Two-mirror systems of the type conventionally used as astronomical objectives can be used noncoaxially to avoid the usual central obscuration. Third-order aberration theory is used to derive the conditions under which the primary and secondary mirrors can be tilted and decentered.
Rubin Gelles, "Unobscured-aperture two-mirror systems," J. Opt. Soc. Am. 65, 1141-1143 (1975)