Development of precision double corner cubes for the Space Interferometer Mission

NASA's Space Interferometer Mission (SIM) PlanetQuest requires, among other things, very precise retroreflectors. The CSIRO Australian Centre for Precision Optics (ACPO) has developed Double Comer Cubes (DCCs) to meet the requirements. The DCC consists of an assembly of three 300 wedged prisms optically contacted to a 132 min diameter flat base plate. The material for all components was Zerodur. The specifications for the DCC were extremely challenging and posed considerable difficulties in the fabrication, coating, assembly, alignment and metrology. Some of the key specifications included: flatness of all reflecting surfaces to be similar to 10 nm peak to valley (P-V); dihedral angle errors < 0.5 arc seconds; collocation of the vertices of the two corner cubes within a circle of 5 lam radius; all reflecting surfaces to be gold coated for a final microroughness < 0.5 mn rms; the clear or working aperture extended to within 0.2 mm of all physical edges; and the assembly had to withstand large vibrational forces. CSIRO delivered to JPL a DCC that was used as the primary unit in the so-called Kite testbed to satisfactorily meet the demonstration requirements of the SIM Milestone 8. This paper will discuss some of the procedures used to realize the DCCs and will show examples of results achieved.