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.
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.
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.
A end-to-end model of an Extreme Adaptive Optics system developed is the framework of the project VLT-PF is presented. The different components are exposed with their specificities. Several AO and XAO issues are discussed like scintillation, vibration and calibration effects among others. A full simulation of an XAO system coupled to a coronagraph and designed to detect faint companion in the vicinity of a bright star is shown.
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.
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.