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Merlin66 is right - the ellipses produced by a 45 degree cutting plane through a cylinder and a cone are very similar, but not quite the same.
A bit of simple geometry will show that for the cylinder, the long axis is exactly square root of two (approximately 1.414) times the short axis. E.g. if you slice a cylinder with diameter 100 mm at 45 degrees, the cut face is an ellipse which is 141.4 mm x 100 mm.
When you slice a cone with a 45 degree plane, you create an ellipse which is slightly more elongated (that is, the ratio of the long axis to the short axis is slightly greater than square root of two). The difference is very small for cones with slopes that are practical for Newtonian telescopes (> f/5 say), but real nonetheless.
For those who are playing with 3D CAD to determine the geometry, try modelling a much shorter cone (e.g. f/1 i.e. height to apex = base diameter), and you will see the effect much more clearly. (By my calculation, for an f/1 cone, the ellipse has a ratio of approximately 1.63:1)
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