Astigmatism

The picture of the previous post shows the crude deformation of the mirror. However not all that deformation is abberation. In fact the mirror is tilted and its radius of curvature modified. To evaluate the real abbertions tilt, piston and defocus must be removed (defocus only means that the best focus position must be adjusted).
The picture in this post was obtained by manually tilting and defocusing the surface (you may see the expression for the plotted surface in the picture). Although they are not exactly the best fitting parabola, they are close. Once they are removed what remains is mostly astigmatism: the sides of the mirror are lifted and the vertical diameter pushed back (this is due to the sling which compresses the mirror in the horizontal diameter, which is strictly unnecessary). The amount of astigmatism is thus rougly 60-70 nm PTV (about 1/4 wave on wavefront).