Instrumentation: Progress and Problems

Abstract

The electron microscopes currently available are designed to transfer information down to resolutions of 2–3 Å or in terms of spatial frequencies, 0.3–0.5 Å−1. Our present problems relate to efforts directed toward instrumentation that will provide a better specimen environment, on the one hand, and that will extend the contrast transfer function (CTF) of the instrument to 1 Å−1 or better, on the other. The instrumental problems and their importance related to specimen environment in high resolution electron microscopy will be discussed in the later part of this contribution. Basic optical parameters such as electron wavelength and angular aperture of the objectives lens can be changed to enhance the CTF and the necessary means are well known, but their execution is far from routine. The electron wavelength can be decreased to give greater resolving power by going to higher accelerating voltages. Spherical and chromatic aberration coefficients can be minimized by using a system of multipoles and when corrected, larger angular apertures can be used to obtain better resolution. These are basic instrumental approaches to the problem and are bemg tried by different groups [1, 2, 3]. Special imaging modes such as tilt or rotatory cone illumination, and phase plates have also been tried [4, 5, 6] and involve special problems of instrumentation. In principle, computer or analogue processing of the electron microscope image also can be used to enhance the information obtained, but this whole area is a subject that is being considered in another contribution in this volume.