Optical design of S-band multifeed for the Sardinia Radio Telescope primary focus

We present the optical design of an S-band seven-feed cryogenic radio astronomy receiver for illuminating the 64-m diameter Sardinia Radio Telescope (SRT) dish from the primary focus. The feeds are arranged in a compact hexagonal configuration with a central one and are cryogenically cooled at 20 K inside a cryostat. Each feed accepts two linear polarizations and use a circular waveguide with a single outer corrugated section to achieve a nearly constant beam width and low cross polarization across the 3.0-4.5 GHz band. The simulated radiation pattern of the SRT telescope is obtained by coupling the array of feed-horn beam patterns (optimized with the electromagnetic software CST) with the 64-m parabolic dish (through a physical optics analysis carried out with GRASP). We compare the simulated beam pattern of an isolated feed with those of the same feed embedded in the dense array and analyze the effects of an absorber located inside the cryostat around the cryogenic feeds. We found that the absorber improves the overall system performance by decreasing the cross-coupling effects between the feeds while adding negligible noise to the system.