Aspheres have a special place in optics and a long history as imaging mirrors, as was noted in Chapter 6. As the name implies, any surface that is not a sphere is an asphere.
Of particular interest are conic sections for imaging mirrors because they form stigmatic images, which means that all rays from an axial object point pass through the same image point. Therefore, they do not create spherical aberration. It all started with the use of parabolic reflectors for astronomical telescopes. The ellipsoid and hyperboloid found their place as suitable shapes for the secondary mirror of the Gregorian and Cassegrain telescopes, respectively, as noted in Chapter 6.
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