Real-time fringe contrast measurement in stellar interferometry

We have built a parallel-shear interferometer, which provides the visibility of celestial objects in real time. The interferometer produces two pairs of images of the entrance aperture, and fringes of constant visibility form in the overlap area of each pair. Due to atmospheric phase distortions the fringes have limited spacing and life time. This, combined with the sparsity of the photons, imposes strict limitations on the detectors and integration time. The fringes are modulated internally at 100 kHz, above the typical atmospheric frequencies. The image is transferred via a fiber optic bundle to a bank of photo-multipliers and preamplifiers, off the telescope. In our instrument twenty digital channels operate in parallel to extract the average fringe modulation. In each channel the photoelectron pulses are fed into a simple counter and two quadrature lock-in counters. The results of all sixty counters is read every few milliseconds. The modulation amplitude is found through the sine and cosine counters, regardless of the random phase. The simple counter serves to remove the Poisson noise bias.