By inserting the aperture or tilting the beam, different
types of images can be formed.
The most common conditions are:
- No aperture - the diffraction
pattern is centered on the optical axis.
- Aperture is centred on the optical axis.
- Aperture displaced, selecting a diffracted beam.
- Beam is tilted so that the diffracted beam is on the
optical axis.
Use
the controls to try the different combinations, noting the
effect on the appearance of the diffraction pattern.
The
objective aperture is to be used to allow the undeflected
beam to pass but to stop electrons diffracted from the 200
planes of copper. What is the maximum size the aperture
could be? 
