Segment piston estimation using sequential phase retrieval

Abstract

The Giant Magellan Telescope (GMT) consists of seven 8.4-m diameter circular segments in a close-packed pattern to create a telescope with an effective diameter of 25.4 m. The Laser Tomography Adaptive Optics system for the GMT uses six Shack-Hartmann wavefront sensors observing six laser guide stars (LGSs) to measure high-order wavefront aberrations. LGS-based adaptive optics systems also require one or more tip-tilt sensors, since the LGSs are affected by tip-tilt errors on the uplink. The wavefront errors also contain segment piston, which can be induced by the atmosphere or the telescope but cannot be sensed by the Shack-Hartmann wavefront sensors. If the tip-tilt sensor is a Nyquist sampled imager, we can use phase retrieval to measure segment piston at the full frame rate of the tip-tilt sensor. Two existing algorithms, Gerchberg-Saxton and the Fast & Furious algorithms, are adapted for this purpose. We demonstrate using end-to-end simulations that phase retrieval can be used to measure segment piston and thus improve the delivered image quality over a range of operating conditions. Show more