and the National Aeronautics and Space Administration
this task was begun. An additional $8.8 million was spent
to improve the accuracy of the reflector, improve the
receiver-transmitter platform, and increase the power of
the radar transmitter. The reflector surface now consists of
38,778 specially designed aluminum panels, each of
which has on it a white square used as a target for the
laser surveying system. This laser system permits the
entire surface of the reflector to be surveyed to an
accuracy of better than one millimeter. The panels are
individually adjustable to keep the reflector as spherical as
possible. The reconstruction was completed in November
of 1974.

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