Understand and correct for the low wind effect on the SPHERE and GRAVITY+ adaptive optics
Comprendre et corriger le low-wind effect sur les optiques adaptatives de SPHERE et GRAVITY+
Résumé
The low wind effect is responsible for uncorrected aberrations that reduce the contrast of exoplanet observations during the nights where the atmospheric conditions are the best. This effect arises at the aperture of 8 meter telescopes such as the Very Large Telescope (on SPHERE, Adaptive Optics Facility (AOF)), Subaru (on SCExAO) and Gemini South (GPI). It is a thermal effect occurring at the spiders that hold the secondary mirror. We use numerical simulations to understand why the high-order adaptive optics fail to correct for low wind effect. Our simulations show that the adaptive optics might amplify/create most of the undesired residuals. We propose a mitigation strategy based on both Shack-Hartmann measurements and H-band focal plane images. We speculate that, contrary to a common belief, the low wind effect could be a local effect on the pupil that is spread out by the adaptive optics loop.
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