Analysis of the impact of optical aberrations in en-face full-field OCT microscopy.

Optical coherence tomography (OCT) is a powerful technique for cross-sectioning imaging. However, the lateral resolution may be degraded by optical aberrations originating from the sample or the setup. We present an extensive quantitative study of the impact of aberrations in time-domain en-face full-field OCT (FFOCT). Using an adaptive optics loop integrated in an FFOCT setup, a deformable mirror is used to introduce low-order calibrated aberrations. The experimental analysis of both the line spread functions (SF) and the complex object images has allowed us to measure the loss in contrast and the impact on lateral spatial resolution. We demonstrate that the frequency content of FFOCT image spectra in terms of signal-to-noise ratio and cutoff frequency is degraded by aberrations but remains much higher than in conventional incoherent images. Line SF profiles in conventional imaging display widening, whereas in FFOCT they display oscillations, leading to the possible perception of preserved resolution. Nevertheless, for complex objects, the aberration image blurring is strong due to the convolution process by the point SF, resulting in a significant filtering of the image spatial spectrum.

First on-sky SCAO validation of full LQG control with vibration mitigation on the CANARY pathfinder.

Adaptive optics provides real time correction of wavefront disturbances on ground based telescopes. Optimizing control and performance is a key issue for ever more demanding instruments on ever larger telescopes affected not only by atmospheric turbulence, but also by vibrations, windshake and tracking errors. Linear Quadratic Gaussian control achieves optimal correction when provided with a temporal model of the disturbance. We present in this paper the first on-sky results of a Kalman filter based LQG control with vibration mitigation on the CANARY instrument at the Nasmyth platform of the 4.2-m William Herschel Telescope. The results demonstrate a clear improvement of performance for full LQG compared with standard integrator control, and assess the additional improvement brought by vibration filtering with a tip-tilt model identified from on-sky data, thus validating the strategy retained on the instrument SPHERE at the VLT.

Differential focal anisoplanatism in laser guide star wavefront sensing on extremely large telescopes.

Laser guide stars (LGSs) aim at increasing the sky coverage of adaptive optics (AO) as this is highly restricted when using only natural guide stars. With such three-dimensional extended objects, spot elongation may limit the measurement accuracy of wavefronts. We evaluate the effect of differential focal anisoplanatism, induced solely by the longitudinal extension of a side-launched LGS, on the slope measurements performed by a Shack-Hartmann for a 40 m class telescope. We also take this effect into account in the wavefront reconstruction and derive estimations of the resulting wavefront error in a multi-LGS AO system. We find an error of 100 nm in the worst case at the subaperture level and a small error of the order of 10 nm for six LGSs after wavefront reconstruction.

Intrinsic limitations of Shack-Hartmann wavefront sensing on an extended laser guide source.

In this paper we investigate the behavior of various centroiding methods (weighted center of gravity, matched filtering, and correlation) classically used in Shack-Hartmann wavefront sensing when dealing with an elongated asymmetric spot. We study the impact of model errors on these centroiding methods at high signal-to-noise ratios, and, using a one-dimensional formalism, we show that the associated estimates all suffer from a bias uncorrelated with the actual spot displacement if its shape is not known precisely. Additionally, we show that the correlation method provides an estimate with a unitary gain whatever the parameters used, while the other two methods introduce a non-unitary gain in the estimation process. Finally, we show that the sampling of the spot structures after filtering by some convolution kernels is crucial to get an unbiased estimate of the spot displacement.

Tomography approach for multi-object adaptive optics.

Multi-object adaptive optics (MOAO) is a solution developed to perform a correction by adaptive optics (AO) in a science large field of view. As in many wide-field AO schemes, a tomographic reconstruction of the turbulence volume is required in order to compute the MOAO corrections to be applied in the dedicated directions of the observed very faint targets. The specificity of MOAO is the open-loop control of the deformable mirrors by a number of wavefront sensors (WFSs) that are coupled to bright guide stars in different directions. MOAO calls for new procedures both for the cross registration of all the channels and for the computation of the tomographic reconstructor. We propose a new approach, called "Learn and Apply (L&A)", that allows us to retrieve the tomographic reconstructor using the on-sky wavefront measurements from an MOAO instrument. This method is also used to calibrate the registrations between the off-axis wavefront sensors and the deformable mirrors placed in the science optical paths. We propose a procedure linking the WFSs in the different directions and measuring directly on-sky the required covariance matrices needed for the reconstructor. We present the theoretical expressions of the turbulence spatial covariance of wavefront slopes allowing one to derive any turbulent covariance matrix between two wavefront sensors. Finally, we discuss the convergence issue on the measured covariance matrices, we propose the fitting of the data based on the theoretical slope covariance using a reduced number of turbulence parameters, and we present the computation of a fully modeled reconstructor.

Optimal method for exoplanet detection by angular differential imaging.

We propose a novel method for the efficient direct detection of exoplanets from the ground using angular differential imaging. The method combines images appropriately, then uses the combined images jointly in a maximum-likelihood framework to estimate the position and intensity of potential planets orbiting the observed star. It takes into account the mixture of photon and detector noises and a positivity constraint on the planet's intensity. A reasonable detection criterion is also proposed based on the computation of the noise propagation from the images to the estimated intensity of the potential planet. The implementation of this method is tested on simulated data that take into account static aberrations before and after the coronagraph, residual turbulence after adaptive optics correction, and noise.

Calibration and precompensation of noncommon path aberrations for extreme adaptive optics.

Noncommon path aberrations (NCPAs) are one of the main limitations of an extreme adaptive optics (AO) system. NCPAs prevent extreme AO systems from achieving their ultimate performance. These static aberrations are unseen by the wavefront sensor and therefore are not corrected in closed loop. We present experimental results validating what we believe to be new procedures of measurement and precompensation of the NCPAs on the AO bench at ONERA (Office National d'Etudes et de Recherches Aérospatiales). The measurement procedure is based on refined algorithms of phase diversity. The precompensation procedure makes use of a pseudo-closed-loop scheme to overcome the AO wavefront-sensor-model uncertainties. Strehl ratio obtained in the images reaches 98.7% at 632.8 nm. This result allows us to be confident of achieving the challenging performance required for direct observation of extrasolar planets.

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.