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

TL;DR

This paper analyzes the low wind effect on VLT/SPHERE, showing how it degrades image quality, and evaluates the effectiveness of a mitigation strategy involving low-emissivity coatings on telescope spiders.

Contribution

It provides a comprehensive statistical analysis of the low wind effect and demonstrates the success of a novel mitigation approach implemented at VLT.

Findings

Significant degradation of AO-corrected images due to low wind effect.

Mitigation with low-emissivity coatings improved image quality and contrast.

Quantitative assessment of wavefront error reduction after mitigation.

Abstract

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.