Aberrations induced by side-projected laser guide stars in laser tomography adaptive optics systems

A laser tomography adaptive optics (LTAO) system is currently under design for the Giant Magellan Telescope (GMT). This system has six laser guide stars (LGSs) in a regular hexagon. For systems engineering reasons, it is preferable to project the LGSs from the side of the telescope. Experience with the Keck II adaptive optics system and analytical modeling have shown that side-launched LGSs result in aberrations, called LGS aberrations, with a lot of power at low-spatial frequencies. This is caused by the elongation of the LGS due to the finite thickness of the sodium layer. In this paper, we model the LGS aberrations for the GMT's LTAO system. We find that the main aberration induced by the structure of the sodium layer is a focus term, which can be measured with a natural guide star and corrected. For subapertures with large numbers of pixels, other aberrations are negligible. However, if there are few pixels available, centroid errors resulting from pixelation and truncation of the spot lead to significant wavefront errors. Working with the assumption that the LGS aberrations affect all of the wavefront sensors in the same way, we propose a method that filters the LGS aberrations directly from the Shack-Hartmann centroids. This method is shown to be effective at removing the bulk of the LGS aberrations while having a small effect on the turbulence estimation.