The Gemini South 8-meter telescope’s Multi Conjugate Adaptive Optics System GeMS is about to enter a new era ofscience with an entire new upgrade for its Natural Guide Star wave front sensor (NGS2). With NGS2 the limitingmagnitude of the natural guide stars used for tip/tilt sensing is expected to increase from its current limit of 15.4 to 17+in R-band. This will provide a much greater sky coverage over the current system. NGS2 is a complete replacement ofthe current Natural Guide Star wave front sensor (NGS). This presents an interesting challenge as the current NGSincludes a Slow Focus Sensor (SFS) used to compensate for the sodium layer mean altitude variations. With the newNGS2 setup, this SFS will be removed and a suitable replacement must be found. Within the Gemini environment thereexist two facility wave front sensors, Peripheral Wave Front Sensors one and two (PWFS1 and PWFS2), that could actas an SFS. Only one of these (PWFS1) is located optically in front of the GeMS Adaptive Optics (AO) bench (Canopus).We are currently preparing this wave front sensor as the new SFS for GeMS under the NGS2 setup. The results ofseveral nighttime and daytime tests show that PWFS1 will be an adequate SFS for GeMS in the NGS2 setup providingexcellent sky coverage without compromising the GeMS Field of View (FoV).

Adaptive Optics (AO) systems aim at detecting and correcting for optical distortions induced by atmospheric turbulences on ground based telescopes astronomical images. They are also extremely sensitive to extraneous sources of perturbations such as vibrations, which degrade their performance. A new well defined vibration at 37Hz has been detected in January 2015 and is still currently affecting the Gemini South telescope secondary mirror. We show how its existence limits the performance of the operational systems at Gemini South: The Gemini Planet Imager (GPI) and the Gemini Multi-Conjugated AO system (GeMS). We further focus on how these vibrations are affecting GeMS performance and propose to implement the Tip-Tilt control strategy first tested on CANARY and routinely used on SPHERE. It combines identification of a sum of auto-regressive models of order 2 with a Linear Quadratic Gaussian (LQG) control. LQG is now routinely used for Tip-Tilt and focus control for GPI and has been successfully tested on all modes on CANARY. We show that the expected gain in performance brought by this LQG Tip-Tilt control strategy on GeMS compared with an Integral Controller is on the order of 15 to 20mas. The analysis was conducted in ”replay mode” using GeMS Tip-Tilt on-sky data. This allows realistic performance assessment before implementation inside the Real-Time Computer (RTC) of GeMS and on-sky tests during the first semester of 2016.