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The 25.4 m diameter Giant Magellan Telescope will have four first-generation observing modes: Natural seeing, ground- layer AO, natural guide star AO, and laser tomography AO. These control modes are enabled by a suite of wavefront sensors and metrology systems that provide feedback to a segmented active primary mirror and a segmented adaptive secondary mirror. There have been several changes made to the GMT AO design since the last conference. The Natural Guide Star Wavefront Sensor now incorporates both a pyramid wavefront sensor for high-order sensing and a Holographic Dispersed Fringe Sensor to increase the segment piston sensing dynamic range. The On-Instrument Wavefront Sensors of the GMTNIRS and GMTIFS instruments have been further developed and now include real-time phase retrieval to sense segment piston errors at high frame rate. We are developing laboratory optical testbeds and prototype wavefront sensors to validate active optics and AO algorithms. We are also fabricating the first off-axis adaptive secondary mirror segment to retire fabrication risk and verify its performance.
