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Coma 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.
Comatic images in the focal plane of a telescope, due to misalignment. Their size is constant over the field, and all the images point in the same direction.
Off-axis coma due to the optical design of the telescope (for example, a classical casssegrain telescope). The images point inwards, and their magnitude increases with field angle.
coma due to