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The low wind effect is a phenomenon disturbing the phase of the wavefront inthe 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 airtemperature inhomogeneities across the pupil. Because it is unseen bytraditional adaptive optics (AO) systems, thus uncorrected, it significantlydegrades the quality of AO-corrected images. We provide a statistical analysisof the strength of this effect as seen by VLT/SPHERE after 4 years ofoperations. We analyse its dependence upon the wind and temperature conditions.We describe the mitigation strategy implemented in 2017: a specific coatingwith low thermal emissivity in the mid-infrared was applied on the spiders ofUnit Telescope 3. We quantify the improvement in terms of image quality,contrast and wave front error using both focal plane images and measured phasemaps.

Low Wind Effect on VLT/SPHERE : impact, mitigation strategy, and results