The basic principle of mirror grinding is: if you rub two pieces of glass together (with grit in between) the top one will loose glass from the center outward and the bottom will loose glass from the edge in so that the top one is concave and the bottom is convex.
Before you start
Rough grinding starts with #80 grit with the mirror on top, then you grind until you are close to the right depth or saggita (the elevation change between the rim (not the bevel) and the center in the center of the mirror) which determines the focal length. The saggita can be determined either by reflecting the light on a wall and moving back ad forth until the light is concentrated into a spot or by measuring the depth with a micrometer.
