Celestron C8

The venerable C8 isn't a field-corrected telescope per se so if you mean field correction rather than field curvature then for an all-spherical sample you get the same coma of an equivalent newton @ f/5 approximately.  Some of the Meade variants had, reportedly, a slight aspherization of the secondary  yielding about 15 mm diameter of usable field and the lore goes that the same applied to some batches of the older C8s. Field curvature should hang at around -300mm, depending on the effective BFL. I am not aware of any field flattener for the old C8 as it seems antithetic its design principles.