The twin objective lens

We now cover a couple of subjects in more detail, starting with the heart of the electron microscope, the objective lens.

Everything we learnt in the earlier sections about the objective lens is only half the story. As a model in your mind for how to line it up, it is an accurate and useful picture. However, the whole truth is somewhat more complicated.

A modern electron microscope has many different and elaborate 'modes', including STEM mode, which we will cover later. In order to accommodate all these different operations, lens design has become a subtle art. We hinted earlier that some of the objective lens 'leaks over' the specimen, and so changes the focus of the beam before it hits the sample. In fact, this is a slight understatement. In a TEM/STEM machine, the objective lens pre-field can be as strong as the lower half of the objective lens. It depends on what type of microscope you are using. Look at the schematic diagram again. Is half the objective lens above the specimen? Ask the demonstrator if in doubt. A symmetric objective lens, which is now very common, has as much lens above as below the specimen.

A symmetric objective has an important consequence: the first (upper) half of the objective lens acts like a third condenser lens, and so in truth there are two beam cross- overs between C2 and the specimen, as shown in the next diagram. (Note that the exact details of the objective pre-field can be affected a mini-lens mounted above the specimen: this varies between makes of microscope, but the general principles are the same.)

Notice that the objective lens pre-field actually does two things: it forms the second cross-over below C2 and then can be used to make the beam parallel before it hits the specimen.

All this may seem to make things appear much more complicated. However, if you think about it, the behaviour of the cross-over at the specimen (i.e. the first experiment we ever did changing C2), goes ahead as before, except there is now one other cross-over. As C2 is lowered in strength, both cross-overs descend in the column, until the second cross-over is a sharp point on the specimen plane, focussed through the lower part of the objective pre-field, and therefore forming an image of the filament. Ray diagrams for increasing values of C2 are shown schematically below:

Assume that the horizontal dotted line is at the focal length of the lower part of the objective lens pre-field: we can tell this from the right-most diagram, where the illumination in the specimen place is parallel and hence, by definition, the previous cross-over is at the focal length .

In the left-most diagram, C2 is low, and the beams are entering the top of the objective parallel. The beam cross- over between the two parts of the pre-field lens is slightly higher than in the normal setting (right-most diagram), and so the beam on the specimen is slightly convergent. As C2 becomes more excited, the first cross-over passes through the top part of the pre-field: remember that rays through the centre of a lens pass through it in a straight line. At some point, a second cross-over is formed at the specimen plane (which is when we see the filament in focus), which in turn passes up through lower part of the pre-field.

Of course in reality, the objective pre-field is one continuous lens, and so it is quite wrong to think of straight rays passing through two discrete lens, but the net effect is the same. To all intents and purposes, thinking of a single objective mounted below the specimen is conceptually equivalent to the to what happens with a twin objective, except that there is an extra cross-over, and at the final setting the illuminating beam is parallel, not divergent.

As far as alignment of the condenser system and objective is concerned, everything we said in the previous chapters still applies.

Remember: the system is usually designed in such a way that the illumination is parallel at the specimen when the intensity (C2) knob is high.

Copyright J M Rodenburg