Artificial-Guide-Star Tilt-Anisoplanatism: Its Magnitude and (Limited) Amelioration

We shall be concerned here with the problem of tilt anisoplanatism in the use of an artificial-guide-star adaptive optics system in imaging some astronomical object of interest through the turbulent atmosphere. It is well known that because the position of the back scatter region is determined by the direction in which the laser beam was transmitted an artificial-guide-star can not provide any pointing information pertaining to turbulence induced tilt— at least not in any direct manner. (Indirect methods, such as those involving second harmonic generation closely coupled with the back scatter process are being considered but the feasibility of such approaches is still unclear.) If the object of interest is sufficiently bright it can serve as the guide star for correction of turbulence induced tilt—or if there is a sufficiently bright star close enough to the object of interest then it can serve as a natural-guide-star for the correction of turbulence induced tilt. The farther away that natural-guide-star is from the object of interest the greater the discrepancy between the turbulence induced tilt for the object of interest and that for the natural-guide-star, and the lower the Strehl ratio of the image of the object of interest will be—even though the artificial-guide-star may be providing a basis for perfect adaptive optics correction of the higher-order wave front distortion.