Sodium Laser Guide Star Technique For Adaptive Imaging In Astronomy

Abstract

Atmospheric turbulence severely limits the resolution of ground-based astronomical telescopes. In good seeing conditions at the best observatory sites, resolution at visible wavelengths is typically limited to ~1 sec of arc. During the past 15 years adaptive optical systems using electrically deformable mirrors have been developed to compensate for turbulence effects. Unfortunately, these systems require bright reference sources adjacent to the object of interest and can only be used to observe the brightest stars. Artificial guide stars suitable for controlling an adaptive imaging system can be created in the upper atmosphere by using a laser to excite either Rayleigh backscattering or the mesospheric sodium layer. The design requirements for an adaptive telescope utilizing Na laser guide stars are discussed in this paper and those for the Rayleigh guide stars in a companion paper. The brightness of the Na guide star depends upon many factors including Na abundance, laser bandwidth, laser pulse energy, pulse length, zenith angle and seeing cell diameter. Based upon our initial experiments at Mauna Kea Observatory and our detailed theoretical calculations, it should be possible to obtain near diffraction limited images for zenith angles down to 30° from ground-based telescopes as large as 10m in diameter with a laser power of less than 30 watts.