Modal decomposition of the wavefront measured by the LGS WFS. The tip-tilt modes have been removed because they are sensed by the NGS WFS. In open loop (OL) the integral of the measured WF amounts to 860 nm rms in daytime, using the prime focus calibration unit (CU) (blue line), and to 830 nm rms on sky (black line). When the AO loop is closed the WFE are lowered to 55 nm rms (green) and 120 nm rms (red) respectively. The dashed lines show the modal decomposition of the covariance matrix for the SH WFS measurement error evaluated considering the daytime (green) and on-sky (red) parameters of seeing, flux, and spot dimension.