Operation of high performance interferometers and high contrast imaging systems often depends on extraordinarily precise knowledge of locations/displacements of certain critical elements. For example, on SIM, differential changes in optical path between the left and right arm of a 10-meter baseline interferometer must be known to within 50 pm if the interferometer is to perform astrometric measurements with resolution on the order of one micro-arcsecond. Such measurements of optical path difference are performed with picometer-class metrology gauges. These gauges are in turn realized as assemblies of frequency-stable laser light sources, high-precision, dimensionally stable beam launchers and similarly high-precision, dimensionally stable fiducials, the latter often consisting of multiple cube corner retroreflectors arranged and fabricated in such a way as to share a common optical vertex.

Clearly, successful design, development, fabrication and calibration of these picometer metrology gauges requires expertise spanning a range of technical disciplines as well as an understanding of all the potential error sources and interactions that impact gauge design and performance.