High resolution scanning aberrations

A corrector based on this principle has been incorporated into a scanning transmission electron microscope by O. Krivanek at the University of Cambridge (Fig. 17). In the second class of corrector, the nonrotationally symmetric elements are sextupoles. A suitable combination of two sextupoles has a spherical aberration similar to that of a round lens but of opposite sign, and the undesirable second-order aberrations cancel out (Fig. 18). The technical difficulties of introducing such a corrector in a high-resolution transmission electron microscope have been overcome by M. Haider (Fig. 19). 

Three factors may limit the resolution of an image the diffraction limit, lens aberrations and noise. Noise can be a problem in scanning microscopy. A very bright source is required if rapid, TV scan rates are to be used at high resolution. In CTEM noise is a problem for radiation sensitive polymers (Section 3.4.4) because a limited number of electrons can be used to form the image. 

The cracial parts of the system are the polychromator and the transfer optics. Polychromators and monochromators are usually optimised for high spectral resolution. This requires keeping the optical aberrations on the path through the polychromator smaller than the slit width. The result is a relatively low f-number, typically 1 3.5 to 1 8. The f-number limits the fraction of the fluorescence light that can be transferred into the entrance slit (see Sect. 7.2.4, page 279). Moreover, the efficiency of any grating is far less than 100%. Therefore some loss of photons on the way from the sample to the detector in unavoidable. A multiwavelength system based on a polychromator is less efficient than a system based on dichroic beamsplitters, but by far more efficient than a system that scans the spectrum by a monochromator. 

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