A distributed Brillouin sensor system is provided. The apparatuses including reflectors for altering the optical distance between a pair of lenses passing a wavefront without changing the physical distance between the lenses, thereby compensating for defocus in the wavefront; and cylindrical mirrors for adding and removing curvature from a curved wavefront, thereby compensating for astigmatism in the wavefront. The plunger seal has an annular flange that provides an OD seal against the gland.