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Hi Annette,
The focal length of the primary (lens or mirror) will not change if a flat secondary mirror or diagonal is used. The curved focal plane is produced by the curvature of the primary. The light path will simply be deflected in the direction set by the secondary mirror or diagonal. The focal plane will similarly be shifted. The curvature of the focal plane can be changed by adding another curved reflective surface to the light path.
The light entering the primary (lens or mirror) is effectively 2D to start with. Example: the flow of photons off the side of a sphere (planet or Sun) or from the centre of its face is constant so there is no depth perception which can be measured as a function of time. Even though photons off one side of the Sun may take about 2 seconds longer to reach us than those emitted at the same time from the middle of its face (radius 700,000km).
I think most telescopes produce an image that has positive curvature i.e. the edges of the field are curved towards the primary. An EP with negative curvature for its focal plane (relative to the observer) will have a focal plane preferably bending in the same direction as the telescope primary.
Regards, Rob
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