Overview and status of the GMT wavefront control system

The Giant Magellan Telescope (GMT) wavefront control system provides active optics control and optical turbulence correction for every instrument on the 25.4 m diameter GMT. The GMT has four first-generation wavefront control modes that balance image quality, field of view, sky coverage, and development risk: Natural Seeing, Ground-Layer AO, Natural Guide Star AO, and Laser Tomography AO. Several aspects of the GMT wavefront control design have been recently updated. The Acquisition, Guiding, and Wavefront Sensing Subsystem, used in all control modes, has completed final design and a full-scale prototype sensor is being assembled. A Holographic Dispersed Fringe Sensor has been developed to improve the segment phasing capture range and stability of the Natural Guide Star AO mode. In the Laser Tomography AO mode, a high-speed infrared imager in each instrument will measure segment phasing disturbances using phase retrieval on a faint natural guide star, replacing an inter-segment differential laser metrology truss as the primary phasing sensor. High-fidelity simulations of all wavefront control modes have been developed, and we are developing wavefront sensor prototypes on laboratory testbeds that replicate the GMT optical design. We review the performance expectations in each control mode, and describe our plan to complete the wavefront control system development.