Performance Estimates of the 18-Meter ngVLA Reflector System with Main Reflector Rim Extension

The mechanical design of the ngVLA 18-meter offset Gregorian reflector system is nearing completion, and requires a main reflector rim extending beyond the nominal optical path defined by a circular projected rim. The extra reflecting area may thus be used to try to improve the electrical performance of the system with little to no extra manufacturing cost. This paper investigates the effect of including the extra main reflector area on the expected performance of the reflector system, in the presence of the extended sub-reflector used to shield ground noise in the feed-down tipping configuration. It is shown that the extra area indeed improves the aperture efficiency, and receiving sensitivity, substantially – especially when accounting for the shape of the sub-reflector extension. This sensitivity improvement, however, comes at the cost of reduced main beam symmetry, presenting a trade-off in the system design.