Analysis and Implementation of an Optimal Controller for Tip-tilt Correction on the Gemini Multi-Conjugated AO System

Thursday, 17 September 2015 noon — 1 p.m. MST

AURA Lecture Hall

NOIRLab South Colloquia
LUCIE LEBOULLEUX (Institut d’Optique Graduate School (France) and Gemini South Observatory Intern Student)

AO systems aim at detecting and correcting for optical distortions induced by atmospheric turbulences. 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 already 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 in November 2015.