Abstract: An electron beam instrument having an objective lens coil 14 and stigmator coils 18 has its astigmatism corrected for by measurement of the contrast in the final image. The currents through the objective lens and the stigmator coils are adjusted in sequence iteratively to optimize the contrast.The optimum contrast may be either a maximum contrast or a minimum contrast depending on the nature of the instrument and its mode of operation. Measurement of contrast may be made by measuring the magnitudes of successive points of an image and calculating the variance. Alternatively two measurements of magnitude at each point of an image may be made and the covariance calculated.

Abstract: In a scanning transmission electron microscope 1 the image of the diffraction pattern formed on a phosphor screen 6 as a result of electron beam impingement on a point on the specimen 4 is converted by a camera 7 into a video signal and digitized in an ADC convertor 9 and stored in a digital store 10. The stored signal is then modified by a weighting factor representing a notional pattern overlaying screen 6. The weighting factor may have one of two binary values, representing a notional opaque and transparent pattern, or may have a multiplicity of different values. The modified signals are then added together to provide a picture value for the point of impingement. By this means a complete picture is built up point-by-point.To speed up the picture taking operation diffraction images from different points may be displayed on different parts of screen 6 so that several images can be scanned together by camera 7.