Experimental procedures of crystallographic image processing
The practical experimental procedures of crystallographic image processing are described with ample figures from actual HRTEM images. A thin area of an image is Fourier transformed. The amplitudes and phases of all reflections are directly read out from the Fourier transform. The projected symmetry is determined from these extracted amplitudes and phases. Once the symmetry is found, it can be imposed exactly on the experimental data. Thereby several distortions including crystal misalignment are eliminated or at least made less. By inverse Fourier transformation the projected potential map is calculated and interpreted in terms of atomic coordinates. Images taken away from Scherzer resolution and/or with significant astigmatism can be corrected for and used equally well as images taken under perfect conditions. The atomic coordinates are typically correct to within 0.1–0.2 Å.
Keywords: crystallographic image processing, Fourier transformation, crystallographic structure-factor amplitudes and phases, symmetry determination, imposing symmetry, contrast transfer function, defocus, astigmatism, crystal misalignment, reconstructing projected potential, atomic coordinates, Scherzer resolution
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