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Very fast wavefront sensing, on the order of 1-2 kHz correction rate is needed for exoplanet detection from the ground. In this paper we describe a wavefront sensor which utilizes direct interferometry to measure the wavefront. The converging beam of the bright star to be observed is passed through a Mach-Zender interferometer. By placing a pinhole in one of the arms at the focus of the star, a spherical reference wave can be produced. The beams from the two arms are then combined to obtain an interference pattern of the wavefront. By introducing a 1/4 wave phase difference between the beams, small phase errors are encoded as intensity differences between the two interferometer outputs which map the wavefront at the telescope pupil. This interferometer builds on a related phase shifting interferometer built by D'nardo Colucci. The new interferometer is aimed at 25 nm accuracy over 5 cm subapertures at 2 kHz rate, given CCD detectors operating at high speed with very low noise.
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