The sandwich mirror is an experiment to see if  mirrors can be made by using epoxy to glue plate glass together to make a light weight, thin, but stiff structure that  can quickly come to thermal equilibrium.  The front surface is 5/8" thick by 18 1/2", the backing is 3/8" X 16", and the webs are 1/4" X 1 1/2". The mirror is ground out to F4 making the front glass about 3/8" thick in the center.  The converging spokes give closer support for this thinner glass.  The mirror is designed for a six point cell.  The center of the backing plate is trepanned for a 3" muffin fan and air can circulate throughout the cells  The plate glass circles were cut out using a tile saw and then the diamond blade was mounted on a pendulum grinder and ground out to a radius of about 156".  The mirror weighs in at 10 kilograms (22 lbs). Machine grinding and polishing progressed with no unusual problems till figuring commenced when it became obvious that the webs were imprinting through as low areas on the surface due to the pressure required for polishing depressing the glass between the webs and then the glass rebounding after the lap is removed, ( my theory ).  To surmount this problem I'm experimenting with using vacuum to generate the pressure between the lap and mirror and at the same time supporting the over arm and lap so that no weight is put on the mirror.  See Vacuum Polishing

Epoxying ribs to front plate