Shearing stellar interferometer. 2: Optoelectronic phase-locked system

A parellel-shear interferometer was built which is able to provide the visibility of celestial objects in real time in the presence of atmospheric turbulence. The interferometer consists of two beam splitters and a modu-lating/shearing element and is stable and efficient. At each shear value the fringes have constant visibility, but their atmospheric-induced phase is random in time and space. A water cell modulates the optical path at a frequency above that of most atmospheric fluctuations. The modulated interference pattern is trans ferred via fiber optics to a bank of photomultipliers off the telescope. A set of digital lock-in amplifiers and a microprocessor give the (incoherent) sum of the fringe visibilities every few milliseconds. First observa tions show the promise of the technique and reveal needed refinements.