Wavefront control of the large optics test and integration site (LOTIS) 6.5 m Collimator

The LOTIS Collimator provides scene projection within a 6.5 m diameter collimated beam used for optical testing research in air and vacuum. Diffraction-limited performance (0.4 to 5 μm wavelength) requires active wavefront control of the alignment and primary mirror shape. A hexapod corrects secondary mirror alignment using measurements from collimated sources directed into the system with nine scanning pentaprisms. The primary mirror shape is controlled with 104 adjustable force actuators based on figure measurements from a center-of-curvature test. A variation of the Hartmann test measures slopes by monitoring the reflections from 36 small mirrors bonded to the optical surface of the primary mirror. The Hartmann source and detector are located at the f/15 Cassegrain focus. Initial operation has demonstrated a closed-loop 110 nm rms wavefront error in ambient air over the 6.5 m collimated beam.