Comparison of closed loop and sensorless adaptive optics in widefield optical microscopy

We report on a closed loop widefield adaptive optics, optical microscopy system in which the feedback signal is provided by backscattered light from the sample acting as a guide star. The improvement in imaging performance is compared to an adaptive optics system controlled via an image optimisation routine commonly described as sensorless adaptive optics. The samples viewed were imaged without fluorescence to ensure that photobleaching and other potential variations did not affect the comparisons in system performance though the method is equally applicable for fluorescence microscopy. The closed loop system is self-optimising for different areas of the sample, using a common reference wavefront, with the accuracy of the loop being limited by variation across the sub-aperture images induced by guide star elongation. Optimisation using an image sharpness metric gives slightly sharper images but takes significantly longer. We thus believe that both wavefront sensor based closed loop AO and metric based optimisation have a role to play in AO for microscopy and that the method of backscattered light as a guide star has a great potential in the application of AO, particularly to optical coherence tomography.