OComa due to misalignment of optical elements

Coma is a well known aberration. It is usually known as an off-axis aberration (see figure at lower right),
whose magnitude increases with field angle.

However, coma can also arise due to the misalignment of the optical elements, as the first two figures show.
 In that case, one gets constant coma over the field, with all the comatic images pointing in the same direction.
Coma due to decentering of M2. Coma due to tilt of M2.