Optimization of Receiver Optics for Radio Astronomy

In a radio telescope, the sub-reflector is illuminated by a complex feed system, consisting of a feed horn and a pair of optics focusing elements which are usually a pair of mirrors. Although rigorous analysis of this system can be performed using Method of Moments (MoM) or physical optics (POs), design optimization using these methods may not be viable, since it requires lengthy computational time. In this paper, we describe an efficient optimization technique for the optics design which applies the quadratic on a pedestal distribution to compute the taper and aperture efficiencies. In our method, multimode Gaussian optics is employed to calculate the electromagnetic waves which scatter through the optical system. The edge taper associated with the optimum aperture efficiency is first identified. By setting the parameters of this edge taper and also the distance between mirror 2 and the antenna focus as the iteration targets, a root-searching routine is then applied to determine the distances of the optical paths between the mirrors and the feed. When an optimized feed design is established, the antenna performance indicators, such as the beam efficiency, co- and cross-polarization levels, and aperture efficiencies, are calculated using PO. In this way, we combine the accuracy of the quadratic function in determining the antenna efficiencies and the computational efficiency of Gaussian optics to optimize the design of the system with the rigor of PO to validate the final parameters of the antenna. The design procedure for the Atacama Large Millimeter/Submillimeter Array (ALMA) interferometric radio telescope’s feed optics system is used as an illustrative example. The results show that the co-polar beam efficiencies achieved with the proposed method are higher than those of the original method used for the ALMA feed optics system, while the cross-polar beam efficiencies are lower. This suggests a substantial improvement offered by the newapproach.