A paraboloid inner surface and a co-paraboloid outer surface can be substantially free from aberrations in forming a parallel beam. A simple approximation for the true co-paraboloid is given, and extended to co-ellipsoids.
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References
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F.A. Benford , Gen. Elec. Rev., 27, 625 (1924).
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2 L.C. Martin, Applied Optics, vol. 1, ch. 2, p. 67, Pitman, London (1948).
