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 backfocal 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 crossover 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 backfocal 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
