Green Bank Telescope

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Green Bank Telescope
Organisation National Radio Astronomy Observatory
Location(s) Green Bank, West Virginia, United States of America
Coordinates 38°25′59″N 79°50′23″W / 38.4331°N 79.8397°W / 38.4331; -79.8397Coordinates: 38°25′59″N 79°50′23″W / 38.4331°N 79.8397°W / 38.4331; -79.8397
Wavelength radio telescope and microwave band
Built 1991-2002
First light 22 August 2000
Telescope style Parabolic off-axis reflector, Gregorian optics
Diameter 100 metre
Collecting area 7,854 m2
Focal length 60 m
Mounting wheel and track mount

The Robert C. Byrd Green Bank Telescope (GBT) is the world's largest fully steerable radio telescope,[1] as well as being the world's largest movable land object. It is part of the National Radio Astronomy Observatory (NRAO) site at Green Bank, West Virginia. The telescope honors the name of the late Senator Robert C. Byrd who represented West Virginia and who pushed the funding of the telescope through Congress.

The Green Bank Telescope operates at meter to millimeter wavelengths. Its 100-meter diameter collecting area, unblocked aperture, and good surface accuracy provide superb 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. 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. The high-sensitivity mapping capability of the GBT makes it a necessary complement to the Atacama Large Millimeter Array, the Expanded Very Large Array, the Very Long Baseline Array, 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 telescope began regular science operations in 2001, making it one of the newest astronomical facilities of the US National Science Foundation. It was constructed following the collapse of a previous telescope at Green Bank, a 90.44 m paraboloid erected in 1962. 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.[2]


The Robert C. Byrd Green Bank Radio Telescope (GBT) has a collecting area of 2.3 acres (0.93 ha) which focuses the radio waves falling on it onto sensitive receivers at the top of the boom attached to the side. It is 485 feet (148 m) tall, nearly as tall as the nearby mountains and much taller than the pine trees in the nearby national forest.

The telescope sits near the heart of the United States National Radio Quiet Zone, a unique area within North America where authorities limit all radio transmissions to avoid emissions toward the GBT and the Sugar Grove Station. The location of the telescope within the Radio Quiet Zone allows for the detection of faint radio-frequency signals which man-made signals might otherwise eclipse. The observatory borders National Forest land, and the Allegheny Mountains shield it from some radio interference.


The structure weighs 8500 tons and is 450 feet tall. The surface area of the GBT is a 100 by 110 meter active surface with 2,209 actuators (small motors 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 µm) RMS. The actuators adjust the panel positions to compensate for sagging, or bending under its own weight, which changes as the telescope moves. Without this so-called "active surface", observations at frequencies above 4 GHz would not be as efficient.[3]

Unusually for a radio telescope, the primary reflector is an off-axis segment of a paraboloid. This is the same design used in familiar home satellite television (e.g., DirecTV) dishes. The asymmetric reflector allows the telescope's focal point and feed horn to be located at the side of the dish, so that it and its retractable support boom do not obstruct the incoming radio waves, as occurs in conventional radio telescope designs with the feed located on the telescope's beam axis.

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.[3]

Because of its height (at 485 feet tall, it's 60% taller than the Statue of Liberty) and bulk (16 million pounds), locals sometimes refer to the GBT as the “Great Big Thing”.[4][5]

It also has the distinction of being the world's largest movable land object.


Composite image of a spectral line observation of star forming region W51, showing the distribution of ammonia in the region. The image of the telescope from a time lapse film of a night of observations

In 2002, astronomers detected three new millisecond pulsars in the globular cluster Messier 62.[6]

In 2006, several discoveries were announced, including a large coil-shaped magnetic field in the Orion molecular cloud,[7] and a large hydrogen gas superbubble 23,000 light years away, named the Ophiuchus Superbubble.[8][9]

Since 2006 numerous discoveries have been made, including the most massive neutron star detected so far, a cloud of primordial gas which surrounds 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.

Engineering priorities[edit]

The capabilities of all radio telescopes are determined by the surface accuracy of the telescope itself and the instrumentation developed by the engineering staff.[citation needed]

Funding threatened[edit]

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.[10] Further information on this divestiture can be found on the AUI webpage and at

In the fiscal year 2014 budget, the US Congress did not recommend divesting the Green Bank Telescope. The Telescope is looking for partners to help fund its $10 million annual operating costs.[11]

Relation to Breakthrough Listen[edit]

The telescope will scan for radio signals, possibly emitted by extra-terrestrials, in the range of 1-10 GHz, a so-called 'quiet zone' uninterrupted by man-made or natural sources. This immense undertaking is for the heavily funded Breakthrough Listen.[12]

See also[edit]