If we look at the cross section of the approximated parabolic mirror with the burden removed, and without showing the colored spherical strike zones, we can view the mirror as a parabolic mirror with a ridge formed by the seam of the two spherical sections.  The smaller the telescope's Focal Ratio, the more pronounced the ridge. This is due to the steeper curves required to make shorter focal length mirrors.  The optician who polishes this ridge away while figuring his mirror, will produce an approximated parabolic mirror with an infinite number of spherical sections,- a true parabolic mirror.
With this mirror at the bottom of a closed telescope tube, off-axis objects are not presented to the mirror, and if the telescope is geometrically collminated, there will be no source of coma in the entire telescope system.  Thus a pure Newtonian Telescope, of any Focal Ratio, can be produced completely free of coma error without the need of a coma correcting eyepiece.
Many telescope makers rely on the myth that coma error is inherant in the design of all Newtonian telescopes so they can sell you a telescope with a cost effective, incompletely figured mirror. A fine optical instrument worthy of their hefty price. Of course you will need their higher priced coma correcting eyepieces to go with it.