Dynamic focussing for recording images from tilted samples in small‐spot scanning with a transmission electron microscope

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Abstract
For three‐dimensional electron microscopical structure research the specimen must be imaged in a tilted position. Specimen tilt is also often needed to achieve an optimal molecular packing orientation. The tilt with respect to the optical axis causes a defocus gradient alongside the imaged area and thus entails the following complications: (1) The phase‐contrast transfer function fades for strong defocus; (2) the Fourier coefficients are split; and (3) the signal‐to‐noise ratio cannot be enhanced by simple averaging. An image procedure with small‐spot scanning and simultaneous defocus compensation is proposed which helps to reduce these problems.