Astigmatism(axial): an electron-optical lens aberration that causes
the defocus to be a function of azimuth, and the contrast transfer
function to deviate from circular symmetry about the optical axis.
As a consequence, the Thon rings deform into elliptic or hyperbolic
patterns, depending on the size of defocus and the size of the
astigmatic defocus difference.
A flag for Y or N for astigmatism estimation