Publications of the Astronomical Society... Publications of the Astronomical Society of the Pacific Vol. 116, No. 817, March 2004 Photometry of a Sodium Laser Guide Star ... Photometry of a Sodium ...

Replacing the guide star with an artificial laser beacon alleviates this dependency on bright stars and therefore increases sky coverage, but it does not eliminate another serious problem for AO ...

This is often done using a laser guide star, a strong laser beam shot into the sky that provides a "perfect" example star whose distortion is measured so that the system can correct for it.