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However, doubling focal length with tubes should still give better quality than doubling with a TC....tubes just magnify "flaws" (aberrations?) already in the lens compared to a TC which magnifies the same flaws and also adds some new ones. Hope I at least got that right

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Weeeeeellllllll............some might argue that doubling the distance that the light cone spreads over will magnify any aberrations present in the lens. On the other hand using a TC will do the same AND quite probably add a few of it's own.... so on the whole I'd say you're right...