Mirror arrangement in a focusing interferometer

A focusing interferometer comprising a light source, a first mirror for collimating the light emitted from the light source, a second mirror for focusing the collimated light emitted from the first mirror, a beam splitter arranged in the focus of the light emitted from the second mirror in order to split the light into a reflected part and a transmitted part, a third mirror for collimating the light reflected from the beam splitter, a fourth mirror for collimating the light passed through the beam splitter, a fifth mirror for reflecting the collimated light emitted from the third mirror back to said third mirror and further focusing it on the beam splitter, and a sixth mirror for reflecting the collimated light emitted from the fourth mirror back to said fourth mirror and further focusing it on the beam splitter, the fifth and sixth mirrors being arranged back-to-back to reflect to opposite directions such that their optical axes join, whereby the light returned from the fifth mirror and passed through the beam splitter and the light returned from the sixth mirror and reflected from the beam splitter interfere with each other, wherein the second mirror and the third mirror are combined to provide a single uniform mirror surface, and each of the fifth and sixth mirrors comprises three flat mirror surfaces that are perpendicular to one another and are arranged to reflect to a direction of a where normals of the mirror surfaces intersect.