Focussing the diffraction pattern

When in diffraction mode, the focus knob will almost certainly be controlling the setting of the diffraction lens: the first lens below the objective lens. However, a little thought (or experiment) will show that you can equally well focus the diffraction pattern with C2. The generalised term 'intensity' or 'brightness' knob also still applies for C2, in that because the number of electrons going through the part of the specimen chosen by the selected area aperture changes as a function of C2, the brightness of the diffraction pattern also changes with C2.

But beware. Whether you have a twin objective or not, focussing C2 in diffraction mode changes the range of angles incident over the area of specimen illuminated by the beam. Clearly, if there a range of angles incident on the specimen, then each diffraction spot will spread out into a little disc. Focussing these discs into points using the diffraction lens is possible, but it means we end up with a diffraction pattern which is sampling a range of different angles of illumination on the specimen, and which is not, formally, focussed on the back-focal plane of the objective lens. If you are doing an experiment where the exact diffraction condition matters, then take care.

One way of establishing the correct focus value of the diffraction lens is to form a focussed spot in image mode (as if you were trying to image the filament). Now go over into diffraction mode. Each diffraction spot will be a disc, because we have a wide range of angles going through the specimen. Focus on the edge of the central disc, which is a shadow image cast by the condenser aperture. Because we know we have a cross-over in the specimen plane, the shadow image of the condenser aperture exists purely in angle space (i.e. as a function of the angle of the ray passing through the specimen, and not as a function of its position). By definition, focusing on the angle distribution will identify the back-focal plane of the objective.

After focussing the diffraction lens, now turn up C2 until the diffraction pattern is in perfect focus. You may find that almost all the intensity in the pattern is lost before you reach the focus. Of course, for truly parallel illumination, the beam is infinitely spread out. In practice, it is easier to work with slightly convergent illumination (C2 set rather low) and a wrong setting of the diffraction lens - in which case, don't pretend that your interpretation of the diffraction intensity in any given pattern will necessarily fit into the theory of electron diffraction! (Most material scientists or biologists do not need to worry about these details much.)

Here ends the elementary introduction on how to use a TEM

Copyright J M Rodenburg