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The Green Bank Interferometer (GBI) is a former
radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming f ...
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
located at
Green Bank A green bank (sometimes referred to as green investment bank, clean energy finance authority, or clean energy finance corporation) is a financial institution, typically public or quasi-public, that uses innovative financing techniques and market ...
,
West Virginia West Virginia is a state in the Appalachian, Mid-Atlantic and Southeastern regions of the United States.The Census Bureau and the Association of American Geographers classify West Virginia as part of the Southern United States while the Bur ...
(
USA The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
) and operated by the National Radio Astronomy Observatory (NRAO). It included three on-site radio telescopes of 85-foot (26m) diameter, designated 85-1, 85-3, and 85-2 (85-1 is also known as the Tatel Telescope) and a portable telescope.


History

The first telescope (85-1, Tatel) was built in 1959 at a fixed location. It was used in
Project Ozma Project Ozma was a search for extraterrestrial intelligence (SETI) experiment started in 1960 by Cornell University astronomer Frank Drake, at the National Radio Astronomy Observatory, Green Bank at Green Bank, West Virginia. The object of the e ...
in 1960 without Interferometer. In 1963, in anticipation of adding movable telescopes for interferometer, the second telescope kit identical to 85-1 was ordered. The construction of the second telescope (85-2) was completed in 1964 along with a track from 85-1. At that time, the 85-2 telescope was placed at the end of the track and cables were connected between the two telescopes. The GBI began operation that year as a two element interferometer in order to test large
aperture synthesis Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection. At each separation an ...
arrays and study radio
astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. His ...
and interstellar scintillation. In 1967 the array was upgraded with construction of the third element (85-3) to be located in the middle of the track. Both 85-2 and 85-3 had truck tires mounted on either side to allow them to be moved along the track to test different baselines. The limitation of the 3-element interferometer along a short track became apparent. A portable telescope was procured. The portable telescope was placed away from Green Bank site and then moved to forming a T shape with the length of the bottom arm of the T to be similar to the length of each arm of the Y configuration at
Karl G. Jansky Very Large Array The Karl G. Jansky Very Large Array (VLA) is a centimeter-wavelength radio astronomy observatory located in central New Mexico on the Plains of San Agustin, between the towns of Magdalena and Datil, ~ west of Socorro. The VLA comprises twent ...
(VLA) which was still in a design phase. The portable telescope was later placed on a mountaintop in
Huntersville, West Virginia Huntersville is a census-designated place (CDP) located in Pocahontas County, West Virginia, United States in the Alleghany Mountains. As of the 2010 census, its population was 73. It is located six miles east of Marlinton and four miles west ...
, away from Green Bank, which is the same distance of the longest baseline of VLA. The portable telescope was replaced in 1973 with portable telescope which was in use until 1983. From 1978-1996, the GBI was operated in support of
USNO United States Naval Observatory (USNO) is a scientific and military facility that produces geopositioning, navigation and timekeeping data for the United States Navy and the United States Department of Defense. Established in 1830 as the Depo ...
and
NRL The National Rugby League (NRL) is an Australasian rugby league club competition which contains clubs from New South Wales, Queensland, Victoria, the Australian Capital Territory and New Zealand. The NRL formed in 1998 as a joint partnership ...
geodetic and astronomy programs. In 1979, the GBI configuration had 85-3 and 85-2 at and away from 85-1 respectively, and the portable telescope at from Green Bank. In 1983, the portable telescope was moved back to Green Bank for another function to become a tracking station for
Space VLBI HALCA (Highly Advanced Laboratory for Communications and Astronomy), also known for its project name VSOP (VLBI Space Observatory Programme), the code name MUSES-B (for the second of the Mu (rocket family), Mu Space Engineering Spacecraft series) ...
satellites. The new portable telescope was put in place for interferometer operation until 1988 when the GBI reconfigured to 2-element interferometer with 85-3 taken out to become geodetic VLBI and
pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
monitoring telescope. The GBI resumed operation as a radio monitoring instrument on Nov. 22, 1996, operated by
NRAO The National Radio Astronomy Observatory (NRAO) is a federally funded research and development center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc. for the purpose of radio a ...
and supported by the
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
High Energy Astrophysics program. The GBI was then used as a two telescope interferometer that operated simultaneously at 2.25 and 8.3 GHz to monitor transient, galactic
X-ray binaries X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively normal star), to the other component, called the ''accretor'', which ...
, AGN's and Gamma-ray sources. Amongst the prime sources were
GRS 1915+105 GRS 1915+105 or V1487 Aquilae is an X-ray binary star system which features a regular star and a black hole. It was discovered on August 15, 1992 by the WATCH all-sky monitor aboard Granat. "GRS" stands for "GRANAT source", "1915" is the ...
, GRO J1655-40, Cyg X-3,
Cyg X-1 Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus and was the first such source widely accepted to be a black hole. It was discovered in 1964 during a rocket flight and is one of the s ...
, GRS 1716-249, SS 433, and LS I +61 303. On October 6, 2000 the GBI monitoring program has ceased due to lack of funding.


Technical Data: 2-element interferometer

* Baseline: 2400 meters at an azimuth of 62 degrees (E of N). * Bands: 8.3 GHz (X-band) and 2.25 GHz (S-band) with 35 MHz bandwidth. * Receivers: Cryogenically cooled, dual frequency, dual polarization. Both X and S bands simultaneously observed in both right and left circular polarizations. * System temperature: About 35 K in Sband and 45 K in Xband. * Sensitivity: RMS noise in a 5-minute scan is about 6 mJy in S-band and 10 mJy in X-band for point sources. * Minimum integration time: 30 seconds. * Resolution: About 3 arcseconds fringe at X-band and 11 arcseconds at S-band.


See also

*
List of radio telescopes This is a list of radio telescopes – over one hundred – that are or have been used for radio astronomy. The list includes both single dishes and interferometric arrays. The list is sorted by region, then by name; unnamed telescopes are in r ...


References


External links

*
Green Bank Interferometer home page
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