Subreflector Position Model Depending On Elevations And Performance Measurement For Shanghai 65m Radio Telescope

In the course of astronomical observations, with changes in angle of pitch,the large radio telescope will have different degrees of deformation in the subreflector support, back frame, main reflector etc, which will lead to the dramatic decline of antenna efficiency in both high and low elevations. A subreflector system of the Shanghai 65 m radio telescope has been installed in order to compensate for the gravitational deformations of the sub-reflector support and the main reflector. The position and attitude of the subreflector are variable in order to improve the pointing performance and the efficiency at different elevations. In this paper, the influence on studied that the changes of position and attitude of the subreflector are studied efficiency of antenna in X and Ku bands due to. A model has been constructed to determine the position and attitude of the subreflector with elevation.In addition, the pointing deviation caused by the offset of the subreflector is analyzed and the model of pointing deflection caused by the displacement of subreflector is established,which can be used to improve the pointing accuracy, by observing the radio source. An eight-parameter pointing model is built after fitting all collected errors. The model is applied and has been proven to work well in X band and other bands with lower frequencies. The final pointing accuracy in X band is 12.36 arcsecond. Holography is used to obtain the aperture surface phase and calculate position offset in X, Y, Z directions,which are compared with subreflector offset modes built by using radio source amplitude scanning method. Both are in great agreement with each other.Especially the fitting situation in Z direction is very consistent.In addition, antenna efficiency was tested with subreflector position adjusted and fixed. The results show that the model of subreflector can effectively improve the efficiency of the Shanghai 65 m radio telescope. In X band, the aperture efficiency of the radio telescope reaches more than 60% over the entire elevation range.