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Originally Posted by IanW
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.
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I am not sure where you have gone with this. If I place an imaging device at the focal plane, why would I not call this prime focus astrophotography/astroimaging, whether or not the light cone from the primary had been diverted by a secondary mirror placed in the light path?
Prime focus = first focus = the focal plane unadulterated by a negative or positive projection lens = placing your camera at the focal plane without a (camera) lens or ocular in the lightpath. A secondary mirror merely diverts the light cone, doesn't alter the point of focus.
simple really
remindes me of the day I spent in the Federal court observing a flock of barristers arguing over the meaning of a single word.