Design considerations and performance of an analytical stem

A VG Microscope HB5 has been modified in collaboration with the manufacturers. The major modifications are the addition of three post specimen lenses, a second condenser lens and a z-lift on the specimen stage. The requirements of the various parts of the microscope are reviewed and the compromises made necessary their interaction are discussed. The performance of the double condenser system, the z-lift stage and the high excitation objective lens are discussed briefly. A more detailed discussion of the expected performance of the post-specimen optics is given and preliminary experimental results are presented. The discussion is in two parts. The first part considers the recording of diffraction patterns and the use of detectors in the diffraction plane. The major effect is the distortion introduced by the lens system. By a suitable choice of operating conditions, this can be minimised. The second part considers the use of an electron spectrometer. Large chromatic effects can occur if a wide range of electron energies is involved. Provided that the correct lens configuration is used, these may prove to be an advantage since it is possible to produce an increase in collection efficiency with increasing energy loss. Finally, the effect of using an electron spectrometer with lower aberrations in conjunction with the post-specimen lenses is considered briefly.