Rayleigh laser guide stars for extremely large telescopes

Rayleigh laser guide star technology is discussed here with particular attention paid to the effects of laser pulse length, a parameter that becomes more significant to the design when telescope apertures are greater than 10 meters. After reviewing the relative return signal for Rayleigh versus sodium laser guide stars, a brief review of the pulse length characteristics of sodium lasers is given. Two sodium laser systems are pulsed in a way that will make them useful for Extremely Large Telescopes (ELTs). To insure star-like sources at the wavefront sensor with FWHM < 1.0 arcsec, Rayleigh lasers must have short pulse formats when used with the largest ELTs. A relatively simple Rayleigh laser guide star method is described for Ground Layer Adaptive Optics (GLAO). This method provides a way to average out the effects of high altitude turbulence with a single Rayleigh laser guide star while leaving intact the wavefront signal needed to correct ground-layer wavefront perturbations.