Chapter One: Introduction
Peter Hawkes
Quadrupole studies have now reached a point at which we know that corrector units for reducing the spherical aberration of a round lens can be successfully designed and that good quadrupole projector lenses can be built. We know too that quadrupoles can be made achromatic and that their chromatic aberration can be made opposite in sign to that of a round lens. Furthermore, quadrupole multiplets can be built that, especially at high accelerating voltages, rival or surpass the focusing properties of good round objective lenses and have lower spherical and chromatic aberrations. Why then are quadrupole correctors not in common use and why have the potentialities of quadrupoles not been exploited more? There ...
