Fine grinding is the process of smoothing the surface of an already shaped mirror. It does not significantly change the geometry of the mirror, but may change the focal length by a few inches. The principle is the same as when sanding down wood. Initially, a rough paper with course grains is used and gradually, finer and finer sandpaper is used until the surface of the wood is smooth and uniform. Much in the same way, a various grits are placed between the concave-shaped mirror blank and convex-shaped mirror tool. The roughest material is silicon carbide, the finest is aluminum oxide.
A suitable workstation must also be created so that the mirror blank and tool are held at a comfortable position and so that they do not slide during the grinding. The roughest grains will smooth out all big scratches, but create their own as well. The next grade of grit must remove these pits, but it also creates some smaller ones of its own. Gradually, the size of these pits is worked down until the glass appears smooth. Along the way, the glass must be examined with an eyepiece to be sure that all of the pits have been removed.
Once a suitable smoothness is attained, the mirror is ready for the process of polishing.
