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Measurement of the non-common vertex error of a double corner cube

The Space Interferometry Mission (SIM) requires the control of the optical path of each interferometer with picometer accuracy. Laser metrology gauges are used to measure the path lengths to the fiducial corner cubes at the siderostats. Due to the geometry of SIM a single corner cube does not have sufficient acceptance angle to work with all the gauges. Therefore SIM employs a double corner cube. Current fabrication methods are in fact not capable of producing such a double corner cube with vertices having sufficient commonality. The plan for SIM is to measure the non-commonalty of the vertices and correct for the error in orbit. SIM requires that the non-common vertex error (NCVE) of the double corner cube to be less than 6 μm. The required accuracy for the knowledge of the NCVE is less than 1 μm. This paper explains a method of measuring non-common vertices of a brassboard double corner cube with sub-micron accuracy. The results of such a measurement will be presented.