thickness can also affect the HOLZ line thickness (Jones

et al., 1977).

 Third, tilting the crystal away from normal

perpendicular incidence means a different thickness may be

encountered across the diameter of the beam with a

consequent change in intensity across the pattern.

(Recall that the beam is convergent.) This is not usually

a problem since only the intensity, not the actual line

position, is affected.

 Fourth, the distortion in the pattern introduced by

the objective lens can affect the accuracy of direct

measurements in the Higher Order zone (Ecob et al.,

1981). Since one almost always uses zone axis patterns to

generate HOLZ information, the final tilt necessary to

obtain an exact zone axis pattern can be accomplished by

tilting the beam rather than tilting the sample. This

tilting can be done in two ways. The beam can be tilted

electronically using the deflector system of the

instrument, or the beam can be tilted by displacing the

second condenser aperture. Either of these procedures is

easier than tilting the sample using the goniometer

controls of the microscope stage. However, a beam

entering the objective lens off axis is subject to the

inherent spherical abberation effects of the lens. The

abberation increases with increasing off axis angle,

degrading the accuracy of the pattern. For this reason it

is best to tilt the sample as close to the exact zone