SUMMARY Focusing and correcting the astigmatism of an electron microscope can be time consuming and difficult to achieve accurately. An automatic method of performing these tasks using an on‐line computer system and a digital framestore has been developed. The method is applicable to noisy images in both scanning and conventional transmission electron microscopes. It is capable of setting focus and correcting astigmatism at least as accurately as can be achieved by an experienced operator.

SUMMARY Digital framestores can be used to reduce noise in TV rate electron microscope images. Three operating modes: averaging, recursive filtering and Kalman filtering have been investigated. It is shown that there is an optimum choice of filter parameters.

K E Y W O R D S . HREM, focusing, stigmating, alignment, on-line. S U M M A R Y* The mean square image contrast is proportional to j IF(k)t(k)P(k) l2 d*k where F(k) is the structure factor of the carbon film-here modelled by a negative exponential curve (Brunger & Menz, 1965); t(k) the weak-phase contrast transfer function (Frank, 1973;McFarlane & Cochran, 1975;Saxton, 1978 p. 15) gives a concise form including tilt and astigmatism); and P(k) the modulation transfer function of the image recording system. In our case, P(k) falls linearly to zero with k, so that low spatial frequencies are over-emphasized; the cut off occurs at a periodicity of about 0.15 nm at our normal operating m a dfication of 600K x (Catto et al., 1982).

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