A New Method For Calibrating Focusing Steps Of Electron Microscopes
Focusing steps of an electron microscope, i.e., the change of focus, Δf, corresponding to one click on any one of the focus dials must be calibrated. Two methods have been used; measuring the distance between bright field and dark field images of small crystalline particles and measuring the diameters of optical diffractograms obtained from the image of an amorphous specimen.A new method utilizing the large depth of field in reflection electron microscopy (REM) provides a direct means of measuring the focusing steps on the micrograph. The in focus area in an REM image can be recognized as the “least grainy” part or the minimum contrast of images of steps. Using surface steps, particles, or other features as References, the shift of this in focus position due to the variation of objective lens current can be readily measured on the micrograph. This distance is then converted to f by taking account of magnification (calibrated) and the foreshortening factor (calculated from the diffraction pattern).