Diffraction and Image Formation

In the last chapter, we discussed telescopes as optical devices able to form images in a way that could be explained by geometrical optics, at least to first order in the light intensity distribution. The wave nature of electromagnetic radiation produces diffraction effects that modify this distribution and introduce a fundamental limitation on the angular resolution of telescopes. Since astronomers always want to obtain images containing more and more detail, it is essential to come to grips with these effects, whose amplitude is directly related to the wavelength of the radiation. Using the notion of coherence discussed in Chap. 3 (see Sect. 3.2), we begin by examining the process by which images are formed in the presence of diffraction, and translate the results in terms of spatial frequency filtering.