Good morning everyone.
A radio telescope is a type of steerable radio antenna that is used in astronomy for studying celestial radio sources. The same types of antennas are also used for tracking and communicating with satellites and spaces probes. When used as telescopes for astronomy, they collect electromagnetic radiation in the radio frequency spectrum, from ~3kHz to 300Ghz, as opposed to optical telescopes which collect visible light. They are used for the study of many celestial objects that optical telescopes either cannot or have difficulty in observing, such as the objects in the center of our galaxy. Radio telescopes are typically very large parabolic, or dish antennas, and are used singly or in arrays. The diameter of the antenna dish is called the aperture of the telescope, and just like optical telescopes, a larger aperture means that a telescope can detect and study fainter objects. Radio telescopes that are thousands of miles apart can be linked together in a technique called Very Long Baseline Interferometry which gives the resolution of a single telescope that is thousands of miles in diameter. Radio telescopes are the giant constructs of astronomy, the largest being the Arecibo Radio Telescope in Puerto Rico at 1,000 feet in diameter.
The Green Bank Telescope in West Virginia is the world’s largest fully steerable radio telescope and the world’s largest land-based movable structure. It’s now known as the Robert C. Byrd Green Bank Telescope, but that’s a bit wordy, so I’ll refer to it as the GBT.
The present GBT is 485 feet tall and its dish measures 100 by 110 meters and has an active surface. There are thousands of actuators that adjust the surface panel positions to correct for the effects of gravity distortions which change as the telescope moves. The dish, or reflector of the GBT is an off-axis segment of a paraboloid, which is the same design used in direct-broadcast satellite antennas, like the one below:
The reflector is a section of a much larger paraboloid figure and was chosen because it gives the main reflector an unobstructed view of the sky. The secondary mirror, receivers and support structures are completely out of the ‘light path’ of the telescope, eliminating reflection and diffraction that ordinarily complicate a telescope’s pattern of response. The GBT weighs 8,000 tons and can be pointed with the accuracy of one arcsecond, or the equivalent to the width of a human hair seen from 6 feet away. Its operating range is 0.1GHz to 116GHz, which corresponds to wavelengths of from over 9 feet (3 meters) to ⅛ inch (3mm).
If that chart is overly complex, here is a simpler one, courtesy of XKCD:
The receiver tower was specially designed to allow (relatively) quick changes in the secondary mirror and receiving instrumentation, making the telescope extremely versatile.
The current GBT saw first light in August of 2000, and was constructed after the collapse of the previous Green Bank telescope, a 90.44 meter paraboloid instrument. The old telescope collapsed on November 15th, 1988, when a gusset plate on the main box girder assembly snapped.
One gusset plate broke and the entire telescope just collapsed? Sounds like sketchy engineering to me. The locals, of course, had different theories as to why the telescope collapsed:
The GBT is used to study objects such as neutron stars, pulsars, active galaxies, gravity wave and the formation of stars, galaxies and galaxy clusters, among other things. In 2002 it found three new millisecond pulsars in M62 and in 2010 it discovered the largest neutron star yet found. Cool stuff.
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