Silicon particle reinforced aluminum containing 42 wt.% silicon (Al-42Si) offers higher specific stiffness, higher resistance to thermal gradients, and an athermal behavior combined with electroless nickel, in comparison to state-of-the-art aluminum alloys (e.g. Al6061). Nevertheless, typical surface modifications (e.g. black anodization) lead to a nonsufficient optical stray light suppression due to the high amount of silicon particles on the surface. Therefore, a coating technique is required that fulfils the optical and the mechanical (e.g. adhesive strength) specifications for stray light reduction on Al-42Si. Different components like housings and barrels made of Al-42Si were coated with amorphous NiP and with Fractal Black™ by Acktar Ltd. The optical performance is analyzed by angle-resolved light scattering (ARS) at two different wavelengths (532 nm and 1064 nm). Fractal Black™ offers a total scattering level of lower than 2 % at 532 nm and 1064 nm. Fractal Black™ has been proven as a suitable coating for components made of Al-42Si.
The low wind effect is a phenomenon disturbing the phase of the wavefront in the pupil of a large telescope obstructed by spiders, in the absence of wind. It can be explained by the radiative cooling of the spiders, creating air temperature inhomogeneities across the pupil. Because it is unseen by traditional adaptive optics (AO) systems, thus uncorrected, it significantly degrades the quality of AO-corrected images. We provide a statistical analysis of the strength of this effect as seen by VLT/SPHERE after 4 years of operations. We analyse its dependence upon the wind and temperature conditions. We describe the mitigation strategy implemented in 2017: a specific coating with low thermal emissivity in the mid-infrared was applied on the spiders of Unit Telescope 3. We quantify the improvement in terms of image quality, contrast and wave front error using both focal plane images and measured phase maps.
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