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29 June 2006 The LINC-NIRVANA fringe and flexure tracker: image analysis concept and fringe tracking performance estimate
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The correction of atmospheric differential piston and instrumental flexure effects is mandatory for interferometric operation of the LBT NIR interferometric imaging camera LINC-NIRVANA. The task of the Fringe and Flexure Tracking System (FFTS) is to detect and correct these effects in real-time. In the fringe tracking concept that we present, differential piston information is gathered in the image plane by analyzing the PSF of a reference star anywhere in the large field of view of the LBT. We have developed and tested a fast PSF analysis algorithm that allows to clearly identify differential piston even in the case of low S/N. We present performance estimates of the algorithm. Since the performance of the FFTS algorithm has a strong impact on the overall sky coverage of LINC-NIRVANA, we studied the required limiting magnitudes of the fringe tracking reference star for different scenarios. As the FFTS may not necessarily operate on the science target, but rather uses a suitable reference star at a certain angular distance to the science target, differences between piston values at the two positions add to the residual piston of the FFTS. We have dealt with the question of differential piston angular anisoplanatism and studied a possible improvement of the isopistonic patch size by the use of multi-conjugate adaptive optics (MCAO). In its final stage, LINC-NIRVANA will be equipped with such a system.
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Thomas Bertram, Carmelo Arcidiacono, Christian Straubmeier, Steffen Rost, Yeping Wang, and Andreas Eckart "The LINC-NIRVANA fringe and flexure tracker: image analysis concept and fringe tracking performance estimate", Proc. SPIE 6268, Advances in Stellar Interferometry, 62683P (29 June 2006);

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