21 July 2010 The LINC-NIRVANA fringe and flexure tracker: first measurements of the testbed interferometer
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LINC-NIRVANA is the near-infrared Fizeau interferometric imaging camera for the Large Binocular Telescope (LBT). For an efficient interferometric operation of LINC-NIRVANA the Fringe and Flexure Tracking System (FFTS) is mandatory: It is a real-time servo system that allows to compensate atmospheric and instrumental optical pathlength differences (OPD). The thereby produced time-stable interference pattern at the position of the science detector enables long integration times at interferometric angular resolutions. As the development of the FFTS includes tests of control software and robustness of the fringe tracking concept in a realistic physical system a testbed interferometer is set up as laboratory experiment. This setup allows us to generate point-spread functions (PSF) similar to the interferometric PSF of the LBT via a monochromatic (He-Ne laser) or a polychromatic light source (halogen lamp) and to introduce well defined, fast varying phase offsets to simulate different atmospheric conditions and sources of instrumental OPD variations via dedicated actuators. Furthermore it comprises a piston mirror as actuator to counteract the measured OPD and a CCD camera in the focal plane as sensor for fringe acquisition which both are substantial devices for a fringe tracking servo loop. The goal of the setup is to test the performance and stability of different control loop algorithms and to design and optimize the control approaches. We present the design and the realization of the testbed interferometer and comment on the fringe-contrast behavior.
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L. Moser, A. Eckart, M. Horrobin, B. Lindhorst, S. Rost, C. Straubmeier, E. Tremou, I. Wank, J. Zuther, T. Bertram, "The LINC-NIRVANA fringe and flexure tracker: first measurements of the testbed interferometer", Proc. SPIE 7734, Optical and Infrared Interferometry II, 77342X (21 July 2010); doi: 10.1117/12.856654; https://doi.org/10.1117/12.856654

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