![]() ![]() The subreflector provides the secondary Gregorian focus, and up to eight receivers ride in a turret that can put any of them at the Gregorian focus. Low-frequency receivers (0.29 GHz–1.23 GHz) can be placed at the prime focus. The large feed arm supports the receivers at the focus. The axis of the parabola is at the edge of the dish’s surface. 1) has an off-axis parabolic design with an unblocked aperture, which results in higher aperture efficiency and a cleaner diffraction pattern (which enables high-fidelity mapping), minimizes out-of-beam radio frequency interference (RFI), and avoids reflections that can compromise spectral baselines. In this paper we provide a summary of the GBT’s pointing model and associated corrections, as well as a discussion of relevant metrology systems and an analysis of its current nighttime pointing accuracy. The model utilizes standard geometrical corrections along with metrology data from the GBT’s structural temperature sensors and data from measurements of the track levels. A pointing model was developed for the GBT to correct for these effects. However, factors such as gravitational flexure, thermal deformation, azimuth track tilt and irregularity, and small misalignments and offset errors within the telescope’s structure cause pointing inaccuracies. The GBT’s off-axis design is advantageous in that it eliminates blockage of the dish and reduces sidelobe interference, and there is no evidence that the resulting asymmetric structure adversely affects pointing accuracy. The required pointing accuracy is dependent upon the half-power beamwidth, and for the higher-frequency end of GBT observing, this means that pointing must be accurate to within a few arcseconds RMS. Accurate pointing requires the ability to blindly acquire source locations and perform ad hoc corrections determined by observing nearby calibrator sources in order to obtain a starting position accurate to within a small margin of error of the target’s location. A major challenge facing large ground-based radio telescopes is achieving sufficient pointing accuracy for observing at high frequencies, up to 116 GHz in the case of the GBT. With a 100 m × 110 m off-axis paraboloid dish, the Green Bank Telescope (GBT) is the largest fully steerable radio telescope on Earth. West Virginia University, Morgantown, WV, USA Ray 2Į-mail: Bank Observatory, Green Bank, WV, USA Astronomical objects: linking to databasesĮ.Including author names using non-Roman alphabets.Suggested resources for more tips on language editing in the sciences Punctuation and style concerns regarding equations, figures, tables, and footnotes ![]()
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