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

TL;DR

The paper reviews the Green Bank Telescope's metrology systems and pointing performance, highlighting how structural and environmental factors affect accuracy and how a model corrects these errors for high-frequency observations.

Contribution

It presents a comprehensive overview of the GBT's pointing model, integrating metrology data to improve accuracy, which is a novel approach for large steerable radio telescopes.

Findings

Pointing accuracy is maintained within a few arcseconds RMS at high frequencies.

The pointing model effectively corrects for structural and environmental errors.

Metrology data significantly enhance the telescope's pointing precision.

Abstract

With a 100mx110m 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.