The Laser Guide Star commissioned in 2007 at the WHT on La Palma is based on Rayleigh backscattering of a 515 nm beam provided by a diode pumped Q-switched doubled frequency Yb:YAG laser launched from behind the WHT secondary mirror. At the time the laser beam is focused at a distance of 15km above the telescope ground and its power just under 20W. With such a pulsed laser, careful fine tuning of the range gate system is essential to isolate the most focused part of the LGS and eliminate parts

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... f the laser plume which would degrade the Shack-Hartmann spots and consequently AO correction. This is achieved by an electro-optic shutter using Pockels cells, triggered by a delay generator synchronised on the laser pulses, and by spatial filters. Images of 0.15" resolution in J and H bands, very close to expected performance, have been routinely taken as soon as the third and fourth commissioning runs. Here we show the performance of the range gate system as measured and improved over the successive commissioning runs, as well as the off sky and on sky calibration procedures of the LGS AO system.