Abstract: Apparatus (20) provides a stable optical reference in the presence of optical source jitter. It comprises a beamsplitter cube (21) having an input surface 22, an output surface 23, a totally reflecting surface (24), and a partially reflecting diagonal surface (26) with a corner cube 27 disposed on a surface (25) opposite to the input surface. An input beam (30) applied to the beamsplitter cube results in two output beams (33, 33'), coplanar with an output optical axis (31), if the input beam deviates at 32 from an input optical axis. The two output beams deviate equally but oppositely from the output axis. This provides signals from which a stable output optical axis may be determined. A polarization-sensitive embodiment is also disclosed which eliminates problems associated with the use of coherent light sources.