Precision glass molding technology for the MODE lens telescope

The primary lens of our multi-order diffractive engineered (MODE) lens telescope combines traditional lens design and a diffractive element to mitigate longitudinal chromatic aberration (LCA). This design uses a 24 cm diameter aperture. In order to make the primary in molded glass, the lens is constructed in 9 segments, 1 radially symmetric center segment, and 8 identical ring segments. A monolithic 24cm aperture MODE lens is not possible at this time, due to limitations of our 14 cm diameter molding cavity. The ring segments each subtend a 45° angular subtense of the ring around the center segment, combining to form a 360° ring around the center segment. Due to the irregular shape of the ring segments and the high precision diffractive surfaces within the design, the lenses are fabricated using precision glass molding (PGM). This presentation considers mold insert design, preform selection, and molding process development. Beyond the overall structure of the molds, the design of the mold insert requires considerations for thermal expansion of the mold and mitigation of adhesion between the mold and the lens using an antiadhesion coating. The preform selection considers both the thermal and optical properties of the glass to be molded and the proper shape of the preform for the easiest material flow during the mold cycle. The general molding process is summarized as heating the preform above the glass transition temperature, applying a force to the mold inserts, and cooling the mold assembly before release.