Abstract: A high precision composite optical mirror substrate comprising a lightweight reticulated foam core which is less than about 25 percent dense, an initially separate dense or small pore size faceplate mounted to the core, and a continuous formed in situ encapsulating coating. The coating fully encapsulates the ligaments throughout the reticulated foam core, the juncture between the core and the faceplate, and the faceplate, including the exposed face of the faceplate (the mirror surface). The exposed face of the dense faceplate is shaped to about the desired mirror surface shape. The deposit of coating material on the exposed face of the faceplate is near net shape to the desired mirror surface. The physical characteristics of the composite substrate are dominated by the properties of the coating material. When the coating material is a chemical vapor deposit of silicon carbide, the interlayer is preferably graphite with a thermal expansion coefficient that closely matches that of the silicon carbide.