What is Astigmatism?

Astigmatism is the monochromatic aberration of off-axis points that affects sharpness.
  When the field of view is large, the object point on the edge is far from the optical axis, and the beam is inclined greatly, causing astigmatism after passing through the lens.
  Astigmatism causes the original object point to become two separate and mutually perpendicular short lines after imaging, which form an elliptical spot after being integrated on the ideal image plane.