Correction of pistonic aberrations at the LBT: a near-infrared fringe and flexure tracker for LINC

The I. Physikalische Institut of the University of Cologne is participating in an international collaboration with the Max-Planck-Institut fur Astronomie in Heidelberg and the Osservatorio Astrofisico di Arcetri for the development of LINC/NIRVANA, the Near-Infrared/Visible Interferometric Camera for the Large Binocular Telescope (LBT). LINC/NIRVANA will be one of the two interferometric camera systems of the LBT and will operate at wavelengths from 0.6 μm to 2.4 μm, with the long wavelength regime between 1.0 μm and 2.5 μm being covered by LINC (LBT INterferometric Camera} and the shorter wavelengths part from 0.6 μm to 1.0 μm being processed by NIRVANA (Near-InfraRed/Visible Adaptive iNterferometer for Astronomy}. The main contributions of the Cologne institute to this camera will be the 77K dewar and the Fringe and Flexure Tracker (FFT) for the near-infrared part on the system. Detecting and correcting the fast pistonic aberrations of the atmosphere and the slow flexure of the instrument in a closed-loop operation, the presence and proper function of the FFT is mandatory for a time-stable image quality at highest interferometic resolutions. In order to get the best possible image correction for LINC, the FFT will be located inside the camera dewar at an interferometric focus close the one of the near-infrared science detector. Using simple optical elements it will continuously monitor the time-variable phase difference and pupil locations of the incoming wavefronts from the two arms of the twin-telescope. In this article we give a short overview of the camera concept of LINC and present the current status of the design and development of the FFT going on at our institute at the University of Cologne.