2018/06/30 by Julien Milli, Markus Kasper, Pierre Bourget +14 · 1 citation
Physics and Astronomy · #astro-ph.IM
paper · pdf · doi:10.1117/12.2311499
published as Proc. SPIE 10703, Adaptive Optics Systems VI, 107032A (10 July 2018) · 20 pages, 21 figures, SPIE 2018 conference proceeding
arxiv created 2018/07/22 · arxiv updated 2018/07/24
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.