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Simulations of Laser Guide Star Adaptive Optics Systems for the European Extremely Large Telescope

Abstract 190

Submitted by Richard CLARE


R. Clare, M. Le Louarn


European Southern Observatory


Current adaptive optics systems on 8-10m class telescopes generate a single sodium laser guide star (LGS) to increase the signal-to-noise ratio for the wavefront sensor (WFS), and hence improve the sky coverage of the science instrument.

For a Shack-Hartmann (SH) WFS with a LGS at a finite height and non-zero sodium layer thickness, the image (“spot”) at each subaperture is elongated due to the parallax effect. The elongation of the SH spots is approximately proportional to the distance between each subaperture and the laser launch telescope. The European Extremely Large Telescope (EELT) is a 42m diameter adaptive telescope that is currently in the design phase. The spot elongation for the EELT will be 4-5 times that observed on the current 8-10m class of telescopes.

In addition, the wavefront error due to the cone effect, which arises as the WFS can only sense the cone of turbulence below the LGS situated at 90km, increases as a factor of the telescope diameter to the power of 5/3. The cone effect will therefore be prohibitive for a single LGS on an EELT, and multiple LGS will be required to overcome it.

In this paper, we present end-to-end simulations incorporating spot elongation and the cone effect made in the ESO tool Octopus. The simulations are performed for multiple typical LGS systems: Ground Layer AO (GLAO), laser tomography AO (LTAO), as well as an on-axis cone effect-free single conjugate AO (SCAO) system to test the spot elongation only. A number of design trade studies are also completed for each system with respect to laser power, WFS centroiding algorithm, sodium profile, detector read noise, and the laser launch telescope location.

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