A column for directing a beam of charged particles with a finite energy spread onto a specimen surface under an oblique beam landing angle comprises: a particle source; an objective lens; a deflection unit for deflecting the beam of charged particles away from the optical axis such that the beam. A reflection coefficient has a representation as a bilinear or sesquilinear form. A first voltage drop may be measured between the first tab group and the second tab group and a measured cell group resistance may be determined based on the measured first voltage drop. The external placement of the bearing assembly also isolates the bearing assembly from the abrasive grit in the tank thereby minimizing the damage to the bearing assembly caused by the grit.