We describe a novel heterodyne interferometer currently under construction at Oxford. The instrument employs new techniques in heterodyne interferometry, with the aim of achieving very high brightness sensitivity in the millimetre band. It is a single-baseline tracking interferometer for operation in the frequency range 185-275 GHz with two 0.4m offset parabolic antennas separated by a 0.5 m baseline. Each antenna feeds an SIS mixer with a 2-20 GHz IF band, driven by a phase-switched LO source. The IF signals from the mixers are processed by a 2-20 GHz analogue complex correlator.The primary science goal of this instrument is to measure the spectrum of the Sunyaev-Zel'dovich effect in galaxy clusters. In particular we intend to measure the frequency of the S-Z null near 217 GHz, which allows the gas temperature of the cluster to be determined. Measuring the spectrum of the S-Z effect requires very high brightness sensitivity with moderate spatial and spectral resolution.
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