Magnetic spectrometers: approximate and ideal designs.

A produce for determining spectrometer parameters on the basis of specified criteria is illustrated by the design of an energy loss spectrometer for The John Hopkins University high resolution scanning transmission electron microscope. The design tutorial not only includes general expressions for uniform field magnets but also shows how published first and second order transfer coefficients which are available for both uniform field and field gradient magnets may be applied. Within the context of the sharp cutoff fringe field approximation, expressions are derived which enable one to construct ideal pole contours for comparison with prospective second order designs. Moreover, exact expressions for certain first order transfer coefficients permit one to study the aberrations of a proposed design without the need for high precision ray tracing.