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Leo
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Milds # Conic constant

In geometry, the conic constant (or Schwarzschild constant, after Karl Schwarzschild) is a quantity describing conic sections, and is represented by the letter K. The constant is given by

$K=-e^{2},$ where e is the eccentricity of the conic section.

The equation for a conic section with apex at the origin and tangent to the y axis is

$y^{2}-2Rx+(K+1)x^{2}=0$ alternately

$x={\dfrac {y^{2}}{R+{\sqrt {R^{2}-(K+1)y^{2}}}}}$ where R is the radius of curvature at x = 0.

This formulation is used in geometric optics to specify oblate elliptical (K > 0), spherical (K = 0), prolate elliptical (0 > K > −1), parabolic (K = −1), and hyperbolic (K < −1) lens and mirror surfaces. When the paraxial approximation is valid, the optical surface can be treated as a spherical surface with the same radius.

Some[which?] non-optical design references use the letter p as the conic constant. In these cases, p = K + 1.

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