I make my telescope mirrors usually by hand using usually two equal size thick window glass like 3/8" or 1/2"and 6 or 8" diam, I lay 1 disk on a wet cloth to keep it from slipping. some grinding grit powder and water,on it, place the other disk on the first (tool) and start grinding with only circular stroking , 35 to 40% of the top (mirror-blanc) over-hanging the tool, all around, holding it with both hands circling but not twirling or spinning, that causes the edge of the tool to grind all around and the center of the mirror grinds out, keep grit and water between and in a few hours the whole mirror face will be ground dull to the edge, and you have likely a deep enough sagitta-dip in the mirror, next use some finer (or recycled grit), 20% overhang and keep stroking around. when the face becomes somewhat smooth rince with water and while dripping wet it can reflect sunlight to a focal point, measure the distance focal length if it is about what you planned, do some more finer and finer grinding until ready for polishing with a "pitch-lap" another time.
Peter-circular-stroke-mirror-on-top
