The LINC-NIRVANA fringe and flexure tracker: control design overview

The Fringe and Flexure Tracker System (FFTS) of the LINC-NIRVANA instrument is designed to monitor and correct the atmospheric piston variations and the instrumental vibrations and flexure at the LBT during the NIR interferometric image acquisition. In this contribution, we give an overview of the current FFTS control design, the various subsystems, and their interaction details. The control algorithms are implemented on a realtime computer system with interfaces to the fringe and flexure detector read-out electronics, the OPD vibration monitoring system (OVMS) based on accelerometric sensors at the telescope structure, the piezo-electric actuator for piston compensation, and the AO systems for offloading purposes. The FFTS computer combines data from different sensors with varying sampling rate, noise and delay. This done on the basis of the vibration data and the expected power spectrum of atmospheric conditions. Flexure effects are then separated from OPD signals and the optimal correcting variables are computed and distributed to the actuators. The goal is a 120 nm precision of the correction at a bandwidth of about 50 Hz. An end-to-end simulation including models of atmospheric effects, actuator dynamics, sensor effects, and on-site vibration measurements is used to optimize controllers and filters and to pre-estimate the performance under different observation conditions