Optical and Infrared Interferometers

Abstract

Stellar interferometers achieve limiting angular resolution inaccessible to even next-generation single-aperture telescopes. Arrays of small or modest apertures have achieved baselines exceeding 300mproducing submilliarcsecond resolutions at visible andnear-infrared wavelengths. The technical cost and challenge in building interferometric arrays is substantial due to the very high tolerance imposed by optical physics on the precision of beam combination and optical path length matching for two or more telescopes. This chapter presents the basic theory and overall design considerations for an interferometer with an emphasis on the practical aspects of constructing a working instrument that overcomes obstacles imposed by the atmosphere, submicron path length matching requirements, limitations on number of telescopes and their layout, light losses throughmultiple reflections and transmissions necessary to superimpose telescope beams in the beam-combining laboratory, and other realities of the art of interferometry. The basic design considerations for an interferometer are laid out starting with site selection and telescope placement and then followed through to beam combination and measurement of interferometric visibility and closure phase after the encountering of numerous subsystems by incoming wavefronts. These subsystems include active wavefront sensing for tip/tilt correction or even full-up adaptive optics, telescope design for directing collimated beams over large distances, diffraction losses, polarization matching, optical path length insertion and active compensation, correction for atmospheric refraction and differential dispersion in glass and air, separation of light into visible and near-infrared channels, alignment over long optical paths, high-precision definition of the three-dimensional layout of an interferometric array, and, finally, a variety of beam-combining schemes from simple two-way combiners to multitelescope imaging combiners in the pupil and image planes. Much has been learned from a modest but robust collection of successful interferometers over the last 25 years or so, and interferometry is poised to become a mainstream technique for astrophysical research.