An apparatus is provided that includes a field reflector and a plurality of pairs of object reflectors. The apparatus also includes a plurality of source and detector port pairs, where each source port is configured to pass a beam of radiation, and each detector port is configured to receive a beam of radiation. The source and detector ports of each pair are positioned proximate an outer edge of the field reflector such that an optical axis of the field reflector lies between the respective source port and detector port. The object reflectors and source and detector port pairs are arranged such that each source and detector port pair is associated with a respective pair of object reflectors forming a distinct channel, where the source and detector port pair, and centers of the associated pair of object reflectors, of each channel lie in a distinct plane.