<title>Metrology for spatial interferometry IV</title>

The proposed Space Interferometry Mission (SIM) spacecraft carries high resolution stellar interferometers for micro-arc- second accuracy astrometric measurements. These stellar interferometers require picometer accuracy one dimensional metrology gauges, surface metrology gauges and 3-dimensional metrology gauges. The absolute metrology gauges required by these interferometers can be considerably less accurate due to the careful design of the astrometric interferometers on the spacecraft. Open-faced, hollow corner cube retro-reflectors are used as fiducials in the one-dimensional relative and absolute metrology gauges and the 3-dimensional metrology gauge. The diffraction caused by the assembly and the component defects of these hollow retro-reflectors affects the accuracy of these metrology gauges. A simulation quantifying some of the effects of the component and assembly defects of hollow retro-reflectors on the accuracy of a picometer linear metrology gauge is presented. An auto-aligning, 3-dimensional metrology gauge constructed using the sub-picometer linear metrology gauges was described in earlier papers. The functioning automatic alignment and the sub-nanometer, in-air tracking results from this 3-dimensional metrology gauge are presented.