Download Advances in Electronics and Electron Physics, Vol. 76 by Peter W. Hawkes (Ed.) PDF

By Peter W. Hawkes (Ed.)

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In case of a small angular divergence of the beam and of a comparatively large object size, the main role is played by distortion, which distorts the image shape but does not cause blurring. Aberrations make different contributions to image distortion; this depends on the construction of the electron optical device and on the function of each of its lenses. For example, an electron microscope objective creates the image of an object of small size but with large angular divergence of the emerging particles, so here the spherical aberration predominates.

6): lo = z ( H 0 ) - 20, li zi - z(Hi). From the triangle similarity and the fact that the magnification M = - h i / h o , we have THE OPTICS OF ROUND AND MULTIPOLE ELECTROSTATIC LENSES 29 Note that linear magnification for the inverted image is negative. To get larger magnifications one should use a short focal-length lens, placing the object near its focal point. Eq. (51) gives a simple relationship between the positions of the object and image (10 - fo)(li - 11)= fob. (52) This expression can be changed into the useful form fo 1 1 -+-=l.

For this reason, the image defects they describe are called third-order uberrations. In the general case, aberrations of an electron optical system with two planes of symmetry depend on 20 coefficients,some of which may be zero in particular systems. When a coefficient is not zero, there is a certain kind of distortion, so each coefficient is associated with its own aberration. ' Different ways of writing down the aberration broadening in terms of aberration coellicients have been suggested. A common factor is introduced in some of them-linear magnification.

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