Along with the expected ELTs comes the diversification of Adaptive Optics (AO) systems. Most AO are beginning to make extensive use of laser guide stars (LGS), to increase sky coverage and produce a bright beacon in order to reduce wavefront measurement errors and improve performance. However, because of the larger size of the next generation of telescopes, the elongation seen in a Shack-Hartmann Wavefront Sensor is larger, i.e up to 7" for a 30m telescope. With such an elongation, both photon

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... nd read noise will increase and distribution variations of sodium atoms in the sodium layer start to matter. In this paper, we conduct Shack-Hartmann simulations at the sub-aperture level made with real sodium profiles taken at Lick Observatory. We will compare focus on two methods of centroiders: the matched filter and the correlation. We then compare results with data taken form a real bench at Uvic in the particular case of NFIRAOS AO system for TMT. The goal is to better understand the impact of such variations on the final error budget for the WFS.