comparison between image size for the near-perfect image found by diffraction
theory and that from geometric aberration theory as given in Chapter 5.
Before discussing the nature of a perfect image as formed by a telescope with a
circular or annular aperture, we discuss Huygens' principle and its extension by
Fresnel. This principle is the basis for diffraction theory and the Fresnel-
Kirchhoff diffraction integral. We first apply this theory to a rectangular aperture.
The mathematics is a bit simpler in this case and the results, although of interest
in their own right, are especially usefiil in situation ...
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