Gun lens design in a charged particle microscope

A method of operating a charged particle microscope comprising:

• providing a specimen on a specimen holder;
• using a source to produce a beam of charged particles;
• passing said beam through an illuminator, provided between said source and specimen holder, and comprising in a propagation direction of said beam:
• a source lens;
• and
• a condenser system;
• irradiating the specimen with the beam emerging from said illuminator;
• using a detector to detect radiation emanating from the specimen in response to said irradiation;
• wherein said source lens is a compound lens, comprising in said propagation direction:
• a magnetic lens, comprising permanent magnets disposed outside said vacuum enclosure but producing a magnetic field within said enclosure, the magnetic lens matched to the source;
• and
• a variable electrostatic lens matched to the condenser system,
• wherein a combination of the magnetic lens and the variable electrostatic lens adapts the source to the illuminator.