Green Bank Telescope (GBT)

The Robert C. Byrd Green Bank Telescope (GBT) was dedicated in 2000 and recently began full-time operation as the world's most sensitive fully steerable radio telescope. Located in the National Radio Quiet Zone (NRQZ) and shielded by mountains, the GBT enjoys unique protection against radio-frequency interference (RFI). In addition, the 100 m diameter GBT is the only large radio telescope having a clear aperture. Eliminating blockage by feed-support structures increases its aperture efficiency, greatly reduces sidelobe responses to stray radiation from extended radio sources and RFI, and lowers the noise pickup from ground radiation. The GBT surface consists of 2,004 high-precision panels which are continuously adjusted by computer-controlled actuators to remove deformations caused by gravity, thermal gradients, and setting errors. The rms surface error of the GBT is currently 0.32 mm (λ/16 at λ = 5 mm, or ? = 60 GHz) in benign conditions, initial operation at λ = 3 mm (ν = 100 GHz) has commenced with the introduction of the Mustang bolometer array.

GBT receivers provide nearly continuous frequency coverage from λ = 1 m to 6 mm (0.29 to 52 GHz) with on-sky system temperatures as low as 20 K. A single-pixel correlation receiver for λ = 3–4 mm (68–92 GHz) and the Penn Array 64-element bolometer camera for λ = 3 mm (100 GHz) are under construction. The GBT spectrometer is a multilevel digital correlator providing 2,048 channels in each of eight 800 MHz spectral bands or up to 262,144 channels at 50 MHz bandwidth. The Zpectrometer, a redshift machine for the GBT having a 14 GHz instantaneous bandwidth at 26–40 GHz, was recently funded by the NSF ATI (Advanced Technologies and Instrumentation) program. Three fast-sampling backends provided by university collaborations are available for pulsar observers.

Unique characteristics of the GBT include:

• Largest fully steerable single-dish telescope in the world.
• The only large telescope having an unblocked aperture.
• Low elevation limit makes 85% of the entire celestial sphere accessible and allows long observations for monitoring transient events, pulsar timing, and VLBI.
• Low-RFI environment thanks to terrain shielding and the unique National Radio Quiet Zone, allowing unique HI and pulsar observations. This offers access to frequencies which might not otherwise be observable, gives greater sensitivity for continuum and pulsar observations, and greater opportunity to observe spectral lines both at rest and redshifted throughout the spectrum.
• Highest pulsar sensitivity of any fully steerable telescope.
• Continuous frequency coverage from 290 MHz–52 GHz (λ = 6 mm to 1 m) currently, to 115 GHz (λ = 2.6 mm) in the future.
• Large effective collecting area (~2,000 m2) and focal plane capable of accepting feed arrays having thousands of pixels at λ = 3 mm.
• Possibly the best imaging capability of any single-dish radio facility owing to the offset optics; high-fidelity wide-field HI imaging capability.
• The large diameter (in wavelengths) of the filled aperture results in a unique combination of high sensitivity and resolution for point sources plus high surface-brightness sensitivity for faint extended sources.