Reflecting telescope

Résumé

A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an . The reflecting telescope was invented in the 17th century by Isaac Newton as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Many variant forms are in use and some employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a catoptric telescope. From the time of Newton to the 1800s, the mirror itself was made of metal – usually speculum metal. This type included Newton's first designs and the largest t

Source officielle

https://en.wikipedia.org/wiki/Reflecting_telescope

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Finite Element Modeling of Acoustic Shielding via Phononic Crystal structures

Clémentine Sophie Sarah Lipp, Andrea Lozzi

Quality factor of Contour Mode Resonators (CMR) are mainly affected by energy losses due to acoustic waves leaving the resonator through the anchors. An engineering of the anchors in order to create a periodic variation in the acoustic impedance of the material, structures known as Phononic Crystals (PnCs), can help improve the Q factor by reflecting part of the acoustic waves. During this project, FEM models have been validated for both 1D and 2D PnCs. The behavior of the band diagram and quantification of transmission has been studied for three different PnC geometries: circle, cross and square unit cell. Parameters such as unit cell size, filling factor, material composition, thickness and number of repetitions have been varied and comparisons between transmission graph and band diagram allowed to understand better the nature of the modes. It has been found that a linear approximation can predict with a good precision the position of the band gap as a function of the unit cell size. These studies have been done in order to select the best configuration for a future integration on the substrate of a CMR whose frequency range of operation is situated between 200MHz and 500 MHz. The results obtained report a better attenuation and broader peak of attenuation for a cross unit cell made out of AlN and an attenuation increasing linearly with the number of repetitions.

2016

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Validation of Landsat-8 and Sentinel-2 algorithms for atmospheric correction on Lake Geneva

Sonia Joëlle Dupuis

Remote-sensing of water surfaces such as large lakes is a useful tool for monitoring and understanding large-scale processes. Suspended particles in lakes can have various sources such as autochthonously produced organic matter, urban or agricultural runoff or rivers inflows. Understanding their sources and impacts on natural waters is highly important as they are the mirror of the ecological well being. As suspended solids can be retrieved by remote-sensing based on their reflectances, satellite images are an important tool to study the lake and detecting important processes and events. Before using the reflectances measured by the sensor for monitoring lakes, atmospheric corrections have to be applied on the image for retrieving accurate data. This project compares three different algorithms for atmospheric corrections, namely Acolite developed in Belgium, OPERA developed at Vito and C2RCC designed by Brockmann Consulting, applied on the case study, the Lake of Geneva. The resulting reflectances from the different algorithm settings are compared to the in-situ water-leaving reflectance measurements. Additionally, the total suspended matter concentrations derived from the output reflectances were validated with in-situ concentrations. C2RCC with elevation indication performs a good atmospheric correction and yield good particulate concentrations by non-ideal atmospheric conditions. Acolite with a linear parameter of 9.5 gives good results by perfect weather conditions. This projects depicts guidelines and recommendations for performing atmospheric corrections over the Lake Geneva with these algorithms.

2017

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The present invention relates to a reflective optical objective (1) comprising: —a first reflecting element (3) including a front surface (3a) and a hack surface (3b), the front surface (3a) including a convex reflecting surface (11); and —a second reflecting element (5) including a concave reflecting surface (13) facing the convex reflecting surface (11) of the first reflecting element (3), the second reflecting element (5) including a transmissive section (7) permitting electromagnetic radiation to pass through the concave reflecting surface (13) of the second reflecting element (5) to the first reflecting element (3). The reflective optical objective (1) is characterised in that the reflective optical objective (1) includes a carrier material (9) embedding at least the front surface (3a) of the first reflecting element (3) and defining the distance (d) between the first (3) and second (5) reflecting elements.

2015

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