The Polychromatic Laser Guide Star for tilt measurement: progress report of the demonstrator at Observatoire de Haute Provence
2007
The Polychromatic Laser Guide Star aims at providing for the tilt measurement from a LGS without any
natural guide star. Thus it allows adaptive optics to provide us with a full sky coverage. This is critical in
particular to extend adaptive optics to the visible range, where isoplanatism is so small that the probability is
negligible to find a natural star to measure the tilt.
We report new results obtained within the framework of the Polychromatic LGS programme ELP-OA. Natural
stars have been used to mimic the PLGS, in order to check the feasibility of using the difference in the tilt at
two wavelengths to derive the tilt itself. We report results from the ATTILA experiment obtained at the 1.52 m
telescope at Observatoire de Haute-Provence. Tilts derived from the differential tilts are compared with direct
tilt measurements. The accuracy of the measurements is currently ≈ 1.5 Airy disk rms at 550 nm. These results
prove the feasibility of the Polychromatic Laser Guide Star programme ELP-OA. New algorithms based on
inverse problems under development within our programme would lead to smaller error bars by 1 magnitude,
as soon as they will run fast enough.
We describe the ELP-OA demonstrator which we are setting up at the same telescope, with a special emphasis
on the optimization of the excitation process, which definitely has to rely on the two-photon excitation of sodium
atoms in the mesosphere. We will describe the implementation at the telescope, including the projector device,
the focal instrumentation and the NdYAG pumped dye lasers.
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