Adaptive vibration cancellation on large telescopes for stellar interferometry

In stellar interferometry fringe-tracking is a method of stabilizing the Optical Pathlength Difference (OPD) from the observed astronomical source to the instrument detector via different telescopes in an interferometric array. At the ESO Very Large Telescope Interferometer, which includes four 8.2 m class Unit Telescopes (UTs), stabilization to better than a tenth of the observing wavelength is required in order to improve the quality and sensitivity of fringe measurements on the interferometer's scientific instruments. Unfortunately, fast mechanical vibrations due to myriad sources in the observatory infrastructure propagate to various mirrors in the optical path and must be compensated for in real time. Due to its limited bandwidth the fringe tracking loop cannot be used for this purpose. Alternative approaches must therefore be adopted. Vibrations imparted to the primary, secondary and tertiary mirrors of the UTs are currently measured by a grid of suitably placed accelerometers, converted to optical pathlengths and cancelled by a wideband feedforward compensation algorithm to a downstream optical delay line. Although very effective, it is obvious that this system can not compensate for vibrations originating elsewhere on the optical path. We present here an adaptive narrow-band cancellation algorithm that can compensate remaining vibrations measured on the stellar signal on condition that they are sufficiently stable in amplitude and frequency.