Prototyping the GMT phasing camera with the Magellan AO system

The future diffraction-limited performance of the 25.4 meter Giant Magellan Telescope (GMT) will rely on the active and adaptive wavefront sensing measurements made by the Acquisition, Guiding, and Wavefront Sensor (AGWS) currently being designed by SAO. One subsystem of the AGWS, the phasing camera, will be responsible for measuring the piston phase difference between the seven GMT primary/secondary segment pairs to 50 nm accuracy with full sky coverage using natural guide stars that are 6-10 arcmin off-axis while the on-axis light is used for science operations. The phasing camera will use a dispersed fringe sensor to measure the phase difference in rectangular subapertures spanning the gaps between adjacent mirror segments. The large gap between segments (>295 mm, compared to 3 mm for the Keck telescope) reduces the coherence of light across the subapertures, making this problem particularly challenging. In support of the AGWS phasing camera technical goals, SAO has undertaken a series of prototyping efforts at the Magellan 6.5 meter Clay telescope to demonstrate the dispersed fringe sensor technology and validate atmospheric models. Our latest on-sky test, completed in December 2015, employs a dual-band (I and J) dispersed fringe sensor. This prototype uses an adaptive optics corrected beam from the Magellan AO adaptive secondary system. The system operates both on-axis and 6 arcmin off-axis from the natural guide star feeding the MagAO wavefront sensor. This on-sky data will inform the development of the AGWS phasing camera design towards the GMT first light.