The LINC-NIRVANA Fringe and Flexure Tracker: Testing Piston Control Performance

LINC-NIRVANA is the NIR homothetic imaging camera for the Large Binocular Telescope (LBT). Its Fringe and Flexusre Tracking System (FFTS) is mandatory for an efficient interferometric operation of LINC-NIRVANA: the task of this cophasing system is to assure a time-stable interference pattern in the focal plane of the camera. Differential piston effects will be detected and corrected in a real-time closed loop by analyzing the PSF of a guide star at a frequency of 100Hz-200Hz. A dedicated piston mirror will then be moved in a corresponding manner by a piezo actuator. The long-term flexure tip/tilt variations will be compensated by the AO deformable mirrors. A testbed interferometer has been designed to simulate the control process of the movement of a scaled piston mirror under disturbances. Telescope vibration and atmospheric variations with arbitrary power spectra are induced into the optical path by a dedicated piezo actuator. Limiting factors of the control bandwith are the sampling frequency and delay of the detector and the resonance frequency of the piston mirror. In our setup we can test the control performance under realistic conditions by considering the real piston mirrors dynamics with an appropriate software filter and inducing a artificial delay of the PSF detector signal. Together with the expected atmospheric OPD variations and a realistic vibration spectrum we are able to quantify the piston control performance for typical observation conditions. A robust control approach is presented as result from in-system control design as provided by the testbed interferometer with simulated dynamics.