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Elliptical v'd cylinderical diagonals
The intersection of a line at 45 deg to a cone gives an ellipse ( by definition), however in the real world to cut an accurate ellipse from a lump or sheet of glass is very difficult, so they normally use a biscuit cutter ( ie a rotating cylinder) to cut the secondary mirror at 45 degrees, this DOESN'T give an ellipse.
To make this cylinderical section better fit the cone of light in a fast reflector ( say < f5) it must be offset away from the eyepiece to maximise the amount of beam picked up without causing excessive shadowing of the primary.
You can see the above for yourself if you do a scale drawing of a cone with a line at 45 degress; one side of the line relative to the centre line is longer than the other, whereas if you do it for a cylinder both sides are the same length!!.
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