Self-coherent camera as a focal plane wavefront sensor: simulations

Direct detection of exoplanets requires high dynamic range imaging. Coronagraphs could be the solution, but their performance in space is limited by wavefront errors (manufacturing errors on optics, temperature variations, etc.), which create quasi-static stellar speckles in the final image. Several solutions have been suggested for tackling this speckle noise. Differential imaging techniques substract a reference image to the coronagraphic residue in a post-processing imaging. Other techniques attempt to actively correct wavefront errors using a deformable mirror. In that case, wavefront aberrations have to be measured in the science image to extremely high accuracy. We propose the self-coherent camera sequentially used as a focal-plane wavefront sensor for active correction and differential imaging. For both uses, stellar speckles are spatially encoded in the science image so that differential aberrations are strongly minimized. The encoding is based on the principle of light incoherence between the hosting star and its environment. In this paper, we first discuss one intrinsic limitation of deformable mirrors. Then, several parameters of the self-coherent camera are studied in detail. We also propose an easy and robust design to associate the self-coherent camera with a coronagraph that uses a Lyot stop. Finally, we discuss the case of the association with a four-quadrant phase mask and numerically demonstrate that such a device enables the detection of Earth-like planets under realistic conditions. The parametric study of the technique lets us believe it can be implemented quite easily in future instruments dedicated to direct imaging of exoplanets.Comment: 15 pages, 14 figures, accepted in A&A (here is the final version

Electrochemical behaviour of N-acetyl-l-cysteine on gold electrode—A tentative reaction mechanism

The electrochemical behaviour of N-acetyl-l-cysteine (NAC) has been investigated by linear and cyclic voltammetry on gold electrode at room temperature. The results showed two oxidation peaks under acid and neutral conditions and only one in basic medium. For each oxidation, as many electron was exchanged as proton. The influence of both the concentration and the potential scan rate on the peak currents highlighted a diffusion-controlled phenomenon for the first peak and an adsorption-limited reaction rate for the second one. The diffusion coefficient of NAC in solution and the surface concentration of the adsorbed species at pH 3 and 7 were close to 2 × 10−4 to 2 × 10−5 cm2 s−1 and 6 × 10−9 to 6 × 10−10 mol cm−2, respectively. Film transfer experiments resulted in an irreversible adsorption of NAC on gold electrode, and the formation of a self-assembled monolayer (SAM)

Leveraging the two timescale regime to demonstrate convergence of neural networks

We study the training dynamics of shallow neural networks, in a two-timescale regime in which the stepsizes for the inner layer are much smaller than those for the outer layer. In this regime, we prove convergence of the gradient flow to a global optimum of the non-convex optimization problem in a simple univariate setting. The number of neurons need not be asymptotically large for our result to hold, distinguishing our result from popular recent approaches such as the neural tangent kernel or mean-field regimes. Experimental illustration is provided, showing that the stochastic gradient descent behaves according to our description of the gradient flow and thus converges to a global optimum in the two-timescale regime, but can fail outside of this regime.Comment: 33 pages, 7 figure

Simulation of radio wave propagation in arched cross section tunnels

5 pagesInternational audienceSince several years, wireless communication systems are developed for train to infrastructure communication needs related to railway or mass transit applications. The systems should be able to operate in specific environments like tunnels. In this context, specific radio planning tools have to be developed to optimise system deployment. Realistic tunnels geometries are generally of rectangular cross-section or arched shape. Furthermore, they are mostly curved. In order to calculate electromagnetic wave propagation in such tunnels, specific models have to be developed. Several works dealt with retransmission of GSM or UMTS [1], [2]. Few theoretical or experimental works focused on 2.4 GHz or 5.8 GHz bands. In this paper, we will propose an approach to model radio wave propagation in these frequency bands in arched shape cross-section straight tunnels using tessellation in multi-facets. The model will be based on a Ray-Tracing tool using images method. Work reported in this paper will also show the propagation loss variations according to the shape of tunnels. A parametric study on the size of facets to model the cross-section will be realized. Finally, the influence of some parameters like the dimensions of tunnels and the frequency of signals will be examined

Technostress in Work-Related and Non-Work-Related Usage Contexts: A Systematic Literature Review

Technostress, defined as the “as stress that individuals experience due to their use of information systems (IS)” (Tarafdar, Cooper and Stich, 2019:2), investigates how and why IS use causes various demands on individuals. Research showed that while the experience of technostress can stimulate positive outcomes, it can also significantly harm users and organizations (Tarafdar et al., 2019). It is thus essential to comprehend how to design IS to prevent negative consequences and increase positive outcomes. To this end, research must understand what individual and technological factors influence the experience of technostress depending on the usage context. Like other emotions, technostress is part of a spectrum of interactions with other emotions or types of stress (Godbold, 2015). It is thus crucial to explore the nature and extent of the relationship between technostress and other forms of stress. As any human phenomenon, technostress is a multidimensional situation, complex, and situated (Godbold, 2015), hence requiring investigating what factors influence the intensity of the stressful event and the probability of adopting specific coping strategies, and what coping strategies lead to what neuropsychophysiological and behavioral outcomes. This systematic literature review aims to contribute to the advance of that knowledge by summarizing existing studies and examining: 1) approaches and measures used to evaluate technostress outcomes; 2) antecedents, factors, coping strategies, neuropsychophysiological and behavioral technostress outcomes identified in work-related mandatory and non-work-related voluntary technology usage context; 3) studies that have explored the interaction technostress and non-technological stress