Abstract: A subsea sonar unit includes an acoustic transducer. The acoustic transducer transmitting an acoustic beam defining an acoustic propagation path for acoustic signals to or from the transducer, and the unit also includes a housing at least a part of which being oil filled and positioned in the propagation path of the beam. The housing is acoustically transparent in the direction of the acoustical beam and has an outer surface with a known shape in the propagation path. The unit also includes a corrective lens, the corrective lens being mounted in the propagation path between the transducer and the housing part the interface between which defining a first surface having a shape relative to the cross section of the acoustic beam in the propagation path essentially corresponding to the shape of the housing surface relative to the beams cross section at the housing surface in the propagation path.