Results from the GMT wide-field phasing testbed

We report on results from the laboratory wide-field phasing testbed that has been constructed to validate the active optics and piston sensing hardware and algorithms to be used on the Giant Magellan Telescope. The GMT is comprised of seven primary (M1) mirror segments, and seven secondary (M2) segments. To maintain a high quality wavefront across the full LTAO field-of-view, telescope aberrations must be controlled at M1 and M2 independently. This will be done on GMT using the 4 off-axis wavefront sensors of the Acquisition, Guiding and Wavefront-sensing System (AGWS). With the double segmented nature of the GMT, the most challenging aberration to control is field-dependent piston, which results from tilts of M1 segments that are compensated by tilts of the corresponding M2 segment. We report here on wavefront sensing experiments conducted with a full-scale prototype AGWS wavefront sensor fed by a GMT optical simulator called the Wide-Field Phasing Testbed (WFPT). With the WFPT we introduce aberrations on M1 and M2, including simulated atmospheric turbulence and a variety of guide-star magnitudes. The AGWS measures the resulting off-axis aberrations, and then corrects the M1 and M2 aberrations. We describe the wavefront reconstructors used to generate the corrections and the performance over a range of conditions.