Pivot points
There are two sets of difflection coils one above C1 (Gun shift -tilt )and another below C2 and
above specimen gap(Bean shift -tilt).When the coils are used to either shift or tilt the beam, the
two pairs of coils work in opposite senses, but the ratio of the individual coil excitations is
different for these operations.
Beam tilt should just rotate the beam through a point in the specimen, like a lever pivoting about
a point. This process is sometimes called rocking the beam about a ‘pivot point’ or ‘rocking
point’. Since it depends on specimen height, then whenever you change the specimen height, you
must re-adjust the pivot points.
If the pivot points are wrong, you see two beams separated laterally: just two blobs of intensity
on the phosphor screen. It is then a simple matter to adjust the two correction knobs (which may
well be the multi-function knobs again) until the two beams are coincident. The correction knobs
adjust the ratio of excitation of the two sets of deflection coils. There are two ratios, because
sometimes the x- and y- coils have cross-talk between them as a result of residual misalignment
and the rotation effects of the objective pre-field.