A theoretical and experimental study of the aperture-bandwidth characteristics of the electronically tunable acousto-optic filter shows that (a) optical divergence in the ordinary plane of the acousto-optic crystal results in a broadening of the filter frequency-transmission characteristic toward longer wavelengths; (b) optical divergence in the extraordinary plane results in a broadening toward shorter wavelengths; (c) divergence of the acoustic beam has the same effect as optical ordinary divergence, and generally will not be of consequence; and (d) roughly, the resolution times the solid-angle acceptance of the acousto-optic filter is approximately the same as that of a solid Fabry-Perot interferometer made of the same material.
S. T. K. NIEH and S. E. HARRIS, "Aperture-Bandwidth Characteristics of the Acousto-Optic Filter," J. Opt. Soc. Am. 62, 672-676 (1972)