Skew error is tilting of the image field axes compared to the ideal axes.
What causes skew error?
Skew error is caused by imprecise bonding of the mirrors or incorrect skew compensation.
Ideally, both image field axes should be perpendicular or parallel to the beam entrance axis. Tilted axes distort the rectangular image field into a parallelogram.
Analog controled systems can compensate skew via a motion of the other axis. The specification is the angle between the actual and theoretical axis in mrad.
Skew error can be eliminated by applying image field correction (e.g. with CorreXionPro).