  Sodium Laser Guide Stars (LGSs) are elongated sources due to the thickness
and the finite distance of the sodium layer. The fluctuations of the sodium
layer altitude and atom density profile induce errors on centroid measurements
of elongated spots, and generate spurious optical aberrations in closed--loop
adaptive optics (AO) systems. According to an analytical model and experimental
results obtained with the University of Victoria LGS bench demonstrator, one of
the main origins of these aberrations, referred to as LGS aberrations, is not
the Centre-of-Gravity (CoG) algorithm itself, but the thresholding applied on
the pixels of the image prior to computing the spot centroids. A new
thresholding method, termed ``radial thresholding'', is presented here,
cancelling out most of the LGS aberrations without altering the centroid
measurement accuracy.
