The Spot of Arago and Its Role in Aberration Analysis.
AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH
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The Spot of Arago is the bright diffraction spot in the center of the shadow behind a circular obstacle. Because this pattern is the result of diffraction at the edge of the obstacle, the spot contains information about the incident beam. If the incident beam contains aberrations, the shape and intensity of the diffraction pattern should change. For points on and near the optical axis, analytical solutions to the Rayleigh-Sommerfeld diffraction integral are possible for a circular obscuration and an annular aperture illuminated by a plane wave of uniform or gaussian intensity distribution and containing defocus andor spherical aberration. Computer studies of these cases show excellent agreement with experiment. The amount of defocus and spherical aberration can be deduced by intensity measurements and shift in positions of on axis intensity extrema behind an annular aperture. An empirical study of astigmatism revealed a predictable change in the diffraction pattern allowing for the verification of the aberration to within .05 wavelengths. The Spot of Arago is inherent in many optical systems due to the geometry of components. The technique of aberration determination by changes in the diffraction pattern should be useful in such systems. Author