An experimental submillimetre heterodyne array receiver

We describe an experimental array receiver intended for radioastronomical observations of the J=3→2 line of carbon monoxide. It consists of six indium antimonide mixers cooled to ～2K in a pumped helium cryostat, with rectangular feed horns arranged on a 3×2 grid at the f/9 focus of the UK Infrared Telescope (UKIRT). Each detector is mounted in a length of fundamental mode waveguide and has an independently tunable backshort. Local oscillator power is provided by a frequency tripler pumped by a klystron, and is coupled to the array via a “lossless” quasi-optical coupler which is described in detail. The beam patterns and performance of the coupler have been measured and are in good agreement with theory.