Ultralight-weight mirror and method of manufacturing it
The invention relates to an ultralight-weight mirror and to a method of manufacturing it. 
     BACKGROUND OF THE INVENTION 
     One known way of manufacturing a light-weight mirror is to fix plates on a honeycomb core and to put a reflecting deposit on one of the plates (IBM Technical Disclosure Bulletin, Vol. 20, No. 12, May 1978, page 5121; U.S. Pat. No. 3,912,380). 
     It is also known that plastic materials and fibers may be used to make low mass mirrors or other optical components (Laser Focus, April 1985, pages 40 to 42; U.S. Pat. No. 4,463,055; French patent 2 215 631). 
     More precisely, French patent 2 215 631 describes a method of manufacturing a low mass optical component in which a stack is made of plies constituted by fibers impregnated with a polymerizable resin, with such a stack being applied and glued to at least one of two opposite faces of a core of cellular structure, with the resin being polymerized by heating and by applying pressure, and with a reflecting surface being formed on the stack. 
     A fundamental difficulty in the manufacture of a light-weight mirror lies in the fact that light weight is often obtained to the detriment of the mechanical properties of the mirror, and in particular of its stiffness. 
     It is thus very difficult to obtain a mirror which is simultaneously both light in weight and which has good mechanical properties. 
     The present invention seeks to provide an ultralight-weight mirror having improved mechanical qualities, in particular improved stiffness. 
     SUMMARY OF THE INVENTION 
     This is achieved, according to the invention, by a method in which a stack is made of plies constituted by fibers impregnated with polymerizable resin, one such stack being applied to at least one of two opposite faces of a cellular structure core, the resin being polymerized by heating while pressure is applied, and a reflecting surface being made on the stack, the method including the improvement whereby each ply is made in such a manner that the fibers of the ply extend in one direction only and in that said polymerization is performed prior to applying and fixing the stack on the cellular structure core. 
     The orientations of the fibers in various plies are preferably determined in such a manner as to increase the sheer modulus and the Young&#39;s modulus of the stack and/or in such a manner as to reduce the coefficient to expansion of the stack. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An implementation of the invention is described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagrammatical perspective view of a cellular substrate for constituting the core of a mirror; 
     FIG. 2 is a theoretical diagram showing the orientations of the fibers in successive plies; 
     FIG. 3 is a diagram of a device used for polymerizing the stack of plies; 
     FIG. 4 is a diagram of a device used for gluing the stack onto the cellular substrate; 
     FIG. 5 is a diagrammatic perspective view of a modified substrate; and 
     FIG. 6 shows the mirror shapes that can be manufactured using the method of the invention. 
    
    
     MORE DETAILED DESCRIPTION 
     FIG. 1 shows a cellular structure 1 (preferably having hexagonal cells and of the type commonly referred to as a honeycomb) which is made of metal or of any other material suitable for obtaining adequate stiffness. 
     On each of the two opposite main faces of said structure, a stack 2 or 2&#39; is fixed, said stack being made by stacking plies of high Young&#39;s modulus fibers (or any other fibrous material) with the plies being pre-impregnated with resin. The fibers in each ply are oriented in the same direction and the orientations of the fibers in different plies are selected so that the coefficient of expansion in the plane is as small as possible while its Young&#39;s modulus is as high as possible. 
     Preferably, the orientations of the fibers in the various plies are at angles of Kπ/n, where K takes all integer values from 0 to (n-1) once, and once only, and n is the number of plies (FIG. 2). 
     In a preferred embodiment, the outside face of the stack is made of metallized fibers extending in a single direction and intended to act as an electrode. 
     Both stacks are polymerized between two parallel steel plates which are optically polished and treated with antiadhesive agents, within a press having exactly parallel heating plates 3 (see FIG. 3). 
     After the two stacks have completely polymerized, they are laminated onto the light-weight substrate using an adhesive film 4 (of the REDUX type sold by CIBA GEIGY) which is polymerizable in the press having hot plates 3 (see FIG. 4). 
     A metal coating is then made on the outside face of one of the stacks to a sufficient thickness to enable it to be polished until it has the desired degree of planeness. 
     The metal deposit may be performed electrolytically, particulary when one or both outside faces 2a and 2&#39;a contain metallized fibers suitable for acting as electrodes. 
     In a variant, the metallization is performed chemically followed by an electrolytic deposit up to a thickness which is sufficient to receive polishing by any of the methods conventional in optics, with the face then being polished until it has the desired degree of planeness. 
     It is also possible to perform metallization by a vacuum deposition technique (e.g. magnetron cathode sputtering) until a sufficient thickness has been obtained for subsequently performing optical polishing to the desired degree of planeness. 
     The reflecting face may be provided directly by the polished metal deposit on the stack, or else by an additional metal coating, e.g. of silver, aluminum, or gold, e.g. made by vacuum evaporation onto the polished metal deposit in the event that it is not possible to obtain the desired reflection factor from the polished metal deposit itself. 
     Finally, if so desired, a protective deposit is provided of the kind conventionally used on optical surfaces (of the SiO, or the MgF 2 , etc. type). 
     All of the metallization operations are performed in such a manner as to ensure that the temperature of the mirror does not exceed the limit temperatures for conserving the mechanical properties of the laminated materials. 
     In applications where pressure variations may occur, it may be necessary to use a cellular structure having cells that intercommunicate by means of holes 5 (see FIG. 5). These holes should be temporarily closed during metallization treatments. 
     The invention is not limited to using special fibers which are suitable for being metallized to constitute an electrode: although carbon fibers are preferably used, it is possible in variants to use other types of fiber, e.g. fibers made of Kevlar, glass, ceramics, pure silica, or boron. 
     The reflecting face may be subjected to a multilayer treatment.