Green Bank Telescope: Overview and analysis of metrology systems and pointing performance

Abstract

With a 100m×110m off-axis paraboloid dish, the Green Bank Telescope (GBT) is the largest fully steerable radio telescope on Earth. 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. 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. 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. 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. 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. A pointing model was developed for the GBT to correct for these effects. 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. 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.

Large-aperture Telescope Tracking Control Based on Time-synchronization Strategy

“…Imaging Atmospheric Cherenkov Telescope (IACT) improves tracking accuracy by means of pointing corrections made by a CCD camera capturing a Cherenkov camera with LEDs and an observational light source in the sky (Zhurov et al 2019). GBT has developed a specialized pointing model to correct pointing accuracy for inaccuracies due to gravitational flexure, thermal deformation, azimuth track tilt, and offset errors (White et al 2022). The Daniel K. Inouye Solar Telescope (DKIST) (Rimmele et al 2020) and the Airborne Lunar Spectral Irradiance Mission (ALSIM) obtain lunar pointing (Newton et al 2020) with the same improved accuracy through modeling.…”

Numerical Simulation and Test Study on Track Welding of QTT

“…Each of the two sections of the track is fxed to the base plate by fllet welding, and a suitable amount of opposite deformation is given by bending the base plate so that the fatness of the entire track is within 0.3 mm after the welding is fnished. Te 100 m radio telescope (GBT) in the USA uses the same welding process as the LMT for its composite track [8][9][10]. After welding, the fatness of the entire track was controlled to within 0.127 mm.…”

Measuring Antenna Elevation Mechanism Pointing Errors with Multiencoder Information Sources

“…At present, the pointing accuracies achieved by large aperture radio telescopes in various countries are at the arcsecond level. Te 110 m radio telescope at Xinjiang [1] under construction has a design requirement of 5″ (blind pointing) for pointing accuracy, the SHAO 65 m radio telescope [2] has a pointing accuracy better than 3″ (after observing a local calibrator), and the 100 m radio telescope at Efelsberg [3] has a pointing accuracy of 10″ (blind pointing), and the Green Bank Telescope [4,5] has a pointing accuracy of 9″ (blind pointing). When higher pointing accuracy is required for radio telescopes to achieve their science goals, it also means that more error factors afecting the pointing accuracy need to be considered, including displacement of the secondary refector, azimuth track fatness, gravity loading, wind loading, and nonuniform temperature felds.…”