Diffraction in astronomical telescopes is caused by the restriction of light to an entrance aperture. Focusing is a convenience imposed on the optical system. This is implied in Fig. 4-6, where the aperture and the focusing power have been divided into two independent functions. One function is the windowing of light by an iris or a rectangular cover—the diffracting aperture. Diffraction occurs whether the light is focused or not; it happens even in pinhole cameras.
All optical work is done on the nondiffractive focusing parts of the optical system. Thus, the optician's task is not to make an existing problem worse by adding additional aberrations to a lens or mirror. An optician cannot correct the window through which the telescope is looking.
The following discussion assumes that the focusing is already done when the wavefront encounters the aperture. This convention does not change the general principles involved.
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