ABSTRACT
A quadrupole-octupole spherical aberration corrector has been designed and built for a modified VG HB5 STEM. The corrector fits between the second condenser and the objective lens, and consists of 6 separate stages. Each stage contains one strong quadrupole, one strong octupole and 12 auxiliary trim coils. The corrector is provided with autotuning software that measures all the relevant aberrations on-line and tunes the corrector automatically. Preliminary tests of the corrector have verified the correction principle, and shown that it can readily compensate for parasitic aberrations arising from misalignment.
