This Ronchi image was made with a 133-line grating, behind center of curvature. The image was taken just after rough polishing was completed. Rough polishing was a high-speed process using a fixed-post polishing machine. The step is performed to rapidly remove the pits left over after fine grinding. The fixed-post method leaves several zonal bands easily visible in the Ronchigram. The mirror is slightly oblate, as seen be the slight outward bulge in the bands. There is also a small "belly button" at the center caused by the overlap of the edge of the polisher and the center of the mirror.
The image above was made about 40% of the way into rough parabolization. In this step, a smaller polisher is used to rapidly remove glass from the center areas of the mirror. This deepening moves the mirror from roughly spherical toward the desired parabola. The narrowing waist at the central part of the mirror demonstrates the deepening of this area, while the more parallel outer areas are still roughly spherical.
In this third image, the mirror is nearly at full depth, just 10-thou short. By now the rough techniques are evident in the poor surface quality, as demonstrated by some zones. The next steps will be performed with smaller polishing tools that are designed to smooth the surface and move toward the correct figure.
Now the mirror is taking on a much better surface figure, as seen by the smoother curve of the Ronchi bands. On a perfect mirror, the bands themselves would be perfect parabolas. This mirror is at about 90% of full correction. This is evident as the bands don't have quite enough curve. The Foucault test rates this mirror as about 1/2-wave P-V on the wavefront at this point.
Now the mirror is rated at about 1/6th-wave. The zones have been smoothed, although this image is slightly marred by air currents moving past the mirror during the test.
This final screen snap from the Ronchi tester shows the finished mirror, which FigureXP rates at about 1/19th wave. The image still suffers a little from thermal effects, but the bands are smooth and correctly shaped, as verified by the multi-zone, multi-diameter Foucault test.