The glass must be thick enough so that the beam does not overlap with itself to produce interference fringes, and if possible that the secondary reflection does not illuminate the active area of the CCD . The Fresnel reflection of a beam from a glass plate is different for the s-and p-polarizations ( s is parallel to the surface of the glass, and p is perpendicular to s ) and changes as a function of angle of incidence keep this in mind if you expect that the two polarizations have different beam profiles.
