The ion beam spectrometer (IBS) is one of the three spectrometers in the Cassini plasma spectrometer instrument aboard the Cassini/Huygens spacecraft. The IBS is a very high energy resolution hemispherical electrostatic analyzer. The design values of the IBS are analyzer gap 2.5 mm, and middle radius 100 mm. Because of the high energy resolution required, special care had to be used in the design and manufacturing of the instrument. A simulation was developed in order to aid the designing process. Here we show that the best fit to the laboratory calibration of the IBS flight model is obtained using the simulation model, where the inner hemisphere is misaligned by about 20 μm, where a maximum asymmetry of 75 μm is included in the inner hemisphere, and where the analyzer gap is increased from the design value by about 0.1 mm. We show here that geometric factors postulated in the theoretical model resulted in a better agreement between calibrations and simulations than the geometric factors calculated from calibration data alone. We find a value of (2.5±0.1)×10-4 cm2 sr for the geometric factor of the flight model. Finally, we also show that the energy and angle responses of the IBS are somewhat lower than the values calculated directly from the calibration data: (2.0±0.1)% is the value of the full width at half maximum (FWHM) of the energy distribution, and (3.1±0.1)°is the value of the FWHM of the azimuth angle distribution.
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