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 the late Senator Robert C. Byrd.
The GBT is designed as a flexible instrument, able to carry out a wide variety of astronomical observations, and has high sensitivity over the wavelength range from 3 mm to 90 cm. Astronomers from all over the world apply for observing time, and if the request is accepted, the NRAO staff assist them in carrying out their observations. The astronomers aim the telescope at a great range of types of targets, from our nearby Moon to the Cosmic Background Radiation, visible at the edge of the Universe. The science goals include the search for water on the Moon's surface, gravitational wave detection using pulsars, the structure of our galaxy, water in nearby galaxies and molecules in extremely distant galaxies.
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 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 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,and a large hydrogen gas superbubble 23,000 light years away, named the Ophiuchus Superbubble.
The capabilities of all radio telescopes are determined by the surface accuracy of the telescope itself and the instrumentation developed by the engineering staff. The NRAO GBT engineering staff priorities are set based on the advice of science user's committee. The priorities set by this committee develop in response to recent discoveries by the astronomical community and based on theoretical predictions published in astronomical journals. The current (December 2012) science priorities include increasing the sensitivity for pulsar observations at high frequencies (> 10 GHz) to allow observations near the galactic center, much higher spectrometer spectral resolution and bandwidth, to improve the efficiency of detection of molecular lines, and bolometer arrays, allowing more sensitive continuum observations.
An additional priority is improving the capabilities for mapping large regions of the sky, to enable studies of the chemical processes in our Galaxy. Recently an array receiver was installed for observations in the frequency range 18 to 26.5 GHz, where several biologically significant molecules are seen by their molecular line emission. These molecules include water, ammonia, methanol, and formic acid.
The National Science Foundation (NSF) Astronomy Portfolio Review committee chaired by Daniel Eisenstein of Harvard University recommended in August 2012 that the Robert C. Byrd Green Bank Telescope should be defunded over a five year period.