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The low wind effect (LWE) refers to a characteristic set of quasi-staticwavefront aberrations seen consistently by the SPHERE instrument whendome-level wind speeds drop below 3 m/s. This effect produces bright low-orderspeckles in the stellar PSF, which severely limit the contrast performance ofSPHERE under otherwise optimal observing conditions. In this paper we proposethe Fast & Furious (F&F) phase diversity algorithm as a viable software-onlysolution for real-time LWE compensation, which would utilise image sequencesfrom the SPHERE differential tip-tilt sensor (DTTS). We evaluated theclosed-loop performance of F&F on the MITHIC high-contrast test-bench under avariety of conditions emulating LWE-affected DTTS images, in order to assessthe expected performance of an on-sky implementation of F&F in SPHERE. Thealgorithm was found to be capable of returning such LWE-affected images toStrehl ratios of greater than 90% within five iterations, for all appropriatelaboratory test cases. These results are highly representative of predictivesimulations, and demonstrate the stability of the algorithm against a widerange of factors including low image signal-to-noise ratio (S/N), small imagefield of view, and amplitude errors. It was also found in simulation thatclosed-loop stability can be preserved down to image S/N as low as five whilestill improving overall wavefront quality, allowing for reliable operation evenon faint targets. The Fast & Furious algorithm is an extremely promisingsolution for real-time compensation of the LWE, which can operatesimultaneously with science observations and may be implemented in SPHEREwithout requiring additional hardware. The robustness and relatively largeeffective dynamic range of F&F also make it suitable for general wavefrontoptimisation applications, including the co-phasing of segmented ELT-classtelescopes.

Laboratory verification of 'Fast & Furious' phase diversity: Towards controlling the low wind effect in the SPHERE instrument