This is necessary because low energy electrons cannot be easily distinguished from low energy X-rays by our CCD detector, and they can provide a large degree of contamination if not deflected by magnetic fields. The six black bars represent the eight identical magnets.
Field strength is indicated by the contour levels, and field lines are also shown.
The two rectangles in the assembly interior show the positions of the CCDs. Rather than attempt to capture electrons in a very strong magnetic field, we take advantage of the geometry of our instrument aperture design to deflect electrons with energy less than 20 keV away from the detectors, assuming that once deflected, they are unlikely to scatter back into the detectors. This requires a fairly uniform field of at least 250 Gauss across the rear opening of the aperture housing. This condition is met by the two central magnet pairs, which provide a field of 800-1000 Gauss over most of the aperture, dropping to about 500 Gauss at the aperture edges. The inner magnets are flanked by two pairs of opposite polarity arranged so that the overall dipole moment of the complete assembly is near zero.