High Precision 10-cm Aperture Penta and Roof-Penta Mirror Assemblies

10-cm aperture penta and penta-roof-type mirror assemblies of unusually high precision and stability for this aperture have been designed and fabricated by Perkin-Elmer Corporation. These assemblies, which are part of the internal optical alignment monitoring subsystem of a large (122-cm) aperture telescope, provide 90 degrees deviations within +/- 1 sec of arc. In order to minimize degradation of the reflected wavefront due to variations of the thermal environment, each assembly is constructed of optical quality Cer-Vit mirror plates contacted to Cer-Vit blocks which establish and preserve the proper angular relationships between the reflecting surfaces. Thermal, vibration, and shock tests of prototype units have confirmed their inherent resistance to adverse environmental influences. In this paper, the method of construction of these prototype units is described and the results of various tests are given. Since the reflecting surfaces of the mirrors are dielectric coatings which maximize reflectivity in the 0.9-0.95-mu wavelength region but have very low reflectivity at visible wavelengths, a real-time, electrooptical interferometric technique was developed for evaluating with near-infrared radiation the wavefront deformation caused by one of these assemblies. This experimental technique is also described.