The Cluster Ion Spectrometer (CIS) instrument is a comprehensive ionic plasma spectrometry package onboard the four Cluster spacecraft, capable of obtaining full three-dimensional ion distributions with good time resolution (one spacecraft spin) and with mass-per-charge composition determination. Since the scientific objectives cannot be met with a single detector, the CIS package therefore consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight ion Composition Distribution Function (CODIF), plus a sophisticated dual-processor based instrument control and data processing system (DPS), which permits extensive onboard data-processing. Both analysers use symmetric optics resulting in continuous, uniform, and well-characterised phase space coverage.
The CODIF instrument is a high-sensitivity mass-resolving spectrometer with an instantaneous 360° x 8° field-of-view to measure full three-dimensional distribution functions of the major ion species (in as much as they contribute significantly to the total mass density of the plasma), within one spin period of the spacecraft. Typically these include H+, He+, He++ and O+, with energies from ~0 to 40 keV/e and with medium (22.5°) angular resolution. The CODIF instrument combines ion energy-per-charge selection, by deflection in a rotationally symmetric toroidal electrostatic analyser, with a subsequent time-of-flight analysis after post-acceleration to ~15 keV/e. The energy-per-charge analyser is of a rotationally symmetric toroidal type, which is basically similar to the quadrispheric top-hat analysers and has a uniform response over 360° of polar angle. In the time-of-flight section the velocity of the incoming ions is measured. Microchannel plates (MCPs) are used to detect both the ions and the secondary electrons, which are emitted from the carbon foil during the passage of the ions and give the start signal, for the time-of-flight measurement, and the positional information (22.5° resolution).
In order to cover populations ranging from magnetosheath/magnetopause protons to tail lobe ions (consisting of protons and heavier ions), a dynamic range of more than 105 is required. CODIF therefore consists of two sections, each with 180° field of view, with geometry factors differing by a factor of ~100. This way, one section will always have counting rates which are statistically meaningful and which at the same time can be handled by the time-of-flight electronics. However, intense ion fluxes can in some cases saturate the CODIF instrument (particularly if data are acquired from the high sensitivity side), but these fluxes are measured with HIA.
The sensor primarily covers the energy range between 0.015 and 40 keV/e. With an additional RPA device in the aperture system of the sensor, and with pre-acceleration for the energies below 25 eV/e, the range is extended to energies as low as the spacecraft potential. The RPA operates only in the RPA mode.
The analyser has a characteristic energy response of about 7.3, and an intrinsic energy resolution of ΔE/E ~ 0.14. The deflection voltage is varied in an exponential sweep. The full energy sweep with 31 contiguous energy channels is performed 32 times per spin. Thus a partial two-dimensional cut through the distribution function in polar angle is obtained every 1/32 of the spacecraft spin (125 ms). The full 4π ion distributions are obtained in one spacecraft spin period. Including the effects of grid transparencies and support posts in the collimator, each 22.5° sector has a respective geometry factor of 2.4 × 10^-3 cm^2 sr keV keV^-1 in the high sensitivity side, and 2.6 × 10-5 cm^2 sr keV keV^-1 in the low sensitivity side, depending on the flight model.
The HIA instrument does not offer mass resolution but, also having two different sensitivities, increases the dynamic range, and has an angular resolution capability (5.6° x 5.6°) adequate for ion-beam and solar-wind measurements. HIA combines the selection of incoming ions, according to the ion energy-per-charge ratio by deflection in an electrostatic analyser, with a fast imaging particle detection system. This particle imaging is based on MCP electron multipliers and position-encoding discrete anodes.
Basically the analyser design is a symmetrical quadrispherical electrostatic analyser which has a uniform 360° disc-shaped field-of-view and narrow angular resolution capability. The HIA instrument has two 180° field-of-view sections with two different sensitivities, with a 20-30 ratio (depending on the flight model but precisely known from calibrations), corresponding respectively to the ''high G'' and ''low g'' sections. The ''low g'' section allows detection of the solar wind and the required high angular resolution is achieved through the use of 8 sectors, 5.625° each, the remaining 8 sectors having 11.25° resolution. The 180° ''high G'' section is divided into 16 sectors, 11.25° each. For each sensitivity section a full 4π steradian scan, consisting of 32 energy sweeps, is completed every spin of the spacecraft, i.e., 4 s, giving a full three-dimensional distribution of ions in the energy range ~5 eV/e to 32 keV/e. The geometry factor is ~8.0 x 10^-3 cm^2 sr keV keV^-1 for the ''high G'' half (over 180°), and ~3.5 x 10^-4 cm^2 sr keV keV^-1 for the ''low g'' half, depending on the flight model. 

This description has been obtained from Section 3.5 of the ''Users Guide to the Cluster Science Data System'', DS-MPA-TN-0015.