Astigmatism in SPIDER

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.

As used by the SPIDER operations TF C, TF CT, TF D, and astigmatism is a vector with magnitude and direction:

Magnitude

The difference between the highest and lowest defocus.

Angle (or azimuth)

The angle of the "unaffected" axis (45 degrees between the low and high defocus axes)

By convention, this angle is clockwise from from the axis of lowest defocus. Zero degrees is defined as the center of the right hand edge of the image.