Particle optical apparatus with a predetermined final vacuum pressure

The invention relates to a particle-optical apparatus with a predetermined final vacuum pressure. To that end a vacuum chamber of said apparatus is via a first restriction connected to a volume where vapor or gas is present at a known pressure and via a second restriction to a vacuum pump. By making the ratio of the two conductances, associated with said restrictions, a calibrated ratio, the final pressure of the vacuum chamber is a predetermined final pressure. This eliminates the need for e.g. vacuum gauges and control systems, resulting in a more compact design of such apparatus.