Differential piston sensing with LIFT: application to the GMT

The Giant Magellan Telescope's primary and deformable secondary mirror are each composed of 7 segments. The Natural Guide Star (NGS) wavefront sensor has the critical task to keep these 7 segments in phase in addition to the classical Adaptive Optics (AO) correction. The baseline defined several years ago has two pyramid wavefront sensors working in the visible. The first one is used to close the AO loop (main channel), but it is not sensitive to differential pistons that are multiples of its central wavelength (lambda(1)), leading to segment ejections. The second pyramid, sensing at a slightly higher wavelength, is then used as a slow "truth sensor" (2nd channel) to derive the sign of a segment ejection and correct it by steps of lambda(1). However, the robustness of this solution with respect to noise and turbulence conditions is not satisfying. We are now in a prototyping phase, for which the first step is to improve the baseline or find an alternative design for the 2nd channel in order to gain robustness. One of the potential solutions is LIFT, a focal-plane wavefront sensor. By making use of the sensor at two different wavelengths, it is possible to derive an unambiguous differential piston measurement. In this work, we describe our piston control strategy and show the results of end-to-end simulations comprising the full AO loop and the 2nd channel correction at the faint end of the NGS mode.