Couple of quotes from
http://www.telescope-optics.net/inde...LE_OF_CONTENTS
"The dominant off-axis aberration in the Newtonian is coma; astigmatism is low in comparison."
"For complete correction of a paraboloid, more complex correctors are required. In general, they need to have three (or more) single lenses, or two or more achromatized doublets. The lenses are more widely separated, creating more degrees of freedom, so that combined aberrations can be brought to a negligible minimum (for instance, the Paracorr-like corrector corrects coma with the front achromat without inducing spherical aberration, but it does come at a cost of inducing enormous astigmatism, then corrected with the rear achromat - something that cannot be done without widening lens separation). Examples of this advanced corrector type are TeleVue's Paracorr and Wynne triplet."
Am no expert, but as I read it, astigmatism like that could come from either the primary (there will be some off axis astigmatism) or from incomplete internal correction of the astigmatism introduced by the front section of the CC. Might be worth playing with the corrector spacing to make sure it is right in the sweet spot for your particular setup. In any event, if the astigmatism right on focus is submerged under the seeing blur, it may not be a major issue.