New method for atmospheric tip-tilt correction: low power UV modeless laser excitation of mesospheric sodium layer

The aim of Polychromatic Laser Guide Star (PLGS) is to ensure 100 % sky coverage for near-diffraction astronomical observations using Adaptive Optics. The technique proposed by Foy et al. used a double resonance laser excitation of the mesospheric sodium layer with two dependent lasers at 589 nm and 569 nm. This process produced a polychromatic Tip-Tilt Laser Guide Star (TTLGS) at 330 nm and 589.6 nm (D₁ sodium transition) for the tip-tilt correction and the Laser Guide Star (LGS) at 589 nm (D₂ sodium transition) for higher order correction. This process is rather complicated because of the need of precise spatial and temporal overlap of the two lasers and separation of chromatic components.