Condenser and objective apertures

In electron microscopy the term aperture is commonly used to describe a diaphragm. The condenser aperture associated with each condenser lens. Condenser aperture 1 is usually a fixed aperture located beneath condenser1 that stops electrons traveling at high angles from the gun crossover point. Condenser aperture 2(CA2) is a changeable aperture located beneath condenser lens 2. A small CA2 increases beam coherence. The second condenser lens is equipped with a physical aperture which limits the beam striking the specimen. (Heath J.P., 2005) This is necessary both to protect the specimen from excessive heating and to limit the generation of X-ray from the microscope which might otherwise reach harmful levels.

The size of CA2 determines the maximum semi-angular aperture of illumination, α_c, as viewed from the specimen (Figure 1). This maximum angular aperture is achieved near the point of condenser focus, i.e. when the beam crossover is imaged on the specimen. The larger the aperture angle, the greater the maximum illumination intensity, however the poorer the image quality. If the second condenser lens is defocused, the illumination semi-angular α is defined not by the condenser aperture but by the size of the crossover image and its distance from the specimen. (Alan W. Agar, 1974, p.22-23).

Figure 1: Condenser aperture

The objective lens is equipped with two important controls: the objective aperture and the astigmatism corrector. Figure 2 show how electron are scattered through different angels by the specimen, and how the objective aperture, placed at the back focal plane of the lens, stops a number of these scattered electrons. By exchanging the aperture for one of a different size, the effective aperture of the objective lens can be varied, thus varying the proportion of electrons from any given object point stopped by the aperture. The semi-angular aperture also affects the resolution. Typically, an objective aperture is 50mm in diameter. For a focal length of 2.5 mm, the defined semi-angular aperture is 10^-2 radian. (Alan W. Agar, 1974, p.26-27)

Figure 2: Objective aperture