The capability of degenerate optical cavities to transmit faithfully incident optical signals with arbitrary wavefronts is limited primarily by geometrical optics aberrations. This capability is expressed by an acceptance factor which is calculated for various types of cavities lacking first-order degeneracy, or suffering from primary aberrations. It is found that the acceptance factors of spherically symmetric cavities are larger, by orders of magnitude, than the acceptance factors of cavities possessing only rotational symmetry (such as the well-known confocal cavity). The correction of primary aberrations for both types of cavities is discussed. Acceptance factors of the order of 107 with finesses of the order of 100 can be obtained. The mechanical accuracy required is, however, a few orders of magnitude higher than in conventional optical instruments.
J. A. Arnaud, "Degenerate Optical Cavities. III: Effect of Aberrations," Appl. Opt. 9, 1192-1200 (1970)