Spherical aberration in concave mirrors [closed]

I saw this posted on the forum with an answer: Cause of Spherical Aberrations

However, the answer helps explain aberration in lenses. Why is there spherical aberration in concave mirrors?

closed as off-topic by Carl Witthoft, ACuriousMind♦, CuriousOne, John Rennie, AccidentalFourierTransformApr 14 '16 at 6:40

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• Please find some basic books or web pages on lenses. A mirror is exactly the same as a simple plan0-convex lens in first approximation. – Carl Witthoft Apr 13 '16 at 19:37
• I'm voting to close this question as off-topic because it shows no research effort. – ACuriousMind Apr 14 '16 at 0:38

It just geometry. If you want all incident rays parallel to the principal axis to reflect through a single point the mirror needs to parabolic in shape. For a focal length f the equation of the parabola (opening upwards) would be $4fy=x^{2}$.