Peter Gordon Martin explains what aberration is.

Aberration is the phenomenon where, quite simply put, light that one had hoped would come to a very fine focus in a telescope image plane is spread out instead. Even if you build a perfect spherical mirror and take a wide field image, you will find that as you look away from the center of the image, the points of lights that you had hoped to create are no longer round. There are ways around this, of course. The easiest way to correct spherical aberration is to make the mirror a slightly different shape: the shape of a parabola. There are other sources of aberration, but generally speaking, they all represent faults in the image that clever opticians are hoping to correct.

ASTROLab/Mont-Mégantic National Park

© 2006 An original idea and a realization of ASTROLAB of the National park of the Mount-Mégantic