Green Bank Telescope
|Green Bank Telescope|
|Location||Green Bank, West Virginia, USA|
|Wavelength||radio telescope and microwave band|
|First light||August 22, 2000|
|Telescope style||Parabolic off-axis reflector, Gregorian optics|
|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. 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 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.
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.
Engineering priorities 
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.
Currently the surface accuracy of the GBT has been measured to be roughly 250 microns.
Funding threatened 
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. Further information on this divestiture can be found on the AUI webpage and at www.savethegbt.org.
See also 
- NRAO 300 foot Telescope Collapse
- "The Proposer’s Guide for the Green Bank Telescope" (PDF). NRAO.
- Newly Commissioned Green Bank Telescope Bags New Pulsars, NRAO, 2002-01-04
- Green Bank Telescope scores big finds in space: Fastest pulsar Slinky magnetics superbubble of hydrogen ID, The Charleston Gazette, 2006-01-17
- The Ophiuchus Superbubble: A Gigantic Eruption from the Inner Disk of the Milky Way, ArXiv
- "Huge 'Superbubble' of Gas Blowing Out of Milky Way". PhysOrg.com. 2006-01-13. Retrieved 2008-07-04.
- Hand, E. (2012). "US telescopes face up to agency cuts". Nature 488 (7412): 440. doi:10.1038/488440a. PMID 22914143.
|Wikimedia Commons has media related to: Green Bank Telescope|
- Official site
- Collection of West Virginia photos including Green Bank from davidmetraux.com