Coma is a geometric defect of the paraboloid. Please consider: light rays striking a paraboloid centrally and on-axis see a perfectly symetrical figure and form an essentially perfect diffraction spot. Rays that come in off-axis see a non symetrical figure and form a distorted diffraction spot. The optical axis of a paraboloid is unique while a sphere has many optical axes.
This is the essential reason behind the search for an all-spherical optical system. Not because spheroids are easier to fabricate than paraboloids, but because they appear symetrical to a much larger number (but not all) of incoming light rays. The Maksutov and Houghtons as well as the FKO family of refractor doublets plus the Christen triplet are all well developed attempts to achieve off-axis symetry.
This is a very meaningful question you pose, one that goes right to the heart of optical design over the last 50 years.
-- Steve Strickland Lensnut@tpoint.net