Green Bank Telescope

The telescope sits near the heart of the United States National Radio Quiet Zone, a unique area located in the town of Green Bank, West Virginia, 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 human-made signals might otherwise mask. The observatory borders National Forest land, and the Allegheny Mountains shield it from some radio interference. The telescope's location has been the site of important radio astronomy telescopes since 1957. It currently houses seven additional telescopes, and in spite of its somewhat remote location, receives about 40,000 visitors each year. == Description ==

The structure weighs and is 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, making the total collecting area of . The panels are made from aluminum manufactured to a surface accuracy of better than 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" adjustment, 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 smaller (eg., 45–100 cm) home satellite television 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 prime focus receiver on a retractable boom in front of a subreflector, and a receiver room. For prime focus operation, the boom is extended to position the feed horn in front of the 8 m subreflector. For Gregorian focus operation, the prime focus boom is retracted. The subreflector, positioned by a Stewart platform with 6 degrees of freedom, reflects incoming radio waves toward eight higher-frequency feeds on a rotating turret located on top of the receiver room. The computerized controlled turret can rotate a particular receiver into the position within a few minutes. Operational frequencies range from 290 MHz to 115 GHz. Azimuth axis is also supported by a pintle bearing at the center point of the azimuth track. File:Green Bank Telescope - surface panel actuators.jpg|Actuators (black) under the surface panels for surface fine-tuning File:GBT Secondary.png|Prime focus receiver enclosed in a cage at the end of the extended boom File:Green Bank Telescope - Gregorian focus operation.jpg|Gregorian focus operation: subreflector (top), rectracted boom (middle), and receiver turret (bottom) File:GBT Receiver.png|Underside of the turret inside the receiver room Because of its height (at 148 meters or 485 feet tall, it is 60% taller than the Statue of Liberty) and bulk (16 million pounds), locals sometimes refer to the GBT as the “Great Big Thing”. The telescope's capabilities include the ngRADAR system which use the dish as a radar transmitting antenna to observe solar system objects such as asteroids. Its low power prototype (700 watts at Ku band), with reception at the Very Long Baseline Array, has already imaged the moon and asteroid (231937) 2001 FO32. == Discoveries ==

, 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. 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. In 2019, the most massive neutron star PSR J0740+6620 to date was detected. Since 2004, 28 new complex molecules have been discovered in the interstellar medium with the Green Bank Telescope. == Funding threatened ==

In response to limited budgetary issues, the Division of Astronomical Sciences (AST) of the National Science Foundation (NSF) commissioned a portfolio review committee, which conducted its work between September 2011 and August 2012. The committee, which reviewed all AST-supported facilities and activities, was composed of 17 external scientists and chaired by Daniel Eisenstein of Harvard University. As part of the committee's August 2012 recommendation for the closure of six facilities, was that the Robert C. Byrd Green Bank Telescope (GBT) should be defunded over a five-year period. In July 2014, the United States Senate Committee on Appropriations approved the NSF's fiscal year 2014 budget, which did not call for divestment of the GBT in that fiscal year. The facility then began looking for partners to help fund its $10 million annual operating costs. On October 1, 2016, the National Radio Astronomy Observatory at Green Bank separated from the NSF and began accepting funding from private sources to stay operational as an independent institution, the Green Bank Observatory. == Relation to Breakthrough Listen ==

The telescope is a key facility of the Breakthrough Listen project, in which it is used to scan for radio signals possibly emitted by extraterrestrial technologies. In late 2017, the telescope was used to scan the interstellar object ʻOumuamua for signs of extraterrestrial intelligence as it passed through the Solar System. == See also ==