22 July 2016 Development and characterization of Four-Quadrant Phase Mask coronagraph (FQPM)
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Abstract
The goal of a coronagraph is to reduce the flux of a bright object (e.g. a star) in order to distinguish its faint neighborhood (e.g. exoplanets and disks). In this context, we proposed one coronagraph that uses a four quadrant phase mask (FQPM). Since 2000, we fabricated several monochromatic FQPM working in visible and near-infrared light at the Paris Observatory. We have developed systematic procedures for fabrication and characterization of the phase masks. Visual inspections with an optical microscope are performed for every component and a coronagraphic performance measurement based on inclination of the component is done on a dedicated bench that is set up in a clean room. This procedure gives a quick feedback on the quality and performance of the component. Depending on the results, images of the central transition can be recorded with an electron microscope to understand the limitations of the fabrication process. This procedure allowed us to understand the influence of various parameters such as the width of the transitions between the quadrants, the alignment of the transitions or the step depth. Based on these results, we modified the mask design and the fabrication process to improve our success rate to nearly 100% when building a FQPM for any given optimal wavelength in visible or near-infrared. Moreover, we improved the performance of the components, reaching attenuations of more than 20,000 on the central peak in raw images for most coronagraphs. The best of these components are now used on the THD bench, an optical/NIR bench developed for the study of high contrast imaging techniques, reaching 10-8 contrast level routinely.
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M. Bonafous, R. Galicher, P. Baudoz, J. Firminy, F. Boussaha, "Development and characterization of Four-Quadrant Phase Mask coronagraph (FQPM)", Proc. SPIE 9912, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II, 99126J (22 July 2016); doi: 10.1117/12.2234497; https://doi.org/10.1117/12.2234497
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