The Green Bank Telescope (GBT) is presently under construction at the National Radio Astronomy Observatory's site in Green Bank, Pocahontas County, West Virginia (79d50m24.2s, 38d25'58.7"). The National Radio Astronomy Observatory is a facility of the (US) National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc. (AUI) .

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.


Operating specifications for the Green Bank Telescope

The performance of the GBT will evolve and improve with time but, even when the telescope is new, the performance will be quite good for sources which can be observed near the so-called "rigging-angle", i.e., an elevation of 51d. This rigging angle is the elevation at which the telescope was originally set in shape, and deformations due to gravity are minimized when the telescope is used there. In Tables 1 and 2 below we show the operating parameters expected of the GBT under benign conditions at the rigging angle and at the zenith. In the early stages of operation, there will be substantial differences between these two situations. Immediately after construction, the telescope surface will have been set to the contractor's specified 0.040" (1.08mm) rms and the telescope rms will vary between 1.1 mm (at the rigging angle) and 1.5 mm (at the zenith or horizon). Soon after delivery, the use of holography is expected to improve the surface setting rms to 0.018". Use of the surface actuators to correct for thermal and gravitational effects according to a physical model of the structure (the so-called open-loop mode) is expected to lower the overall rms to 0.36 mm. We hope eventually to use the laser-ranging system and surface actuators in a fully closed feedback loop to afford observing up to 80 GHz under the best and most benign conditions available at the site [see GBT Memo #119].

Aperture efficiency at 44° elevation (Table 1)
Phase After Telescope
rms
Nominal
maximum frequency
Pointing
(benign conditions)
Aperture efficiency
8 GHz 20 GHz 50 GHz
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%

Aperture effiencies at zenith and horizon (Table 2)
Phase After Telescope
rms
Nominal
maximum frequency
Pointing
(benign conditions)
Aperture efficiency
8 GHz 20 GHz 50 GHz
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.

Beamwidth (Table 3)
Diffraction beamwidth (FWHM)
8 GHz 20 GHz 50 GHz
90" 36" 14"


H. Liszt 06/20/1999 "assume nothing"