Simulation of the photon return of a mono and polychromatic sodium guide star

Summary form only given. Adaptive optics can improve the imaging of ground based telescopes near the diffraction limit in spite of atmospheric turbulence. Laser excitation of the mesospheric sodium layer have been proposed and experimentally investigated as a synthetic beacon for wavefront correction. However, this technique suffers from a major drawback: tip-tilt induced by the atmospheric refraction cannot be measured because of the principle of the inverse return of light. A polychromatic guidestar has thus recently been proposed to measure this tip-tilt effect.