The 40-foot radio telescope at

Green Bank Observatory The Green Bank Observatory (previously National Radio Astronomy Observatory, Green Bank) is an Observatory, astronomical observatory located in the United States National Radio Quiet Zone, National Radio Quiet Zone in Green Bank, West Virginia, G ...

in the U.S. state of

West Virginia West Virginia is a mountainous U.S. state, state in the Southern United States, Southern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States.The United States Census Bureau, Census Bureau and the Association of American ...

was constructed in 1961, and started observing a selection of variable radio sources in the same year, completing its observations in 1968. It became the first automated telescope in 1962. It was subsequently used during summer schools, and in 1987 it was repurposed as an educational telescope, as well as continuing to observe radio sources. In 2021 the telescope celebrated its sixtieth anniversary, having been in continuous use since its 1987 restoration, and used by more than 1,500 students.

Specifications

The radio telescope has a diameter of in the form of a

parabolic reflector A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a Mirror, reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface ge ...

. The surface is made of

steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...

mesh, with a superstructure of

galvanised steel Hot-dip galvanization is a form of galvanization. It is the process of coating iron and steel with zinc, which alloys with the surface of the base metal when immersing the metal in a bath of molten zinc at a temperature of around . When expos ...

. It is a

transit telescope In astronomy, a transit instrument is a small telescope with an extremely precisely graduated mount used for the precise observation of star positions. They were previously widely used in astronomical observatories and naval observatories to m ...

: it only moves in elevation, not in

azimuth An azimuth (; from ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system. Mathematically, the relative position vector from an observer ( origin) to a point ...

(horizontally), and relies on the

Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in progra ...

to observe the full sky. It observes at

L band The L band is the Institute of Electrical and Electronics Engineers (IEEE) designation for the range of frequencies in the radio spectrum from 1 to 2 gigahertz (GHz). This is at the top end of the ultra high frequency (UHF) band, at the lower en ...

, originally at 1340–1580 MHz, more recently at 1355–1485 MHz, both in radio continuum mode. It can also observe

neutral hydrogen The hydrogen line, 21 centimeter line, or H I line is a spectral line that is created by a change in the energy state of solitary, electrically neutral hydrogen atoms. It is produced by a spin-flip transition, which means the directio ...

emission at 1420.41 MHz in

spectroscopy Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum. Spectro ...

. Since 1994 it records the telescope output both digitally and on

chart recorder A chart recorder is an electromechanical device that records an electrical or mechanical input trend onto a piece of paper (the chart). Chart recorders may record several inputs using different color pens and may record onto strip charts or circu ...

History

The 40-foot was ordered in 1961 from Antenna Systems. It was delivered to

in December 1961, and took two days to assemble. Its first light was on 14 December 1961. The original control system was constructed by the

National Radio Astronomy Observatory 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 radi ...

(NRAO). On 1 February 1962, it became the first telescope to be fully automated, ahead of the automation of optical telescopes like that at Washburn Observatory from the late 1960s onwards. It was used to survey a number of variable radio sources from 1962 until 1968. After the completion of the survey in 1968, it was used occasionally as an educational instrument as part of the Green Bank radio astronomy summer schools. In 1987 it was restored and upgraded for full use as an educational telescope, including by local science teachers as part of the Secondary Science Teachers Institute (SSTI). The restoration reused the 1960

dipole antenna In radio and telecommunications a dipole antenna or doublet is one of the two simplest and most widely used antenna types, types of antenna; the other is the monopole antenna, monopole. The dipole is any one of a class of antennas producin ...

and radome from the Tatel Telescope, which was used during the

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 ...

. A substantial part of the

spectrometer A spectrometer () is a scientific instrument used to separate and measure Spectrum, spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomeno ...

was reused from the 300 Foot Telescope. In 1991 this changed to the 'Learning to Investigate the Universe' (LITU) project, changing again in 1994 to Research Experiences in Teacher Preparation (RETP). It has also been used by the annual Educational Research in Radio Astronomy (ERIRA) program since 1992, and Green Bank Observatory's "Radio Astronomer for a Day" program for K–12 students. The 40-foot telescope has since been in continuous use since its restoration, and has been used by over 1,500 students. It celebrated its sixtieth anniversary in 2021 while still in use.

Science

It was originally used to observe eight variable radio sources on a daily basis over the course of five years. The sources were

3C 48 3C48 is a quasar discovered in 1960; it was the second source conclusively identified as such. 3C48 was the first source in the Third Cambridge Catalogue of Radio Sources for which an optical identification was found by Allan Sandage and Thomas ...

, 3C 144 (Taurus A), 3C 218 (Hydra A), 3C 274 (Virgo A), 3C 295, 3C 358, 3C 405 (Cygnus A), and 3C 461 (Cassiopeia A). The project was led by D. S. Heeschen, who had previously been using the antenna for this work. The use of a relatively cheap telescope for this work freed up larger telescopes for other uses. In initial results, the sources were observed on a daily basis at 20 and 40 cm over 28 months, and only Cas A was seen to be variable. As part of its educational use, it has continued to observe some of the original radio sources it was targeting. Data from 1995 to 1999 comparing Cas A to Cyg A was published in 2000, followed in 2017 by a publication combining 20 years of observations of Cas A using new observations between 1994 and 2015. These tracked the fading of the source and improved the

flux density Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications in physics. For transport phenom ...

calibration models that use this source. It has also been used to observe the

H I region An HI region or H I region (read ''H one'') is a cloud in the interstellar medium composed of neutral atomic hydrogen (HI), in addition to the local abundance of helium and other elements. (H is the chemical symbol for hydrogen, and "I" is the Ro ...

in the spiral arms of the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

and the

Andromeda Galaxy The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...

, as well as intensity and polarized

synchrotron radiation Synchrotron radiation (also known as magnetobremsstrahlung) is the electromagnetic radiation emitted when relativistic charged particles are subject to an acceleration perpendicular to their velocity (). It is produced artificially in some types ...

from the galactic plane and the North Polar Spur.

References

{{Reflist, refs= {{cite web , title=40 ft , url=https://greenbankobservatory.org/science/telescopes/40-ft/ , publisher=Green Bank Observatory , date=11 May 2016 {{Cite Q, Q59879147 {{Cite Q, Q68275286 {{cite news , last1=Skeldon , first1=Katherine , title=Green Bank Observatory celebrates 60th birthday of 40-foot telescope , url=https://woay.com/green-bank-observatory-celebrates-60th-birthday-of-40-foot-telescope/ , work=WOAY-TV , date=9 December 2021 , access-date=17 January 2022 {{cite web , title=ERIRA , url=https://www.danreichart.com/erira , website=Dan Reichart {{cite web , title=Green Bank Observatory 40-foot radio telescope operator's manual , url=https://greenbankobservatory.org/wp-content/uploads/2018/02/40-manual.pdf , access-date=17 December 2021 {{cite web , title=40 Foot Telescope , url=https://science.nrao.edu/facilities/gbt/other-telescopes/40foot , website=NRAO , access-date=17 December 2021 , date=8 April 2014 {{Cite Q, Q110368513 {{Cite Q, Q110368819 {{cite web , title=For Students – Research , url=https://greenbankobservatory.org/education/research/ , website=Green Bank Observatory , access-date=1 January 2022 , date=12 May 2016 {{cite news , title=Astronomy and physics students visit Green Bank Telescope , url=https://artsandsciences.osu.edu/news/astronomy-and-physics-students-visit-green-bank-telescope , work=College of Arts and Sciences , date=9 May 2018 , language=en , access-date=17 January 2022 {{cite web , title=Radio Astrophotography , url=https://www.danreichart.com/radiophoto , website=Dan Reichart , access-date=1 January 2022 {{Cite Q, Q110380956 {{Cite Q, Q68565587 Radio telescopes Astronomical observatories in West Virginia 1961 establishments in West Virginia 1961 in science Buildings and structures in Pocahontas County, West Virginia