Aberrations induced by side-projected laser guide stars in laser tomography adaptive optics systems
Marcos A. van Dam, Rodolphe Conan, Antonin H. Bouchez and Brady Espeland
(1) Flat Wavefronts (2) and (4) Australian National University (3) Giant Magellan Telescope Observatory
A laser tomography adaptive optics (LTAO) system is currently under design for the Giant Magellan Telescope (GMT). For systems engineering reasons, it is preferable to project the laser guide stars (LGSs) from the side of the telescope. Experience with the Keck II adaptive optics system and analytical modeling have shown that side-launched lasers 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, which has three launch telescopes projecting six LGSs in a regular hexagon. When the wavefront is reconstructed tomographically, the aberrations largely cancel on-axis. However, an off-axis truth sensor with a dedicated DM on the truth sensing path will see completely different aberrations, introducing an unacceptably large wavefront error. Working with the assumption that the LGS aberrations affect all of the wavefront sensors in the sane way, we propose a method that filters the LGS aberrations directly from the Shack-Hartmann centroids. This method is shown to be very effective at filtering the LGS aberrations while having a negligible effect on the turbulence estimation.