Facility class Rayleigh beacon AO system for the 4.2m William Herschel Telescope

A rationale is presented for the use of a relatively low-altitude Rayleigh Laser Guide Star to provide partial adaptive optics correction across a large fraction of the sky on the 4.2m William Herschel Telescope. The scientific motivation is highlighted and supported by model calculations. An overview the technical implementation of the system is presented.