The influence of the objective lens current in low magnification electron holography
In order to align an electron microscope for low magnification holography we usually completely switch off the objective lens and image the sample by the first intermediate lens. In addition, to achieve a highly coherent electron beam we highly excite the condensor lens resulting in a divergent illumination of the sample and the intermediate lens. Now negatively biasing the fiber of a Möllenstedt type biprism placed between the first an second intermediate lenses of our Hitachi HF-2000 field emission electron microscope creates two virtual sources below the back focal plane of the first intermediate lens. These two sources are necessary to form off-axis holograms. Slightly exciting the objective lens and still imaging the sample by the first intermediate lens results in two major changes in our holograms.First: Due to an electron beam less divergent or even convergent illuminating the first intermediate lens when exciting the objective lens (compare Fig. 1 to Fig.2) the angle β at which object wave and reference wave are superimposed decreases.