A Brief Discussion on the Main Defects Limiting Microscope Performance
Due to defects in the lens, all ideal optical path diagrams must be corrected. These defects limit the resolution of the microscope, but paradoxically, they help to obtain better depth of focus and depth of field from the microscope.
Under paraxial conditions, that is, when electrons with a small angle between their trajectory and the optical axis participate in imaging, all points on the same object plane will be imaged on a Gaussian image plane with single values and no deformation. However, in reality, the electrons participating in imaging do not fully meet the theoretically set paraxial conditions, and the resulting object image may be blurry or distorted.
At present, there are many defects in lenses, and all defects can be observed in images or diffraction patterns. However, in practice, most people do not need to have a comprehensive understanding of them. Here, we will introduce the main limitations of microscope performance, including spherical aberration, chromatic aberration, and astigmatism.
1. Ball difference
Since the term aberration was first mentioned in the main optical components of the Hubble Telescope, its use has become more frequent. This defect is caused by the uneven effect of the lens field on off-axis rays. For electromagnetic lenses, the farther away the electrons are from the axis, the more they are deflected, resulting in the formation of a disk of a certain size after the point like object refracts. This limits the ability to magnify the details of the object, as the details are reduced during the imaging process.
The effect of spherical aberration is shown in the following figure, where a point like object P forms an image on the Gaussian plane P. This doesn't look like a dot two, it's a high-intensity central bright area with a surrounding halo.
Spherical aberration is the most important factor in the objective lens because it reduces the quality of TEM images, and all other lenses magnify the errors it produces. Ball aberration is equally harmful in the focusing mirrors of AEM and STEM, both of which require large excitation currents to form the smallest electron beam spot. The functionality that all forms of TEM can achieve at the resolution limit is almost limited by spherical aberration, which is why people are so excited about being able to correct spherical aberration.
