Vertically configured collimator for cryogenic vacuum testing of meter scale optical systems

Ball Aerospace has constructed a new collimator for interferometric and image quality testing of meter scale optical systems under cryogenic, vacuum conditions. Termed the Vertical Collimator Assembly (VCA), it features 1.5 m diameter off-axis parabolic and calibration flat mirrors. In order to preserve as large a volume as possible for the unit under test, the main platform is suspended inside its vacuum chamber by a hexapod, with the parabolic mirror mounted overhead. A simultaneous interferometer facilitates collimator alignment and monitoring, as well as wavefront quality measurements for the test unit. Diffusely illuminated targets may be employed for through-focus image quality measurements with pinholes and bar targets. Mechanical alignment errors induced by thermal and structural perturbations are monitored with a three-beam distance measuring interferometer to enable mid-test compensation. Sources for both interferometer systems are maintained at atmospheric pressure while still directly mounted to the main platform, reducing vibration and stability problems associated with thermal vacuum testing. Because path lengths inside the ambient pressure vessels are extremely short, problems related to air turbulence and layering are also mitigated. In-chamber support equipment is insulated and temperature controlled, allowing testing while the chamber shrouds and test unit are brought to cryogenic temperatures.