Abstract
Sodium laser guide stars (LGSs) allow, in theory, Adaptive Optics (AO) systems to reach a full sky coverage, but they have their own limitations. The artificial star is elongated due to the sodium layer thickness, and the temporal and spatial variability of the sodium atom density induces changing errors on wavefront measurements, especially with Extremely Large Telescopes (ELTs) for which the LGS elongation is larger. In the framework of the Thirty-Meter-Telescope project (TMT), the AO-Lab of the University of Victoria (UVic) has built an LGS-simulator test bed in order to assess the performance of new centroiding algorithms for LGS Shack-Hartmann wavefront sensors (SH-WFS). The design of the LGS-bench is presented, as well as laboratory SH-WFS images featuring 29x29 radially elongated spots, simulated for a 30-m pupil. The errors induced by the LGS variations, such as focus and spherical aberrations, are characterized and discussed. This bench is not limited to SH-WFS and can serve as an LGS-simulator test bed to any other LGS-AO projects for which sodium layer fluctuations are an issue.
Highlights
Assess the performance of wavefront sensing algorithms when using Na LGS SH-WFS Provide a budget error for the TMT-NFIRAOS facility
The LGS-bench has to reproduce in laboratory the following features:
! the LGS elongation along the telescope optical axis, due to the Na layer thickness ! the spatial and temporal variability of the Na layer, ! the dithering tip-tilt signal required by the matched filter algorithm, ! the residual atmospheric turbulence
Summary
Assess the performance of wavefront sensing algorithms when using Na LGS SH-WFS Provide a budget error for the TMT-NFIRAOS facility. Ringberg 2007 -- Astronomy with LGS AO
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