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Open Access Publications from the University of California

A New Slow Focus Sensor for GeMS


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).

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