The effects of fringing fields at the entrance and exit windows of the parallel plate electrostatic particle analyzer on the trajectory of particles is studied taking into account the finite thickness of the condenser plate bearing the windows. The influence of the physical dimensions of the spectrograph on the field variation is assessed together with that of a grounded plate parallel to the condenser and located in the entrance‐exit region. A set of inequalities expressed in terms of the physical dimensions of the instrument is deduced to define a reference model spectrograph of known characteristics and minimal electric field distortion with the help of which any actual instrument may be studied using the electrostatic scaling laws.