Predictions of expected count rates from ion collectors with circular openings having half acceptance angles of 16¿ and 45¿ are compared for the two extreme cases of (1) a Laplace field and (2) one in which the potential drop between the spacecraft surface and the plasma occurs in a sheath of infinitesimal spatial extent. It is found that for the narrower field of view (16¿) and typical values of the spacecraft potential in the plasmasphere, the predictions are nearly identical in both cases and agree with observed ion data. However, the two cases increasingly deviate as the instrument field of view is made wider. This conclusion, derived here for arbitrary ion Mach numbers and finite Mach angles, applies to particle analysis based on both the retarding potential method as well as on the spin modulation method. We also argue that the method of obtaining plasma parameters (N and T) by analysis of spin-modulated fluxes has several advantages over retarding potential analysis. In the case of narrow aperture instruments, a simple formula for the integral ion flux is put forward, suitable for rapid processing of spin modulated data. It reduces in the appropriate limits to well-known analytic formulas. Applications to the European Space Agency (ESA) ESA/GEOS 1 ion composition data are included. ¿ American Geophysical Union 1987