Thursday, July 5, 2018

Stigmators: Adjusting and Optimizing Astigmatism

The correction of astigmatism is essential to high resolution imaging, particularly at higher magnifications. Astigmatism is an aberration where the electron column has different focal lengths in two orthogonal directions. This means the image will be focused in one direction at one focal length, and focused in an orthogonal direction at a different focal length. In pragmatic terms this can also be understood as the objective lens (OL) being able to focus to a "blob" but not a point at the optimal focus.

Stigmators are octopole lenses (sometimes 12-pole lenses) that compensate for astigmatism at the level of the objective lens.  This series of images shows the effect of stigmator adjustment. The top image shows the best possible focus of a small particle at 40kX. The working distance, reflecting the OL focal length, is 19.4 mm.

The next image the OL focal length is reduced to 19.2 mm and the image shows streaks going from bottom right towards top left. Another way of looking at that is the image is focused along a roughly 45 degree diagonal from bottom left to top right.

The third image shows the OL focal length increased to 19.5 mm and the image shows streaks going from the bottom left to the top right. Another way of looking at that is the image is focused sharply along a roughly 135 degree diagonal from bottom right to top left.

This streaking in one direction followed by streaking in an orthogonal direction at a different focus is a classic sign of a significant amount of astigmatism.

The final image shows the same particle with the stigmators properly adjusted. Without astigmatism correction this would have looked like a particle on a smooth background, not a background of nanoparticles.

The best approach in astigmatism correction is to find the OL focus settings that provide the second and third images, and then focus in between. This should provide the first image. Then adjust the X- and Y-stigmators, followed by fine focus, iteratively, until one arrives at the last image.

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