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.
Phase retrieval is a very promising approach for wavefront sensing in the focal plane of ground-based large telescopes. It is a non-linear problem that must be solved by means of global optimization. Currently only multi-focal phase diversity algorithms are used in adaptive optics. They enable the correction of static aberrations. For speckle imaging the problem is increasingly multi-modal with the ratio <i>D/r</i><sub>0</sub>. Yet thanks to an iterative Newton algorithm with self-adapting Kolmogorov prior information, we show from consistent modeling and simulations, that we could efficiently sense short exposure wavefronts at high <i>D/r</i><sub>0</sub> from a single focal plane. We show that using data at different wavelengths with a proper polychromatic model would even enforce the convergence, thus making it an envisageable method to sense the returned flux of a polychromatic laser guide star (PLGS). For instance, we show that if we suppose the PLGS is not resolved, phase retrieval would enable an improvement in the centroid estimation in agreement with the Cramer-Rao lower bound. As a post-processing technique, our algorithm already has numerous potential applications for astronomy and for other domains. Thanks to the improvement of computing workstations and the optimization of the algorithm, applications involving realtime wavefront corrections should be soon possible.
We give an overview of recent results obtained with the GI2T interferometer. On the technical side, great improvements have been obtained on photon counting detectors, especially in terms of quantum efficiency and of photon centroiding algorithms. Piston measurements with the GI2T dispersed fringes have been made during
coordinate observations with the Generalized Seeing Monitor GSM. These observations have lead to wavefront outer scale determinations. The last topic we will present concerns the polarimetric measurements done with the SPIN device on the GI2T spectrograph. We conclude this paper by a summary of the results obtained with the GI2T during its scientific life.
Since 1974 we develop photon-counting imaging devices for high angular resolution in the visible by means of speckle
and optical interferometry. Our last generation photon-counting camera, CPNG, has been built to benefit from the recent
advances in photonic commercial components. CPNG is an ICCD which uses electron multiplication in microchannel
plates to overcome the readout noise of fast CCD. We achieve optimal performances (sensitivity and resolution) by proper
optical design, by cooling of the first stage photocathode and by careful data processing. Thanks to the power of current
workstations, the processing of the CCD signal can be done by elaborated real-time software at frame rates as high as
262 Hz (72 Mpixel/s). The real-time software is in charge of detecting occurences of photon-events and estimating their
positions. We explain how our dedicated processing improves the detection sensitivity to reach an effective quantum
efficiency of 35%. We also show that our unbiased recentering of detected photons can avoid spurious high-energy events
and nevertheless achieve sub-pixel resolution. In practice, our resolution is limited by the size of a microchannel to about
2000×2000 effective pixels for our 516×532 CCD. The very good performances of CPNG open us new classes of objects
and have proven to be useful for other applications. For instance, several versions of our camera have been developped
(with different spectral ranges) to cover the common needs in Astronomy and biological imaging for an extremely low-light
level and fast imaging detector.