Tilted interferometer

Polarization interferometric apparatus, said apparatus comprising interferometer means for receiving at least two beams and providing first and second measurement legs, separating said two beams for travel along said first and second measurement legs, respectively, and generating exit beams containing information about the respective differences in the optical paths each beam experienced in traveling said first and second measurement legs, said first and second measurement legs having optical paths structured and arranged such that at least one of them has a variable physical length, the optical path length difference between said first and second measurement legs varying in accordance with the difference between the respective physical lengths of their optical paths, said interferometer means comprising a plurality of optical components including a stage carrying a plane object mirror moveable along one of said measurement paths;

• said plurality of optical components other than said stage and said plane object mirror being rotated collectively with respect to said stage and said plane object mirror by a small angle in the same angular direction, for substantially eliminating and reducing subharmonic cyclic error contributions that may otherwise overlap and interfere with interfering components in said exit beams.