Focus anisoplanatism: A limit to the determination of tip-tilt with laser guide stars

One limitation to using laser beacons to measure tilt is focus anisoplanatism. By use of a spectral representation for the atmospheric turbulence, the wave-front error is expressible as an integral over the distribution of turbulence with height. Results are calculated for turbulence profiles typical of astronomical sites. The decrease in Strehl ratio when a single laser guide star is used to measure tip-tilt is shown to be especially significant at visible wavelengths, precisely where tip-tilt is most difficult to obtain from natural stars. (C) 1996 Optical Society of America.