The Wobbler

What am I aligning when I turn on the wobbler?

The wobbler, sometimes called "lens align" or "aperture align," shifts the focal plane slightly above and below the sample surface, assuming it was focused before you engaged the wobbler.

In a properly aligned SEM, the probe emerges normal to the pole piece. 

If the aperture is not aligned, the beam will be tilted. This causes the image to shift because the center of the probe travels along the sample surface.

Aligning the aperture helps ensure consistent beam brightness and focus throughout the imaged area.