An optical aberration that is commonly seen off-axis on an optical system, especially in fast Newtonian reflectors. It makes the stars look like V shapes; they are often described as looking like comets or seagulls. If an optic is aligned properly (see Collimation), then there should be no coma at the center of the field. There may however be coma in the corners of the field, because they are off-axis. If an optical system is not aligned properly, coma may be visible throughout the field.