Compensation of pointing for the parabolic antenna of a radio telescope

Abstract

Taking Shanghai 65 m antenna as a research object, this paper investigates the pointing model and the influence caused by the displacement of subreflector on the performance of the antenna and the corresponding compensation method. The pointing error of antenna is caused by many factors, such as antenna manufacturing and assembling errors, the antenna structure deformation, and so on. For a large radio telescope, relatively difficult is the hardware calibration, which is implemented generally during the antenna installation and assembling, and hardly modified afterwards. In order to meet the accuracy requirement, it is necessary to measure and analyze the pointing errors in different azimuths and elevations, and to build a pointing error correction model. The investigation focuses on X-band frequencies. The data are collected by means of “cross scan”, then, they are fitted with a model including both the Gaussian and nonlinear terms, and we can calculate the pointing deviation. By fitting the whole-sky data of single-point fitting errors in both azimuth and elevation with the least squares method, the different coefficients of pointing model formulas can be obtained. It is verified that the model works well in the X band and other low-frequency bands, the pointing accuracy at the X band attains 13 arcsec. In addition, we need to compensate gravitational deformation of the antenna by adjusting the position and attitude of the subreflector, which varies along the elevation direction. So the pointing deviation caused by the offset of the subreflector need to be analyzed. Finally, we let the subreflector offset multiple displacements in the X and Y direction and then scan the target source in the azimuth and elevation. By gaussian fitting for the power reading to determine the pointing deviation value, the average pointing deviation caused by the unit displacement of the subreflector is calculated.