On-sky demonstration of segment piston estimation using an infrared tip-tilt sensor

Abstract

The Giant Magellan Telescope (GMT) consists of seven 8.365-m diameter circular segments in a petal pattern to create a telescope with an effective diameter of 25.4m. The Laser Tomography Adaptive Optics system for the GMT uses six Shack-Hartmann Wavefront Sensors (SHWFSs) guiding on six laser guide stars to measure high-order wavefront aberrations. Because tip-tilt measurements are corrupted by uplink disturbances, they are measured using a natural guide star with an infrared tip-tilt sensor operating at 500Hz. Segment piston induced by the atmosphere or the telescope is invisible to the SHWFSs. In previous work, we demonstrated through numerical simulations that segment piston can be recovered from tip-tilt sensor images using phase retrieval. In this paper, we present the results of on-sky experiments using the W. M. Keck Observatory as a testbed for GMT segment piston sensing. Short-exposure, near infrared AO-corrected images were taken with the NIRC2 camera on Keck II. Segment piston was applied to three blocks of 12 contiguous segments to emulate the large segments of the GMT. The images were processed using a modified Gerchberg-Saxton phase retrieval algorithm and a non-linear optimization algorithm to recover the applied segment piston. The on-sky experiments confirm the suitability of phase retrieval to maintain telescope phasing for the GMT.