Misleadingly Small Full-Width-Half-Maximum, And Other Grazing Incidence Image Features
Paul E. Glenn · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 1989
Grazing incidence systems, such as extreme ultra-violet and X-ray telescopes, produce images which differ qualitatively from those of conventional normal incidence systems. Some of the more well known effects are increased scatter due to the short radiation wavelength, and pronounced diffraction patterns due to the high obscuration ratio of the system. However, a much less well known effect is the rapidly increasing intensity very near the center of the image. Within a substantial part of the image core, the intensity can be shown to increase inversely with the image radius ("1/r" behavior). This gives rise to an inherently minute Full-Width-Half-Maximum (FWHM) for the image. The measured FWHM in fact depends critically on the detector, all of which suggests that the FWHM is not a suitable image descriptor for grazing incidence systems. In this paper, we first review the well known scatter and diffraction effects. We then give some intuitive explanations for the less well known "1/r" behavior of the intensity, derive the mathematics to define the effect quantitatively, and give some examples.