The design of ultra-lightweight mirrors for large optical space systems is necessitated by spacecraft payload capabilities. The weight of supporting structure is in turn lessened as the optical element weights decrease. Lightweight mirror design, however, must be shown capable of inducing minimal distortion error response from the severe environment of a typical orbital mission. Such includes on-board vibration, gravity release, and a cryogenic operational temperature often below 100°K. Further, the optical elements must withstand the extreme loadings of a launch environment. As such, the mirrors must exhibit excellent stiffness, strength, and thermal expansion characteristics, particularly so in a passive (non-correctable error) system. A symmetrical, fused silica glass lightweight design, composed of fused facesheets and core, which meets these design requirements is presented. The fabrication and manufacturing processes utilized to accomplish the design and its objectives are also discussed.