ABSTRACT
The cathode ray tubes are a class of imaging that have seen wide use in electronic displays, such as the television tube and the oscilloscope. For all the different types of spectrometers, the goal is to achieve high resolution, and in many cases large acceptance at the same time. As resolution improves, the need for better understanding and correction of high order aberrations increases. This chapter discusses the linear theory of various types of spectrometers, followed by the studies of aberrations and their correction. Although the electron mirror itself maintains the rotational symmetry, a magnetic beam separator is needed to guide the electron beam to the detector downstream of the mirror, thus breaking the rotational symmetry of a conventional photoemission electron microscope (PEEM). Consequently, the most challenging part of an aberration-corrected PEEM/low energy electron microscope is the beam separator whose own aberrations have to be small compared to the existing ones.
