Image-plane fringe tracker for adaptive-optics assisted long baseline interferometry

Accurate fringe tracking is essential for sensitive long-wavelength thermal background limited operation of the current Very Large Telescope Interferometry (VLTI) and future Planet Formation Imager (PFI) facilities. We present and simulate a dual fringe tracking and low-order adaptive optics concept based on a combination of non-redundant aperture interferometry and eigenphase in asymmetric pupil wavefront sensing. This scheme can acquire fringes at many wavelengths of path length offset between telescopes, even with moderate tilt offset and pupil shifts between beams. Once locked to fringes, our technique can also be used for simultaneous low-order wavefront sensing, and has near-optimum sensitivity where there is a dominant point-source image component. This concept is part of the Heimdallr visitor instrument currently being investigated for VLTI.