Astronomical Engines, Big Complicated Machines

Big, Complicated machines #12 – The Green Bank Telescope

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The Robert C. Byrd Green Bank Telescope. Image courtesy of NRAO/AUI

Good morning everyone.

Today we’re going to take a quick look at radio telescopes in general and the Green Bank Telescope in particular.

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.

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Underneath the GBT - Image courtesy of NRAO/AUI

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:

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It uses the high definition antenna shape

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).

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Legend γ= Gamma rays MIR= Mid infrared HF= High freq. HX= Hard X-rays FIR= Far infrared MF= Medium freq. SX= Soft X-rays Radio waves LF= Low freq. EUV= Extreme ultraviolet EHF= Extremely high freq. VLF= Very low freq. NUV= Near ultraviolet SHF= Super high freq. VF/ULF= Voice freq. Visible light UHF= Ultra high freq. SLF= Super low freq. NIR= Near Infrared VHF= Very high freq. ELF= Extremely low freq. Freq=Frequency

If that chart is overly complex, here is a simpler one, courtesy of XKCD:

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Image: XKCD

The receiver tower was specially designed to allow (relatively) quick changes in the secondary mirror and receiving instrumentation, making the telescope extremely versatile.

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A view of a GBT receiver assembly. Image courtesy of NRAO/AUI

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.

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The old Green Bank telescope. Image courtesy of NRAO/AUI

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The old GBT after collapse. Image courtesy of NRAO/AUI

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:

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Image: Dark Roasted Blend

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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New millisecond pulsars in M62 and others.

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Image courtesy of NRAO/AUI

 

References

The National Radio Astronomy Observatory

Discovery News

XKCD

Dark Roasted Blend

 

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  • OA5599

    Years ago I was on a cross-country trip and when I approached the Very Large Array in New Mexico I was astonished by the size of the radio telescopes there. I made a comment to my then-girlfriend but she blew it off; she was getting a can of refreshment out of the cooler at that moment. When she turned around to face the front, my previous comment finally clicked, and we pulled off the highway to find the visitor center and see these up close.

    <img src="http://upload.wikimedia.org/wikipedia/commons/6/63/USA.NM.VeryLargeArray.02.jpg&quot; width=500>

    The Green Bank one must be absolutely enormous.

    Professor, do these need to be located away from population centers to avoid radio wave pollution, the way optical telescopes are located away from sources of light pollution?

    • The Professor

      Yes, they need to located away from stray RF as much as possible. The ideal locations turn out to be valleys with nice, high mountains all around to block the noise.
      I'm going to do an article on the VLA, or rather the EVLA as it is now known, in the near future. I'm going to do a series covering many of these wonderful machines.

  • Total failure from the loss of a single gusset does sound flaky, but arguably still better than the official explanation for the collapse of a new set of football bleachers during construction here at UW in 1987:

    <img src="http://www.washington.edu/news/archive/images/20080221_pid39937_aid39936_stadium_w400.jpg"&gt;

    "Erection sequence errors…."

    Bummer.

    • Always an ill timed "ping" before the crashing and the screaming…

      • Happily in this case, no screams of injury or worse:

        http://www.historylink.org/index.cfm?DisplayPage=

        …although the cat probably didn't appreciate the experience.

      • The Professor

        Or the ever popular, "Hey, we have 6 bolts with nuts and washers left over. Is that right?" Then there's a ping and a pregnant pause…

    • The Professor

      Perhaps the instruction sheet was one of those Chinese->English translation horrors.

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