Ask the demonstrator: Show me how to control the condenser
stigmators. Show me how to change the step size (in other
words, the sensitivity) of the condenser stigmators: put the
step size on the coarsest setting.
Once again, actually obtaining control of the stigmators may
involve pressing buttons on the console and the computer
screen, but eventually you should be able to turn two knobs
– quite possibly the ‘multifunction knobs’ – which control
the condenser stigmators. It’s easiest to understand
stigmation by doing an experiment.
Experiment: Go back to our old favourite of imaging the
filament using C2. It doesn’t matter if there is a specimen
loaded, as long as you can find a hole. Turn the spot size
up, i.e. demagnify the filament and focus it into a sharp
point. Turn the magnification up so you can see it clearly.
Now play around with the condenser stigmator and watch what
happens. Change the defocus of the filament – i.e. change
C2 (the brightness knob) – while you play with stigmators.
We are seeing the third important difference between optical
lenses and electron lenses.
Remember: The differences so far: it is easy to change the
strength of an electron lens, which we call defocus.
Electron lenses are also highly aberrated, which in practice
means we should avoid putting large-angle beams through
We now discover that electron lenses are routinely
‘astigmatic.’ In fact, purists would argue that defocus and
astigmatism are themselves aberrations or simply errors in
the lens. That’s true. But its worth thinking of these
things separately in order to understand all the shenanigans
we have to go through in order to get the microscope to work
at its best. Focus is the most important adjustment. High-
angle aberrations make lining up the different lenses
essential. Astigmatism is something that we have to adjust
if we want good images.
Astigmatism occurs when a magnetic lens is not perfectly
round. Every time you switch on or adjust an electron lens,
the magnetisation of the metal in the lens changes. Because
of hysteresis, the lens never quite goes back to where it
was, even though you return the knob that controls it to the
same setting. The lens will have non-round features due to
pockets of different magnetisation around the pole-piece,
which is the focussing part of the electron lens. Also,
apertures tend to charge up if they have dirt on them,
leading to another source of asymmetry.
We can control the astigmatism in the condenser, the
objective, and the diffraction lenses. (We haven’t actually
discussed the diffraction lens yet – it’s the first lens
(i.e. the topmost lens) in the projector system.)
Imagine an oval optical lens. A cross-section through the
longest axis of the oval will look like a weak round lens.
A cross-section through the shortest axis will look like a
stronger round lens. Astigmatism means that the strength of
the lens is different in two different directions. That
means there are now two focus points.
Experiment: Repeat the previous experiment, but check out
the different foci. When the filament appears as a thin
line, it is focussed in the direction at right angles to the
line. As you change C2, you can get to another focus where
the thin line appears at right angles to the first line.
Between the foci, it is possible to get a small circle, but
not a sharp image of the filament. A long way above and
below focus, the thin lines open up into ovals and then,
Can you understand what’s going on? The figure below might help. [Note that
in this diagram, the oval is not a cross-section of the lens as in
previous diagrams, but is meant to represent a perspective view of the top
of the lens - sorry, I will make a better picture one day if I have time...]
The next question is how to correct the astigmatism.
Stigmators work by adding a small quadrupole distortion to
the lens. A quadrupole is an arrangement around the
electron beam of four magnetic poles: two south poles
opposite one another, and two north poles opposite one
another. These have the effect of focussing the electron
beam in one direction, and defocussing it in the direction
at right angles; just exactly what we want in order to
correct astigmatism. In fact, if you think about it, in
order to cope with every possible orientation of
astigmatism, we need two sets of quadrupoles mounted at 45
degrees to one another. When we adjust the two condenser
stigmator knobs, the relative excitation of these two quadrupoles are being changed.
Anyway, luckily you don’t need to understand quadrupoles in
order to correct astigmatism: just turn the knobs until you
minimise the distortion.
Experiment: Practice introducing astigmatism and getting
rid of it by turning the condenser stigmators together with
the C2 focus. A good method is to correct all three knobs
in a series; for example, correct focus, correct first
stigmator, correct focus, correct second stigmator, correct
focus, correct first stigmator, and so on.
A quicker way, which needs a little more thought, is to set the focus
exactly half-way between the two 'maximally streaked' settings. In other
words, focus the lens until you have a sharp streak in one direction. Turn the focus
knob, counting the number of clicks, until there is a sharp streak in the perpendicular
direction (make sure you turn the focus in the correct direction). Now reverse the
direction of focus and move half the number of clicks back again. The spot should appear as a disc.
Now correct both stigmators until the disc is as small as possible.
If you are having difficulties, ask the demonstrator for more help.
Astigmatism occurs in all electron lenses. There are usually three obvious set of stigmators on
a conventional microscope - the condenser stigmators (which are now playing with), the objective lens stigmators
(truly essential for good high resolution images) and the diffraction lens stigmators.
Copyright J M Rodenburg