Recent work done at CSIRO's Australian Centre for Precision Optics has pushed the fabrication limits on optical retroreflectors. To achieve dihedral angle errors well below an arcsecond, the measurement accuracy was required to be 0.1 arcsec or better. On this level, the error introduced by the interferometer's instrument function is not negligible. So-called "collimation errors", or more generally, wavefront aberrations, can significantly falsify the angle measurements. This paper describes and demonstrates the basic concepts for interferometric measurement of dihedral angle errors in retroreflectors from a practical point of view. We discuss obvious and subtle stumbling blocks, summarize practical experiences, and show some results of the recent joint work between JPL and CSIRO.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.