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A ground-layer AO system demonstrator for the William Herschel Telescope

An experimental Ground Layer Adaptive Optics system utilizing a low-altitude Rayleigh Laser Guide Star is presented. This demonstrator is designed for the GHRIL Nasmyth platform of the 4.2m William Herschel Telescope, where it uses a low-altitude (~4km) focused-spot 523nm Rayleigh-scatter beacon, launched from behind the secondary mirror using an independent beam launch telescope. A novel range-gate is used to select the LGS return altitude for wavefront sensing, whilst wavefront correction uses a 97-actuator continuous phase sheet deformable mirror and separate tip-tilt mirror. The performance can be monitored on-axis and off-axis. These and other aspects of the demonstration system are described in detail, including optical design, laser launch technique, laboratory performance, and a preliminary assessment of potential on-sky performance.