Improved Tip-Tilt Correction with Altair, The Gemini North AO System

Although tip-tilt may seem straightforward to correct with a high-order adaptive optics system, tip-tilt errors due to latency in the correction loop usually turn out to dominate the residual wave-fronts in the classical control scheme using a one integrator compensator and a fast tip-tilt mirror. In this paper, we propose a more sophisticated control scheme where the fast tip-tilt mirror as well as the deformable mirror and the telescope secondary mirror are used to corrected tip-tilt, with optimal use of their respective bandwidth and stroke. This scheme is based on a cascade of three integrators and is called multi-nested control loop. Its performance is assessed in the context of Altair, the future Gemini North high order adaptive optics system. It is found that in median observing conditions, this scheme allows to reduce the tip-tilt error rms by a factor of about 4, without signiicantly increasing the sensitivity of the control loop to wave-front sensing noise. In addition, it is found that these results can be achieve without requiring more than 1 Hz of bandwidth from the secondary mirror.