The Giant Magellan Telescope Phasing System

The 25 m Giant Magellan Telescope consists of seven circular 8.4 m primary mirror segments with matching segmentation of the Gregorian secondary mirror. Achieving the diffraction limit in the adaptive optics observing modes will require equalizing the optical path between pairs of segments to a small fraction of the observing wavelength. This is complicated by the fact that primary mirror segments are separated by up to 40 cm, and composed of borosilicate glass. The phasing system therefore includes both edge sensors to sense high-frequency disturbances, and wavefront sensors to measure their long-term drift and sense atmosphere-induced segment piston errors. The major subsystems include a laser metrology system monitoring the primary mirror segments, capacitive edge sensors between secondary mirror segments, a phasing camera with a wide capture range, and an additional sensitive optical piston sensor incorporated into each AO instrument. We describe in this paper the overall phasing strategy, controls scheme, and the expected performance of the system with respect to the overall adaptive optics error budget. Further details may be found in specific papers on each of the subsystems.