First Results of a Polychromatic Artificial Sodium Star for the Correction of Tilt

With the present state of technology, the tilt component of a aberrated stellar wavefront which has been distorted by atmospheric turbulence cannot be ascertained by an artificial guide star. The absolute position of the artificial guide star cannot be determined since it wanders on the uplink portion of its propagation and there is no way, at least with a single receiving telescope, to distinguish an undetermined position of a guide star from tilt in the wavefront. In conventional adaptive optics systems, a natural star is needed to supply the tilt information in addition to the higher order corrections supplied by an artificial guide star. The probability of finding a suitably bright natural guide star for tilt correction is not significantly higher than for higher order correction despite the fact that the entire telescope area is used and both the tilt anisoplanatic angle and integration time are considerably larger. Since the tilt contributes ≈90% of the phase variance, the accuracy in the tilt measurement has to be much greater than for higher order corrections1. In particular at galactic latitudes above the galactic plane, or at visible wavelengths, the sky coverage for a tilt star becomes unacceptable for most applications.