Vector near-field beam scanner for the SMA

Abstract

Here we describe the principles behind the design, construction, and implementation of a vector near-field beam scanner for the antennas of the Submillimeter Array. The Submillimeter Array (SMA) is a radio interferometer array operating at frequencies ranging from 200 { 700 GHz at the summit of Maunakea in Hawaii. A set of 4 receivers cover the key atmospheric windows over which the SMA operates. Each receiver insert is equipped with an ambient optical insert, which is pre-aligned in the lab prior to installation at the summit. However, as a result of receiver upgrades and problems, some receiver inserts may no longer be matched to the original optics inserts. Since the SMA is used extensively in dual-receiver observations, such beam mis-alignments lead to a relative pointing error between a pair of receivers during the observation. In order to address this issue, we have designed a near-field beam scanner which can be used to map out the receiver beam of each antenna. The setup employs the existing radio references available in each antenna for the vector beam measurement. We have successfully used this scanner to improve the on-sky co-alignment of receiver beams. In this presentation, we will describe the system and operational aspect of this in-situ radio frequency alignment technique.