Preliminary on-sky results of the next generation GMT phasing sensor prototype

The Active optics, Guiding, and Wavefront Sensing system (AGWS), currently being designed by SAO, will use J-band dispersed fringe sensors (DFS) to phase the GMT to a fraction of an imaging wavelength. These phasing sensors will use off-axis guide stars to measure phase shifts at each of 12 segment boundaries. The fringes produced at each boundary will be dispersed in the perpendicular direction using an array of high-index doublet prisms. Inter-segment phase shifts will appear as tilts in the dispersed fringes, which can be measured in the Fourier domain. In order to avoid atmospheric blurring of the fringes, we require a J-band detector capable of fast, low-noise readout, which mandates the use of a SAPHIRA e-APD array. We built a DFS prototype that we tested on-sky at the Magellan Clay telescope behind the MagAO adaptive optics system in May 2018.