An Optimal 18 m Shaped Offset Gregorian Reflector for the ngVLA Radio Telescope

Abstract

This article describes the design of the 18 m shaped offset Gregorian reflector system for the Next Generation Very Large Array (ngVLA) radio interferometer and the associated 1.66:1 axially corrugated conical horn antennas. A parametric sweep determined feed horns with the goal of pattern symmetry and low spillover efficiency. Given these feeds, a mapping function for the shaped offset Gregorian reflector system was then determined, by a full-factorial design, to achieve maximum receiving sensitivity with secondary parameters such as plate scale, scan loss, sidelobe, and cross-polarization at acceptable levels. A subreflector extension is included in a feed-down tipping configuration with improved performance over the feed-up case. The resultant optics and feeds achieve very symmetrical patterns with an aperture efficiency of around 95%, a plate scale of 15 arcsec/mm, and first and second sidelobe levels of −18 and −24 dB, respectively. The receiving sensitivity at 30 GHz is 8.7 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /K and it increases to above 12 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /K at 12.3 GHz and reduces to 1.5 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /K at 116 GHz (ignoring the efficiency loss due to nonideal reflector surfaces). These sensitivities are close to the theoretical optimum.