What is astigmatism?
Astigmatism can be one of the hardest adjustments to correct accurately in images, and requires practice.
The image in the centre shows a correctly focused image that has also been corrected for astigmatism. At left and right are examples of poorly corrected astigmatism, seen as streaking of the image.
To allow accurate imaging, the electron beam (probe) should be circular in cross section when it reaches the specimen. The probe cross section can be distorted to form an ellipse, due to a range of factors such as level of machining accuracy and the material of the pole-piece, imperfections in the casting of the iron magnets and the copper winding. This distortion is called astigmatism and causes focus difficulties. Bad or "gross" astigmatism can be seen as "streaking" in the image in an X direction that changes to the Y direction as the image passes through focus from under focus to over focus. At exact focus the streaking vanishes and focus can be correctly achieved if the spot size is suitable.
To make the probe circular, a stigmator is used. This comprises electromagnetic coils placed around in the microscope column in quadruple, sextuple or octagonal orientations. These allow adjustment of the shape of the beam and can be applied to correct for any major lens distortions. An image is generally regarded as free of astigmatism when it does not streak in one direction or the other when the objective lens is adjusted to under or over focus at around 10,000X magnification. Astigmatism is usually negligible in an image at less than 1000X.
The best procedure to correct for astigmatism is to set X and Y stigmators to zero offset (i.e. no astigmatism correction) then fine focus the sample as best as possible. Then adjust either the X or Y stigmator controls (not both) for the best image and refocus the image. When the best image has been obtained with one stigmator, use the other stigmator to get the sharpest possible image. Refocus the image: if the astigmatism has been corrected there will be no streaking of the image as it is focused.