Compensation of Lens Aberrations by Single-Sideband Holography

Abstract

Single-sideband holography has been suggested as a method for extending the resolution of the electron microscope, since it produces contrast for all object spacings simultaneously and avoids the oscillations of the contrast transfer function characteristic of normal imaging. Image processing is still required to compensate for the effects of spherical aberration and defocus, which now show up in lateral shifts of image components. The method involves use of a semicircular objective aperture to stop out half of the scattered electrons. One finds, however, that a charge builds up on or around the aperture soon after it is positioned, introducing still another image defect.In order to compensate for the image defects, one needs to determine the wave aberration corresponding to the point spread function or contrast transfer function. In the case of normal (double-sideband) imaging one can determine the transfer function from measurements of the oscillations of the fourier transform of the micrograph (Fig. 1e).