On-sky performance of the GMT dispersed fringe phasing sensor prototype

The Acquisition, Guiding, and Wavefront sensing System (AGWS) serves multiple active and adaptive optics functions within the GMT. The AGWS uses four identical moveable star probes that perform target acquisition, active optics wavefront control, Shack-Hartmann ground-layer wavefront sensing, segment tip/tilt sensing, and segment phasing. With the exception of segment phasing, these tasks are performed by one of three selectable optical channels feeding an e2v CCD351 EMCCD camera. The most challenging task, segment phasing, is performed by a J-band dispersed fringe sensor (DFS). The DFS uses 1.5-meter square subapertures overlaying each of twelve ~40 cm segment boundary interfaces in a re-imaged pupil plane. A doublet prism array disperses the fringes formed by each segment boundary in the direction parallel to the segment gap. An optical relay images the fringes onto a SAPHIRA eAPD array that is being read out faster than the coherence time of the atmosphere. In-phase segments produce straight fringes while out-of-phase segments produce tilted fringes. This tilt is measured with the Fourier amplitude. To validate our design, we built a DFS using the same optical design, doublet prism array, and C-RED eAPD camera as our GMT design. We tested the prototype DFS on-sky at the Magellan Clay telescope during two observing runs in May and November 2018. The first run used an AO corrected beam from the MagAO system and the second used a seeing-limited beam from the F/11 facility secondary mirror. We present our on-sky results and data analysis. The tests included guide stars of varying magnitudes, AO-corrected and seeing-limited observations, and changing the zenith angle and dispersion orientation to understand the effect of differential atmospheric refraction on the measurements.