HT Wobbler in TEM mode is used to adjust the microscope optical axis to minimize the motion of the image, i.e. the image stays at the center of the fluorescent viewing screen even if the accelerating voltage is changed during TEM-imaging. This condition is converntionally called "Voltage Axis". This adjustment is normally done after the basic TEM alignments have been performed well. The typical steps for such Voltage Axis alignment is:
i) Click the HT Wobbler (or called WOBBLER-HT) switch. While this switch is turned on, the high voltage varies periodically, facilitating alignment of the voltage axis.
ii) Press the Bright Tilt (or called BRIT Tilt or Beam Tilt) switch, and then adjust the relavent knob(s), e.g. beam deflectors (DEF x and y) on some microscopes, to tilt the beam until the image expands and contracts around the center of the fluorescent screen.
By wobbling the condenser lens excitation and then the microscope high tension, the alignment of the illumination electron beam in STEM mode can be very accurately checked. If there is a misalignment of the beam between condenser and objective lenses, there will be a periodic translation of Ronchigram features when the wobbling takes place. This misalignment can be corrected by using the condenser alignment coils (CTEM bright tilt in JEOL systems) so that the features only oscillate in and out symmetrically about the coma-free axis.