The Robert C. Byrd Green Bank Telescope (GBT) is the world's largest fully steerable radio telescope and the world's largest land-based movable structure. It is part of the National Radio Astronomy Observatory (NRAO) site at Green Bank, West Virginia (USA). The telescope honors the name of Senator Robert C. Byrd.
The telescope was built following the collapse of the previous Green Bank telescope, a 300ft paraboloid. This 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 at the heart of the United States National Radio Quiet Zone, a large area where all radio transmissions are either limited or banned outright, to help the telescope function properly.
The surface area of the GBT is 100 by 110 meters 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 less 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.
The telescope is unusual in that the mirror is not a symmetrical dish, but is a section of a much larger parabolic figure with the receiver where the prime focus of the entire mirror would be. As a result, the support for the receiver does not in any way obscure the mirror's view of the sky.
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 49.8 GHz.
In 2006, several discoveries were announced, including a large coil-shaped magnetic field in the Orion molecular cloud, and a large hydrogen gas superbubble 23,000 light years away, named the Ophiuchus Superbubble.