The money needed to build the GBT, $74,500,000, was provided in 1990 by the Major Research Equipment Fund of the Foundation's Division of Math and Physical Sciences. The contractor for the construction work is Comsat/GBT, a newly-created division of Comsat in Herndon, VA. We at NRAO are currently hoping for a provisional acceptance of the instrument early in the next millenium. Operations will begin some time thereafter, following a period of outfitting, testing, and refinement.
The GBT will be the largest fully steerable radio telescope in the world. It is described as a 100 meter telescope but the actual dimensions of the surface are 100 by 110 meters. The overall structure of the GBT is a wheel and track design which allows the telescope to view the entire sky above 5 degrees elevation. The track, 200 ft. in diameter, is level to within a few thousandths of an inch in order to provide precise pointing of the structure while bearing the 17,000,000+ lb. moving weight.
The GBT is of an unusual design. Unlike conventional telescopes which have a series of supports in the middle of the surface, the GBT's aperture is unblocked so that incoming radiation meets the surface directly. This increases the useful area of the telescope and eliminates reflections and diffraction effects which ordinarily complicate a telescope's pattern of response. To accommodate this concept, an off-axis feed arm cradles the dish, projecting upward at one edge, and the telescope surface is asymmetrical. It is actually a 100 x 110 meter section of a conventional, rotationally symmetric 208 meter figure, beginning 4 meters outward from the vertex of the hypothetical parent structure. The GBT's lack of circular symmetry greatly increases the complexity of its design and construction. The GBT is also unusual in that the 2000 panels which make up its surface are mounted at their corners on actuators, little motor-driven pistons. This affords the ability to adjust the surface easily after construction. Such adjustment will be crucial to the high frequency performance of the GBT in which an accurate surface figure must be maintained.
The GBT will be equipped with a novel laser-ranging system. Beams of light will be reflected within the structure and between the telescope and a series of ground stations surrounding the telescope in a broad ring. Monitoring of these beams will show the deformation of the figure under the stresses of gravity, wind, and temperature differences, etc., and will allow the telescope's motors, subreflector, and surface panel actuators to compensate for any ill effects.
A detailed drawing of the telescope provides a guide to some of the more technical terms which are often used to describe it. The system of counterweights shown below will contain nearly 2,700,000 lbs. of concrete.
Phase | After | Telescope rms |
Nominal maximum frequency |
Pointing (benign conditions) |
Aperture efficiency
|
|||||
---|---|---|---|---|---|---|---|---|---|---|
0 | construction | 1.1 mm | 17 GHz | 14" | 63% | 33% | 0.6% | |||
1 | holography | 0.53 mm | 35 GHz | 7" | 69% | 59% | 24% | |||
2 | actuators (open-loop) | 0.36 mm | 52 GHz | 3" | 70% | 65% | 45% | |||
3 | actuators (closed-loop) | 0.24 mm | 78 GHz | 3" | 71% | 68% | 60% |
Phase | After | Telescope rms |
Nominal maximum frequency |
Pointing (benign conditions) |
Aperture efficiency
|
|||||
---|---|---|---|---|---|---|---|---|---|---|
0 | construction | 1.48 mm | 12 GHz | 14" | 55% | 15% | 0% | |||
1 | holography | 1.17 mm | 16 GHz | 7" | 61% | 27% | 0% | |||
2 | actuators (open-loop) | 0.36 mm | 52 GHz | 3" | 70% | 65% | 45% | |||
3 | actuators (closed-loop) | 0.24 mm | 78 GHz | 3" | 71% | 68% | 60% |
For a 65% aperture efficiency, the gain of the GBT will be 1.85K/Jy (by comparison, the gain of the 140' antenna at 23GHz presently is 0.1K/Jy). The beamwidth of the GBT will be 12'/Frequency in GHz or 0.4' x wavelength in cm, e.g.
Diffraction beamwidth (FWHM) | ||
---|---|---|
8 GHz | 20 GHz | 50 GHz |
90" | 36" | 14" |