These are photos of 6" f/8 mirror that tested out on a Foucault test as 1/20 wave.
Reality is that this is at best a 1/8 wave mirror. This is due to the astigmatism. It is revelaed here by an oval shape instead of a perfect sphere. Each wave is 1/8 wave of light. Two fringes shows about 1/4 wave error.
Notice how these appear very similar to the Airy disk when viewing a star at high power.
Looking much like a Ronchi screen, the interferrence patterns above show an undercorrected mirror, with some astigmatism. The lines should be perfectly straight for a perfect parabola. They show the same mirror at different orientations. The small ripples in the center are ghost reflections and are not surface defects. Most commercial mirrors look very similar to this.