An
electrostatic analyzer or ESA is an instrument used in
ion optics that employs an
electric field to allow the passage of only those
ions or
electrons that have a given specific
energy. It usually also focuses these particles (concentrates them) into a smaller area. ESA’s are typically used as components of space instrumentation, to limit the scanning (sensing) energy range and, thereby also, the range of particles targeted for detection and scientific measurement. The closest analogue in
photon optics is a
filter.