National Radio Astronomy Observatory, Green Bank, WV

Faraday Cage demonstration platform, NRAO, May 2012

The Green Bank (Radio) Telescope

The Robert C. Byrd Green Bank Telescope (GBT) is the world’s largest fully steerable radio telescope. It is part of the National Radio Astronomy Observatory (NRAO) site at Green Bank, West Virginia, USA. The telescope honors the name of the late Senator Robert C. Byrd.

The GBT operates at meter to millimeter wavelengths. Its 100-meter diameter collecting area, unblocked aperture, and excellent surface accuracy provide excellent sensitivity across the telescope’s full 0.1 – 116 GHz operating range. The GBT is fully steerable, and 85% of the entire celestial sphere is accessible. Its operation is highly efficient, and it is used for astronomy about 6500 hours every year, with 2000-3000 hours per year going to high frequency science. Part of the scientific strength of the GBT is its flexibility and ease of use, allowing for rapid response to new scientific ideas. It is scheduled dynamically to match project needs to the available weather. The GBT is also readily reconfigured with new and experimental hardware, adopting the best technology for any scientific pursuit. The high sensitivity mapping capability of the GBT makes it a necessary complement to ALMA, the EVLA, the VLBA, and other high angular resolution interferometers. Facilities of the Green Bank Observatory are also used for other scientific research, for many programs in education and public outreach, and for training students and teachers.

The current telescope was built following the collapse of the previous Green Bank telescope, a 90.44m paraboloid. The previous telescope collapsed on 15 November 1988 due to the sudden loss of a gusset plate in the box girder assembly, which was a key component for the structural integrity of the telescope.
The telescope sits near the heart of the United States National Radio Quiet Zone, a large area where all radio transmissions are limited to avoid emissions toward the GBT and the Sugar Grove Research Facility, West Virginia. The existence of the telescope within the Radio Quiet Zone allows for the detection of faint scientific signals which otherwise would be eclipsed by man-made signals. The observatory borders the National Forest and is shielded from radio interference by the Allegheny mountains.
The surface area of the GBT is a 100 by 110 meter active surface with 2,209 actuators (a small motor used to adjust the position) for the 2,004 surface panels. The panels are made from aluminium to a surface accuracy of better than 0.003 inches (76.2 micrometers) RMS. The actuators adjust the panel positions to correct for distortions due to gravity which change as the telescope moves. Without this so-called “active surface”, observations at frequencies above 4 GHz would not be as efficient.

Unusual for a radio telescope, the primary reflector is an off-axis segment of a paraboloid. This is the same design used in the familiar direct-broadcast satellite (e.g., DirecTV) antenna: the reflector is a piece of a much larger paraboloidal figure, chosen such that the entire primary reflector has a clear view of the sky, unobstructed by the secondary mirror, feed system, or support structures.

The offset support arm houses a retractable prime focus feed horn in front of the 8 m subreflector and eight higher-frequency feeds on a rotating turret at the Gregorian focus. Operational frequencies range from 290 MHz to 100 GHz.
In 2002, astronomers detected three new millisecond pulsars in the globular cluster Messier 62.

In 2006, several discoveries were announced, including a large coil-shaped magnetic field in the Orion molecular cloud,[4] and a large hydrogen gas superbubble 23,000 light years away, named the Ophiuchus Superbubble.

Since 2006 numerous discoveries have been made, including the most massive ever neutron star, cloud of primordial gas which surround other galaxies, vast molecular clouds surrounding other galaxies, and complex molecules, such as sugar, in space. A listing of recent discoveries can be found at “Green Bank in the News” and 2012 GBT Publications.