Abstract: A process for making a ribbed light weight composite mirror unit. Preferred embodiments are silicon carbide composite structures. Preferred structures comprise a front smooth silicon carbide surface supported by a silicon carbide ribbed back support. The ribbed back support may be produce by milling out portions of SiC block or by the joining of multiple simple shapes to form the ribbed support. At least the smooth front SiC surface is produced using a chemical vapor composite process as described in the Background Section. These include very large mirrors that resist gravitational sagging and smaller scanning and stepping mirrors that can be pointed quickly and accurately with minimal hysteresis. Preferred milling techniques include precision water jet milling. Special bonding techniques are described to produce ribbed support from multiple parts.