A LEO SUPRA 35 VP FESEM has been modified to significantly improve image quality in the low pressure mode of operation. It is further shown that the same conversion can allow the machine to operate also as a fully fledged environmental scanning electron microscope with specimen chamber pressures in excess of 2000 Pa. This is achieved by a diamagnetic insert with a thin pressure limiting aperture at the bottom of the pole-piece. The insert allows the use of various size apertures in addition to the existing 1 mm differential diaphragm.