Magalie Nicolle

Optimization of a Shack-Hartmann-based wavefront sensor for XAO systems

Optimization of a Shack-Hartmann based WFS is proposed for XAO systems. Both aliasing effects and noise propagation issues is investigated in order to optimize the WFS device. In particular a new estimator of the spot position is proposed and characterized both analytically and using end-to-end simulations. Analytical expressions of the slope measurement errors is derived and the gain brought by our new Weighted Center of Gravity estimator is quantified.

Shack-Hartmann wavefront sensing with extended sources

Shack-Hartmann wavefront sensor is widely used for the measurement of phase perturbations induced by turbulence. Such a wavefront sensor relies on the measurement of the image displacements in the lenslet array focal plane. Different algorithms can be used to estimate this displacement. This paper is dedicated to the analysis and comparison of their performances. Special attention will be paid to correlation techniques which are well suited to extended sources.

Optimization of star-oriented and layer-oriented wavefront sensing concepts for ground layer adaptive optics

Multiconjugate adaptive optics is one of the major challenges in adaptive optics. It requires the measurement of the volumic distribution of the turbulence. Two wavefront sensing (WFS) concepts have been proposed to perform the wavefront analysis for such systems: the star-oriented and layer-oriented approaches. We give a performance analysis and a comparison of these two concepts in the framework of the simplest of the multi-guide-star adaptive optics systems, that is, ground layer adaptive optics. A phase-related criterion is proposed to assess analytically the performance of both concepts. This study highlights the main advantages and drawbacks of each WFS concept and shows how it is possible to optimize the concepts with respect to the signal to noise ratio on the phase measurement. A comparison of their optimized versions is provided and shows that one can expect very similar performance with the two optimized concepts.

Ground layer adaptive optics: analysis of the wavefront sensing issue

Here are presented the basis of an analytical development whose purpose is to give arguments for the evaluation of wavefront sensing concepts for Ground Layer Adaptive Optics. Simple hypothesis make possible the derivation of analytical expressions for the phase measurement error and reveal consequent differences between Star Oriented and Layer Oriented concepts. Influence of key parameters such as guide star statistics or strength of the turbulence in altitude are then studied. In the Layer Oriented case, necessity of reducing the guide stars flux dispersion to achieve a uniform correction in the field of interest is demonstrated.

Optimization of center of gravity algorithms in a Shack-Hartmann sensor

We propose analytical studies supported by simulation of various centroiding algorithms for Shack-Hartmann based wavefront sensor. We focused on the simple center of gravity as well as one of its optimization, the weighted center of gravity. Noise effects, as well as linearity issues and high flux bias induced by sub-aperture size and PSF structures are investigated. For each method, optimal parameters are defined in function of photon flux, readout noise, and turbulence level.

Performance analysis of multi-object wave-front sensing concepts for GLAO

Adaptive optics enable large telescopes to provide diffraction limited images, but their corrected field is restrained by the angular decorrelation of the turbulent wave-fronts. However many scientific goals would benefit a wide and uniformly corrected field, even with a partial correction. Ground Layer Adaptive Optics systems are supposed to provide such a correction by compensating the lower part of the atmosphere only. Indeed this layer is in the same time highly turbulent and isoplanatic on a rather wide field. In such a system the wave-front analysis is a critical issue. Measuring the ground layer turbulence requires multi-object wave-front analysis. Two multi-object wave-front sensing concepts have been proposed so far, derived from multi conjugate adaptive optics. They are the star oriented and the layer oriented approaches. A criterion for the analytical study of both concepts performance had been proposed in a previous presentation. First results on the behavior one can expect from one concept or the other had been given then. Here is presented a study made by improving the analytical model and completing its results with the ones of a numerical model which accounts for AO limitations that are uneasy to insert in an analytical formalism. Results are presented that highlight the advantages and drawbacks of each wave-front sensing concepts and the interest of optimizing them.