  Deep fluorescence imaging in mammalian brain tissues remains challenging due
to scattering and optical aberration-induced loss in signal and resolution.
Correction of aberrations using adaptive optics (AO) requires their reliable
measurement in the tissues. Here, we show that an extended-source
Shack-Hartmann wavefront sensor (ESSH) allows quantitative aberration
measurements through fixed brain slices with a thickness up to four times their
scattering length. We demonstrate in particular that this wavefront measurement
method based on image correlation is more robust to scattering compared to the
standard centroid-based approach. Finally, we obtain a measurement of the
tissue scattering length taking advantage of the geometry of a Shack-Hartmann
sensor.
