The LINC-NIRVANA fringe and flexure tracking system

LINC-NIRVANA is the near-infrared homothetic imaging camera for the Large Binocular Telescope. Once operational, it will provide an unprecedented combination of angular resolution, sensitivity and field of view. Its Fringe and Flexure Tracking System (FFTS) is mandatory for an efficient interferometric operation of LINC-NIRVANA. It is tailored to compensate low-order phase perturbations in real-time to allow for a time-stable interference pattern in the focal plane of the science camera during the integration. Two independent control loops are realized within FFTS: A cophasing loop continuously monitors and corrects for atmospheric and instrumental differential piston between the two arms of the interferometer. A second loop controls common and differential image motion resulting from changing orientations of the two optical axes of the interferometer. Such changes are caused by flexure but also by atmospheric dispersion. Both loops obtain their input signals from different quadrants of a NIR focal plane array. A piezo-driven piston mirror in front of the beam combining optics serves as actuator in the cophasing loop. Differential piston is determined by fitting a parameterized analytical model to the observed point spread function of a reference target. Tip-tilt corrections in the flexure loop are applied via the secondary mirrors. Image motion is sensed for each optical axis individually in out-of-focus images of the same reference target. In this contribution we present the principles of operation, the latest changes in the opto-mechanical design, the current status of the hardware development.