Modern optical astronomy: technology and impact of interferometry

The present “state of the art” and the path to future progress in high-spatial-resolution imaging interferometry is reviewed. The review begins with a treatment of the fundamentals of stellar optical interferometry, the origin, properties, and optical effects of turbulence in the Earth’s atmosphere, the passive methods such as speckle interferometry that are applied on a single telescope to overcome atmospheric image degradation, and various other techniques. These topics include differential speckle interferometry, speckle spectroscopy and polarimetry, phase diversity, wave-front shearing interferometry, phase-closure methods, dark speckle imaging, as well as the limitations imposed by the detectors on the performance of speckle imaging. A brief account is given of the technological innovation of adaptive optics to compensate for atmospheric effects on the image in real time. A major advancement involves the transition from single-aperture to dilute-aperture interferometry using multiple telescopes. Therefore the review deals with recent developments involving ground-based and space-based optical arrays. Emphasis is placed on the problems specific to delay lines, beam recombination, polarization, dispersion, fringe tracking, bootstrapping, coherencing and cophasing, and recovery of the visibility functions. The role of adaptive optics in enhancing visibilities is also discussed. The applications of interferometry, such as imaging, astrometry, and nulling, are described. The mathematical intricacies of the various “postdetection” image-processing techniques are examined critically. The review concludes with a discussion of the astrophysical importance and the prospects of interferometry.