Construction Milestone Announced on Green Bank Telescope

The National Radio Astronomy Observatory announces completion of a major
construction milestone on the world’s largest fully steerable radio
telescope – the National Science Foundation’s Green Bank Telescope
(GBT). The last of 2,004 aluminum surface panels was recently installed
on the GBT’s two-acre (100 m x 110 m) collecting dish. The telescope is
located at NRAO’s Green Bank site, in rural Pocahontas County, West
Virginia.

The GBT will be used to study everything from the formation of galaxies
in the early universe, to the chemical make-up of the dust and gas
inside galaxies and in the voids that separate them, to the birth
processes of stars. In conjunction with other instruments, it will help
make highly accurate radar maps of some familiar objects in our own
solar system.

The GBT is an engineering marvel. At 485 feet tall, it is comparable in
height to the Washington Monument. It weighs 16 million pounds, yet by
swiveling the dish in both azimuth and elevation, it can be pointed to
any point in the sky with exquisite accuracy.

Additionally, the telescope’s two-acre collecting dish has many novel
features. Most radio telescopes in use today use receivers suspended
above the dish by four struts. These struts block some of the surface of
the dish, scattering some of the incoming radio waves from celestial objects
under study. The GBT’s offset feedarm has no struts to block incoming
radio waves.

The GBT also boasts an active surface. The surface of the dish is
composed of 2,004 panels. On the underside of the dish, actuators are
located at each corner (i.e., intersection of four panels). These
actuators are motors that move the surface panels up and down, keeping
the (paraboloid) shape of the dish precisely adjusted, no matter what
the tilt of the telescope.

The combination of its unblocked aperture and active surface promise
that the GBT will display extremely high sensitivity to faint radio
signals.

The GBT itself is not the only precious national resource in Green Bank.
The Observatory also administers the National Radio Quiet Zone, which is
centered on the NRAO Green Bank site. The Zone extends outward from
there, enclosing a land area of approximately 13,000 square miles. This
zone is the only one of its kind in the world. Inside it, anything that
might interfere with a radio telescope’s sensitive receivers – like
cellular phones and radio stations – are strictly regulated. This will
ensure the GBT clearer reception of radio waves from distant galaxies
and the other celestial objects it will study.

What remains before the GBT will begin probing the heavens for new
discoveries? First, the contractor building the telescope will complete
several tasks, including tests of the systems that move the dish.
Following successful completion of these tasks, NRAO can begin
outfitting the telescope with the sensitive radio-wave receivers it
designed and built especially for the GBT

If all goes according to the current plan, astronomers should be doing
exciting new science with the GBT by the first quarter of 2001.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.