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Without knowing much about your 8", they should both be parabolic. At f5 the 12" would be horrible if it was spherical, and my old 12" had the same make mirror and it was pretty good.
As I understand it, and the experts on optics can correct me if I am wrong, on slower scopes a spherical mirror is close enough not to cause major issues, but I think you need to get to f8 or slower to get that.
Malcolm
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