GBT Operating Specifications

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 that can be observed near the so-called "rigging-angle", i.e., an elevation of 44°. 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.

Tables 1 and 2 below 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 the two.

Immediately after construction, the telescope surface will have been set to the contractor's specified 1.08 mm (0.040 inch) 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 0.46 mm (0.018 inch). 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.

NRAO expects eventually to use the laser-ranging system and surface actuators in a fully closed feedback loop to afford observing up to ~100 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"