Above: foucaultgrams, at radius of central, 70%, and edge zones.
Prior to Saturday's STM mirror class I worked the mirror with center-over-center strokes to try to get rid of the rings you can see in the images in the March 11 post below. It took almost 2 hours of polishing away, but all that was left by way of departure from a sphere when I arrived at class was a slight mound in the middle 40% of the mirror. Dave K. suggested I start with a narrow parabolizing W stroke, which added correction to the center, and then with a slight chordal stroke, with extra thumb pressure on the overhang, which pushed the correction toward the edge. When I got home I decided to do some more of the narrow W—about 10 minutes of that brought the mirror to the state depicted above. So now my question is: what next? My guess is work on the edge some more, perhaps with the same slight chordal overhang and thumb pressure. But I'm unsure and wonder how much overhang to use. Below are the zonal knife-edge measurements, made with a 5-zone couder mask.