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Originally Posted by IanW
"Prime Focus" is short for Primary Focus and refers to the focus point of the first mirror or lens in an optical system, thus for example in a Newtonian Primary focus occurs at the location of the diagonal (a.k.a secondary mirror, flat). The focal point where your eyepiece is located is properly the Newtonian Focus.
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This doesnt sound right to me. Light at the the position of the secondary/diagonal isnt at focus. That mirror merely reflects the pre-focused light cone, and doesnt modify the focal length in any way.