Heterodyne interferometry: review and prospects

The recombination of a large number of telescopes in an imaging array represents a major long-term challenge of infrared astronomy, which motivates active instrumental research. In the mid-infrared, as initiated in the early 1980s by the group of C.H. Townes at Berkeley, heterodyne interferometry offers a potential path in complement to classical interferometry, being scalable more easily to a large number of telescopes and by relaxing the requirement on a complex infrastructure, but at the cost of a sensitivity penalty. In this review, we present the current status of heterodyne interferometry and its prospects in light of recent technological developments in this technique. We start by recalling the basic working principles of heterodyne interferometry and the sensitivity budget of this technique. We then present the impact of current technological developments — detectors, local oscillators, correlators, phase synchronization — on the building blocks of a heterodyne interferometer. In the last part, we focus on the interest of developing pathfinders of imaging interferometric instruments, and the synergies with classical interferometry. In particular, the dimensioning of pathfinder instruments highlights the trade-off between angular resolution and sensitivity in the design of large imaging interferometers.