The difficulty of obtaining accurate calibrations of electrostatic analyzers employing large aperture channel electron multipliers is discussed. This is due primarily to the varying efficiency of large aperture multipliers depending on point and angle of impact on the aperture. This varying response makes the assignment of an over‐all detector efficiency uncertain by at least a factor of 2. A laboratory calibration scheme is discussed by which the combined efficiency and geometry factor is obtained for flight detectors using a simulated omnidirectional source of particles.