A CHA (also know as an HDA) consists of two metal hemispheres. One hemisphere being shaped concave, and the other convex. They are arranged such that their centres of curvature are coincident (see the schematic diagram, 8k). Different voltages are placed on each hemisphere such that there is an electric field between the two hemispheres. Electrons are injected into the gap between the hemispheres. If the electrons are travelling very fast, they will impinge on the outer hemisphere. If they are travelling very slow, they will be attracted to the inner hemisphere. Hence only electrons in a narrow energy region (called the pass energy) succeed in getting all the way round the hemispheres to the detector. A series of lenses are placed before the CHA. The lenses enable two operating modes - Constant Retard Ration (CRR), or Constant Analysis Energy. With CRR mode, the electrons are slowed down by an amount which a constant ratio of the electron energy to be analysed (see also RFA - Retarding Field Analyser). That is if the retard ratio is 10, and 1000eV electrons are to be detected, then the electrons will be slowed down to 100eV, and the pass energy will be set to 100 eV. In the CAE mode, the pass energy is fixed. Hence if the pass energy is 50 eV, then electrons of 1000eV will have to be slowed down by 950 eV in order to be detected. The CRR mode gives constant resolving power and the CAE mode gives constant energy resolution.