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The Robert C. Byrd Green Bank Telescope of the National Radio Astronomy Observatory is the world's premiere single-dish radio telescope operating at centimeter to long millimeter wavelengths. This paper describes the history, construction, and main technical features of the telescope.

We report on the design and construction of the Penn Array Receiver, a 64 element array which will be a 90 GHz facility instrument for the 100 m Green Bank Telescope. Each array element is a transition edge sensor bolometer. Detector noise will be background limited by the random arrival of photons and in one hour we expect to be able to map a 5 arcminute square of sky to 80 μ Jy at a resolution of 8 arcseconds. No feed horns are used, instead cooled high-density polyethylene lenses and a cold (3 K) Lyot stop control the illumination of the telescope. Capacitive mesh filters act as IR blockers and define a bandpass of 86-94 GHz. The array elements are spaced by 0.5 f λ so that the sky is fully sampled in a single pointing of the GBT. This significantly simplifies the scanning patterns needed for sparser arrays. The detectors are cooled to 0.25 K using a pulse tube cooler and helium sorption refrigerators. The cryogenics along with all the electronics and the computer system have been successfully operated on the GBT. By February, detailed tests on the response of the detectors will allow us to further develop the data processing techniques needed to remove atmospheric noise from the data and beam measurements of the cryostat will confirm that the optical design is correct. We are also developing better control software to automatically bias the SQUID multiplexed readout.

This paper discusses the design, construction, and measured performance of dual-band, dual-polarization, prime focus receiver systems used on three antennas at the NRAO facility in Green Bank, WV. The receivers operate at Sand X-bands near 2.3 and 8.4 GHz. Described is the development of a new dualband feed horn, a cryogenics package with waveguide input ports using HEMT low-noise amplifiers, and a receiver system using optical fiber microwave links to transmit frequency reference and IF signals. Measured performance of major components and the overall installed system is given. >

The scanning properties of shaped reflectors, both offset and circularly symmetric, are examined and compared to conic section scanning characteristics. Scanning of the pencil beam is obtained by lateral and axial translation of a single point source feed. The feed is kept pointed toward the center of the subreflector. The effects of power spillover and aperture phase error as a function beam scanning are examined for several different types of large reflector design including dual-offset, circularly symmetric large f/D, and smaller f/D dual reflector antenna system. It is shown that the Abbe-sine condition for improved scanning of an optical system cannot, inherently, be satisfied in a dual-shaped reflector system that is shaped for high gain and low feed spillover. The gain loss, with scanning, of a high-gain shaped reflector pair is demonstrated to be due to both aperture phase error loss and power spillover loss. >

A 100-m-diameter, fully steerable, offset dual reflector radio telescope is under construction at the Green Bank, West Virginia, facility of the National Radio Astronomy Observatory (NRAO). Significant performance improvement is expected of the offset design over that possible with symmetrical antennas. The antenna will be capable of operation from below 300 MHz to above 40 GHz. To take full advantage of the offset design, ultra-low-noise cryogenic receivers will be constructed and installed. The trade-offs made in the design of the microwave optics are summarized, and plans for receivers and associated equipment are described. >

Several characteristics of dual offset shaped reflector (DOSR) antennas are examined. Despite the potentially superior performance of DOSR over conventional Cassegrain/Gregorian offset designs and over shaped circular symmetric designs, these characteristics can make the DOSR designs less desirable. In particular, the simultaneous multiple-beam capability of DOSR is found to be inferior to conic (paraboloid-ellipse/hyperboloid) reflectors. This difficulty can possibly be overcome to some degree by the use of overlapping feed arrays. The higher manufacturing cost of a DOSR can be somewhat ameliorated by the use of adjustable panels on a large offset paraboloid if the paraboloid and a shaped reflector are very little different in shape. The adjustable panels may be used for correction of gravity and wind distortions as well as for shaping. It is pointed out that new exact synthesis methods for DOSR permit the design of very low cross-polarization aperture distributions such as are obtainable with Mizugutch designs for conic systems. >