Question

For recording electron diffraction pattern from a crystalline sample, why does the primary electron beam have to be almost a parallel beam? What happens if it is focused on the object plane?

Answer 1

The parallel electron beam gives the high angular resolution for resolving the ditails of diffuse scattering that comes from finite size of the crystal & deviation from the ideal crystal structure .that the reason why primary electron beam have to be almost parallel beam . scattered beam does not give resolution & recording pattern is not up to the mark.

If we focused primary electron beam on the object plane it gives pattern consist of disks.Each disk can be divided into many pixels.& each pixel approximately represents one incident beam direction.