Coma is a defect of an objective mirror or lens
in which rays of light, striking the objective away from the optical
axis are not brought to focus in the same image plane. In the case of
an astronomical telescope, for example, the result is that star images toward
the edge of the field of view appear
to have comet-like tails spreading radially out from the optical axis (negative
coma) or in toward the axis (positive coma).
AND OPTICAL PHENOMENA