Setting up ELP-OA: the polychromatic laser guide star demonstrator

ELP-OA ('Etoile Laser Polychromatique pour l'Optique Adaptative) aims at demonstrating the tip-tilt is measurable with a Laser Guide Star (LGS) without any natural guide star. This allows a full sky coverage down to visible wavelengths. ELP-OA is being setup at Observatoire de Haute-Provence (OHP). To create a polychromatic LGS, we use two pulsed dye lasers (at 569nm and 589nm) to produce a two-photons excitation of sodium atoms in the mesosphere. The chromatism of the refractive index of the air yields a difference of the LGS direction at different wavelengths. The position differences is proportionnal to the tip-tilt. Since the LGS isn't sharp enough to give us a small enough error in the differential tip-tilt, we use an interferometric projector to improve the high spatial information in the laser spot. It requires an adaptive optics working down to 330nm. This one is done by post-processing algorithms. Two two aperture projectors are used. Each one creates a fringe-modulated LGS, and a better RMS error in the LGS position is obtained by measuring the information in a normal direction with respect to the fringes. By using a two aperture projector, we also strongly decrease the negative effect of the laser star elongation in the mesosphere, and the Rayleigh contribution near the LGS. We propose a new optimal algorithm to retrieve the tip-tilt from simultaneous images at different wavelengths. To enhance the RMS error of the measurements, we extend this algorithm to exploit the temporal correlation of the turbulence.