27 June 2006 The LINC-NIRVANA fringe and flexure tracker: piston control strategies
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The Fringe and Flexure Tracking System (FFTS) is designed to correct the atmospheric piston variations and the instrumental flexure during the NIR interferometric image acquisition of the LINC-NIRVANA camera at the LBT. The interferometric image quality depends on the performance of these corrections. Differential piston and flexure effects will be detected and corrected in a real-time closed loop by analyzing the PSF of a guide star at a frequency of up to several hundred Hz. A dedicated piston mirror will then be moved in a corresponding manner by a piezo actuator. The FFTS is expected to provide a residual piston of better then 0.1 λ at the central wavelength of the science band. Thus, the required correction bandwidth is 10-20 Hz as differential piston simulations of different seeing conditions indicate. Therefore, a sampling frequency of 100-200 Hz is required to correct OPD variations. The upper limit for the loop frequency is the resonance frequency of the mirror and the response function respectively. The piston mirror as the actuator and the FFTS detector as the sensor feedback are embedded in a very complex system. Many control loop aspects like sampling frequencies, delays, controller algorithm and control bandwidth have to be identified. With accurate simulations of the system the limits of atmospheric and instrumental conditions for reliable closed loops can be determined against the respective control parameters. We present strategies for the closed-loop control of the piston correction which are suitable to achieve the 0.1 λ requirement and the optimal overall imaging performance with a sufficient "all-purpose" control stability.
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Steffen Rost, Steffen Rost, Thomas Bertram, Thomas Bertram, Christian Straubmeier, Christian Straubmeier, Yeping Wang, Yeping Wang, Andreas Eckart, Andreas Eckart, } "The LINC-NIRVANA fringe and flexure tracker: piston control strategies", Proc. SPIE 6274, Advanced Software and Control for Astronomy, 62741P (27 June 2006); doi: 10.1117/12.671741; https://doi.org/10.1117/12.671741

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