The schematic diagram

Take a look at the schematic diagram of the microscope: there is usually one posted on the wall of the laboratory, or you may be able to find one in the instruction manual.

This next figure is a minimal schemetic, you should be able to find a much better picture of your particular microscope.

On the equivilant diagram for your particular make of microscope, find the most important lens, the objective. Work from there, find the specimen, the condenser lenses (somewhere above the objective), the diffraction and projector lenses (somewhere below the objective). Find as many double deflection coils as you can: between the gun and condensers, the condensers and the specimen, and the diffraction lens and, possibly, within the projector lenses. Find all the aperture mechanisms. Can you see the stigmators (often left out of such diagrams). Does it make sense? Ask the teacher if there is something you don’t understand.

How many variables are there in an electron microscope?

• - The setting or current through each lens (about 7 variables)
• - The setting of each double-deflection coil and their rocking points for both shift and tilt: for three sets, that’s 4 settings for each x and y: a total of 24 settings.
• - At least three sets of stigmators: another 6 variables.
• - The physical height of the specimen, the physical aperture settings, and some physical alignments we haven’t worried about: another 10 or so variables.

Luckily, we don’t have to constantly change all the 50 variables to hand: but it’s worth knowing what variables exist. The main thing is to understand each lens can be shifted or tilted, stigmated and focussed and may or may not have an aperture. Most of the time, we try to leave the most important variables constant, although if someone else has used the microscope before you, don’t count on them being aligned.

Copyright J M Rodenburg