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Originally Posted by Argonavis
That was a bit of a rant...well
Prime Focus is generally used in the amateur literature to denote the placement of the camera body at the focuser of a Newtonian. There are no lenses between the mirrors (primary and secondary) before the film plane in this arrangement. Professionals generally have bigger scopes than amateurs so they can mount the camera (in the days of film) inside the scope where the secondary mirror would normally go. Amateurs are generally reduced to attaching the SLR to the focuser. To me both are Prime (ie first) Focus. As I recall Steve Lee had a similar rant, and I think you are both wrong. DSLR's are generally used in the same position but with thier camera lens, in what used to be called the afocal method. Essentially, words mean what they are popularily accepted to mean.
end rant
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Ahh the sainted Steve Lee, there's a name that brings back memories. Anyway you are 100% incorrect and here's why.
To keep it simple I'll just use a mirror as an example as this applies to any curved mirror surface, not just the parabaloid of a Newtonian.
The focus point is fixed in relation to the surface of the mirror. it is a physcial location about which the surface of the mirror can be plotted both mathematically and physically. Diverting the light (image) cone via a mirror does not alter the focus point of the mirror, it merely diverts the light (image) cone. It's a simple concept yet one that many amateurs these days never come to grips with as so few of us actually make mirrors or lenses these days. It's elementary optical theory as well.