Using a monochromator to improve the resolution in TEM to below 0.5 إ. Part II: Application to focal series reconstruction

We apply monochromated illumination to improve the information transfer in focal series reconstruction to 0.5 إ at 300 kV. Contrary to single images, which can be taken arbitrarily close to Gaussian focus in a CS-corrected microscope, images in a focal series are taken at a certain defocus. This defocus poses limits on the spatial coherence of the illumination, and through this, limits on the brightness of the monochromated illumination. We derive an estimate for the minimum spatial coherence and the minimal brightness needed for a certain resolution at a certain defocus and apply this estimate to our focal series experiments. We find that the 0.5 إ information transfer would have been difficult and probably impossible to obtain without the exceptionally high brightness of the monochromated illumination.