Dual beam system

A method of operating a dual beam system including an electron microscope using a magnetic objective lens and an ion beam system, the method comprising:

• directing a beam of ions toward a work piece;
• applying a current to a magnetic objective lens of the electron beam column concurrently with directing the beam of ions toward the work piece, the current being the same current used when the electron beam column is directing electrons toward the work piece;
• forming a resultant beam focusing field by providing the magnetic objective lens with primary and secondary coil sections, the primary section having an unequal number of coil turns from the secondary section, and
• varying the strength of the focusing field while maintaining approximately constant thermal signature in the coils of the magnetic objective lens.