Fringe detection and piston variability in LINC-NIRVANA

We present the latest status of the fringe detecting algorithms for the LINC-NIRVANA FFTS (Fringe and Flexure Tracker System). The piston and PSF effects of the system from the top of the atmosphere through the telescopes and multi-conjugate AO systems to the detector are discussed and the resulting requirements for the FFTS outlined.