Design of frequency-based controllers for vibration mitigation at the Gemini-South telescope

Reduction of tip and tilt vibrations at the Gemini South MCAO System (GeMS) is addressed in this paper. A frequency framework for the synthesis of controllers is described, with particular emphasis on the search for better closed-loop performances by minimizing a H2 norm of the tilt residuals. Previous results have shown that modeling the turbulence via identification tools using standard AR or Laplace representations can lead to non-optimal solutions, resulting in excessive rejection of certain frequencies or an unbalanced residual spectrum due to poor modeling of vibrations. In this novel approach we reconstruct the open loop slopes (pseudo-open-loop) from on-sky data and then perform a fine tuning of the controller by finding the parameters that minimize the variance of residuals during a sequence of closed-loop runs with increasing controller complexity. Although the method is not optimal, it effectively rejects the main vibrations in the loop and it also improves the overall performance of the system. The method is compared to two standard integrators: one with fixed gain and the other with optimized integral gain. Results show substantial improvements of this new method when compared to the classical integrator.