When the National Radio Astronomy Observatory was first founded, it was with the understanding that, fairly quickly, it should be able to offer the world a large, single dish radio telescope. The radio astronomy community was hoping for a massive moving telescope, perhaps 1000 feet in diameter! However, until engineers could deem such a monstrosity feasible, the National Science Foundation funded the design and installation of a 140-ft radio telescope on a polar-aligned mounting – a telescope that would be unique in its own right.
A polar-aligned telescope, also called an equatorial-mounted telescope, parallels the axis of the Earth for its spin axis. The spinning of the Earth causes the rising and setting of sky objects, and the imaginary northern axis of the Earth’s spin (its North Pole) is below the North Star, appropriately called Polaris.
The 140-foot telescope rides on a giant gear whose axis is also aimed at Polaris. As the Earth spins toward the east, the 140-foot telescope spins to the west to follow the sky’s apparent movement. The 140-foot easily tracks objects in space this way.
A robust design for the world’s largest polar-mounted telescope stymied engineers for years, and a few false starts delayed the progress of this telescope. The original polar mount used a welded steel shaft and a 22-foot diameter welded bearing on top that was assembled like segments of an orange.
However, inspectors found over 70 minute cracks in these critical pieces, a condition known as “brittle fracture.” The cold Green Bank winters would have spelled doom for this bearing, and resulted in the loss of the entire telescope. This design was abandoned in 1962 for a smaller cast bearing. The steel from the sphere was taken to Long Island to shield the particle accelerator at Brookhaven National Laboratory, and the original and ill-fitting polar mount is buried as a culvert under the road leading from the 140-foot to the GBT.
From its earliest observations, this giant telescope proved to be worth the wait.
Tool of Astrochemistry
Chemistry is a scientific exploration of the assembly and reactions of the basic structures of everything we know. Chemists study atoms, molecules, and the energies that are required to break them and make them, in labs on Earth.
In space, however, the conditions for chemistry are cold, full of destructive radiation, and fairly empty. Few chemists even considered it a place where they could explore the same activities they watched in molecules on Earth.
However, astronomers without the burden of knowledge in chemistry wanted to try to hunt for molecules anyway. From a physics point of view, they felt that every time a space-based molecule spins around, its electrons should cough up weak radiation in a radio frequency.
In the mid-1960s, looking at the center of our Galaxy, astronomers in California found the signal of ammonia. Soon, the 140-foot telescope, with its huge collecting dish and receivers that were tuned to many molecules’ predicted frequencies, was put into service to hunt for more species of molecules. In 1968, it found formaldehyde, the first complex molecule ever found in space.
Formaldehyde’s discovery overturned the long-held assumptions about the conditions required for chemistry. In good spirit, chemists joined astronomers to help target new and wonderful signals from space-based molecules. Thanks to this new interdisciplinary pursuit of astrochemistry, the 140-foot went on to discover formic acid (you know it as insect venom), methanol, acetaldehyde (the chemical responsible for hangovers), and cyanoacetylene (a possible precursor to the DNA acid cytosine).
Tool of VLBI
Very Long Baseline Interferometry is a technique that combines the views of two separate telescopes, separated by large distances, to capture the finest details of an object in space. In radio astronomy, the two telescopes observe the same radio source for hours at a time, and their data is written to tape or hard drive. Those data must be stamped with a timecode to make them accurate to within 1/1000th of a second or better, or else they cannot be combined.
Technicians send the data to a central computing facility to be merged. The waves of data are added, and the interference pattern created by their overlap refines true peak signals into sharper points and flattens the random noise. The result is a highly-detailed picture of distant, compact sources such as quasars deep inside the hearts of galaxies and clouds of hyper-glowing gas, called masers.
VLBI experiments with the 140-foot telescope and others around the United States commenced within a couple of years of this giant telescope coming online. In 1967, the 140-foot and the 85-foot Maryland Point telescope, run by the Naval Research Laboratory on the shore of the Potomac River, worked together for the very first time. They formed a telescope 226 miles across!
Later that year, the 140-foot and the 120-foot telescope in Westford, Massachusetts successfully worked together as a single unit, 845 miles long, to observe a quasar. Throughout 1967, the 140-foot paired with American telescopes farther and farther away, eventually succeeding in simulating a telescope the size of the United States when it paired with the 25-meter Mk2 telescope in Jodrell Bank, near Manchester in England.
The first US-USSR experiments, between the 140-foot telescope and a 22-meter USSR telescope in Crimea were conducted in September and October 1969, with follow-up experiments in 1971.
Ionospheric Studies
In 1999, this giant telescope officially ceased observations as an open-access instrument and was mothballed to await a new benefactor. In 2004, it was retrofitted with a new receiver to become MIT’s Lincoln Laboratories receiver station for the study of the Earth’s ionosphere, a high layer of our atmosphere that acts like a mirror for bouncing radio broadcasts around the planet.
MIT’s Millstone Hill Observatory’s radar station in Westford, Massachusetts bounced radar signals off orbiting spacecraft for the 43-meter to receive. Scientists using the data measured the density of the ionosphere from the change in the radar signal.
During breaks in its MIT schedule, the 43-meter was also used for pulsar hunting and monitoring. In particular, it regularly checked on the Crab pulsar, searched for new changing sources, and mapped pulsars across the entire sky.
RadioAstron
When MIT ended its contract with the NRAO for use of the 43-meter in 2011, once again, the giant telescope say waiting – but not for long. For years, scientists from NRAO have been consulting with Russian scientists about a space-based radio astronomy project called RadioAstron.
On July 18, 2011, the Russian Space Agency launched a satellite into orbit that unfolded into a 10-meter (33-foot) dish radio telescope called Spektr-R. This telescope observes as a space-based radio telescope,
but also can be used with other radio telescopes on Earth for what is called Space Very Long Baseline Interferometry.
Our telescopes in Green Bank were the first to participate in Space VLBI decades ago, when we worked with the HALCA radio telescope launched and operated by Japan in the 1990s.
In 2012, telescope technicians adapted the 43-meter to become an Earth station for tracking and downlinking data from Spektr-R, including a high-frequency receiver and a fiber optically-controlled secondary mirror system that could fine tune the focus of the telescope quickly and remotely.
Since then, the 43-meter has served to help the RadioAstro team in Russia to keep track of the position, navigation, and health of their orbiting radio telescope. The GBT has joined Spektr-R in several observations of active galactic nuclei, the supermassive black holes lurking inside galaxies that are bright in radio waves.
Overall:
Height: 60 feet; 41 feet 6 inches to observation deck
Contains: 5700 tons of concrete and 140 tons of steel
Wall thickness: 3 feet. Houses control room, hydraulic and electric equipment, transformer vault, and electronic workshop
Total Moving Weight: About 2,500 tons
Mount: Equatorial – Two mutually perpendicular axes
Pointing Precision: 10 arc seconds – about the diameter of a dime at 400 yards
Receivers: Generally at wavelengths between 2cm and 40cm.
Reflector:
Paraboloid Diameter: 140 feet.
Surface: 1/8 inch aluminum plate.
Surface tolerance: 0.030 inches at zenith.
Contains: 350 tons of aluminum, 35tons of concrete ballast, 5 tons of balancing blocks.
Focus:60 feet above the surface. Carries 1/2 tons of receiving equipment
Stability: Position relative to paraboloid stable to 1/4 of an inch.
Declination Axis:
Shaft: Length: 67-1/2 feet overall; 57 feet between bearings.
Diameter: 2 feet.
Material: composite aluminum and steel shaft running in two spherical roller bearings.
Rotates 145 degrees to -53 degrees north of zenith and 92 degrees south.
Yoke: Serves to support the declination shaft and to rotate the antenna east and west about the polar axis by means of the polar gear.
Polar Axis:
Shaft Length: 67 feet
Diameter: 12 feet
Weight: 555 tons of steel; 170 tons of high density concrete ballast
Rotates 220 degrees (-110 degrees east of meridian and 110 degrees west)
Diameter of polar gear sector: 84 feet
Diameter of declination gear sector: 71 feet
Spherical Bearing Diameter: 17-1/2 feet
Surface tolerance: 0.003 inches
Floats on oil film 0.005 inches thick
