  We describe a setup based on Michelson interferometry for coherent
measurements of the backscattered light from a low roughness optical surface
under test. Special data processing was developed for the extraction of the
useful signal from the various stray contributions to the coherent signal. We
achieve coherent detection of light scattered by a mirror down to -130 dB in
optical power. We characterize the dependence of the backscattered light with
spot position and incidence angle. Results of cross-polarization scattering
coherent measurements and preliminary results of dust deposition experiment are
presented here. This work represents the first step in the experimental
evaluation of the coherent perturbation induced by the scattered light in space
gravitational wave detector of the LISA mission.
