Abstract The ultimate performance of coronagraphic high-contrast exoplanet imaging systems such as SPHERE or GPI is limited by quasi-static aberrations. These aberrations produce speckles that can be ...mistaken for planets in the image. In order to design instruments, correct quasi-static aberrations or analyse data, the expression of the point spread function of a coronagraphic instrument in the presence of residual turbulence is most useful. Here, we derive an analytic expression for this point spread function that is an extension to coronagraphic imaging of Roddier's expression for imaging through turbulence. We give a physical interpretation of its structure, we validate it by numerical simulations and we show that it is computationally efficient. Finally, we incorporate this imaging model into a coronagraphic phase diversity method (COFFEE) and validate by simulations that it allows wave-front reconstruction in the presence of residual turbulence. The preliminary results, which give a sub-nanometric precision in the case of a SPHERE-like system, strongly suggest that quasi-static aberrations could be calibrated during observations by this method.
Structured Illumination Microscopy (SIM) is an imaging technique for achieving both super-resolution (SR) and optical sectioning (OS) in wide-field microscopy. It consists in illuminating the sample ...with periodic patterns at different orientations and positions. The resulting images are then processed to reconstruct the observed object with SR and/or OS. In this work, we present BOSSA-SIM, a general-purpose SIM reconstruction method, applicable to moving objects such as encountered in in vivo retinal imaging, that enables SR and OS jointly in a fully unsupervised Bayesian framework. By modeling a 2-layer object composed of an in-focus layer and a defocused layer, we show that BOSSA-SIM is able to jointly reconstruct them so as to get a super-resolved and optically sectioned in-focus layer. The achieved performance, assessed quantitatively by simulations for several noise levels, compares favorably with a state-of-the-art method. Finally, we validate our method on open-access experimental microscopy data.
Abstract
Quasi-static aberrations in coronagraphic systems are the ultimate limitation to the capabilities of exoplanet imagers both ground-based and space-based. These aberrations – which can be due ...to various causes such as optics alignment or moving optical parts during the observing sequence – create light residuals called speckles in the focal plane. Those speckles might be mistaken for planets. For ground-based instruments, the presence of residual turbulent wavefront errors due to partial adaptive optics correction causes an additional difficulty to the challenge of measuring aberrations in the presence of a coronagraph. In this paper, we present an extension of COFFEE, the coronagraphic phase diversity, to the estimation of quasi-static aberrations in the presence of adaptive-optics-corrected residual turbulence. We perform realistic numerical simulations to assess the performance that can be expected on an instrument of the current generation. We perform the first experimental validation in the laboratory, which demonstrates that quasi-static aberrations can be corrected during the observations by means of coronagraphic phase diversity.
Context.
The detection and characterization of Earth-like exoplanets (exoEarths) from space requires exquisite wavefront stability at contrast levels of 10
−10
. On segmented telescopes in ...particular, aberrations induced by co-phasing errors lead to a light leakage through the coronagraph, deteriorating the imaging performance. These need to be limited in order to facilitate the direct imaging of exoEarths.
Aims.
We perform a laboratory validation of an analytical tolerancing model that allows us to determine wavefront error requirements in the 10
−6
− 10
−8
contrast regime for a segmented pupil with a classical Lyot coronagraph. We intend to compare the results to simulations, and we aim to establish an error budget for the segmented mirror on the High-contrast imager for Complex Aperture Telescopes (HiCAT) testbed.
Methods.
We use the Pair-based Analytical model for Segmented Telescope Imaging from Space to measure a contrast influence matrix of a real high-contrast instrument, and use an analytical model inversion to calculate per-segment wavefront error tolerances. We validate these tolerances on the HiCAT testbed by measuring the contrast response of segmented mirror states that follow these requirements.
Results.
The experimental optical influence matrix is successfully measured on the HiCAT testbed, and we derive individual segment tolerances from it that correctly yield the targeted contrast levels. Further, the analytical expressions that predict a contrast mean and variance from a given segment covariance matrix are confirmed experimentally.
Second harmonic generation (SHG) imaging microscopy of thick biological tissues is affected by the presence of aberrations and scattering within the sample. Moreover, additional problems, such as ...uncontrolled movements, appear when imaging
. Deconvolution methods can be used to overcome these limitations under some conditions. In particular, we present here a technique based on a marginal blind deconvolution approach for improving SHG images obtained
in the human eye (cornea and sclera). Different image quality metrics are used to quantify the attained improvement. Collagen fibers in both cornea and sclera are better visualized and their spatial distributions accurately assessed. This might be a useful tool to better discriminate between healthy and pathological tissues, especially those where changes in collagen distribution occur.
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