The platform of the
Arecibo Observatory telescope is a large triangular scaffolding structure that houses all the receivers and most of the front-end
electronics for processing signals from the telescope. The
platform is
suspended from three towers surrounding the main dish, with each point
of the triangle directly connected by cables to one of the towers. One can
reach the platform either by taking a
cable car up (maximum of 4 people at
a time) or climbing the
catwalk, an at times frighteningly flimsy walkway that guides you up the several hundred feet from the top of a cliff to the base of the platform (maximum of 5 people at a time).
Receivers for the telescope are located in one of two structures affixed
to the base of the platform. The first is a large metal
Gregorian dome that was just installed in 1997, and the second is what is known as the
Carriage House, which is something of a holdover from the original
receiver system implemented when the telescope was first built. A large
arm attached to a circular track at the bottom of the platform allows both
structures to rotate to different locations above the
dish. As an
observer
rotates the receivers, he is actually steering the telescope to look at
different parts of the
sky by pointing at different parts of the dish.
(More explanation about this
later.) Observers can also position the
receivers at different points along the length of the arm, giving him
further steering flexibility.
Inside the dome are a variety of different receivers, each of which is
attuned to a certain range of
frequencies.
Astronomers observing at
the telescope may select from any of these receivers depending on what
portion of the radio wave
spectrum they are looking to study. Most
commonly used is the
L-band receiver which is centered on the 21-cm
neutral hydrogen emission line. Radio waves are focused from the main
reflector, up into a
secondary reflector housed in the roof of the dome,
down onto a
tertiary mirror, and finally up into the receivers.
The Carriage House on the other hand makes use of a much more primitive
technique for observing signals from a
spherical reflector (such as the
main dish of the telescope) by using a single line feed. The line feed
corrects for the fact that spherical reflectors focus incoming rays to a
line instead of to a point (the dome corrects for his using its
complicated system of multiple mirrors). However, the line feed is
only capable of viewing a very limited range of frequencies,
whereas the dome's receivers tend to have a much broader range. Attached
to the line feed are two different
radar transmitters, which astronomers can point out into the sky in order to observe the reflected pulses and thus try to put together pictures of how far-off objects look in space. The Arecibo
transmitter is
the most powerful non-
military-use radar in the world.
Beyond all of its nifty
electronics, though, the platform of the Arecibo telescope is one of the most amazing places to visit in the world. Watching the huge structures rotate, and walking around 450 feet above the ground with little more than a grating to support you is just plain exhilarating (not recommended for people with a
fear of heights). This
tour in and of itself made a
summer's worth of hard work at the observatory worthwhile. (Of course just the coolness of being in
Puerto Rico made it worthwhile, but this was icing on the
cake.)
Wow.