Electron microscopy guide


TEM alignment

STEM alignment

Wave interference


Diffractive imaginging


The Computer

All modern electron microscopes are controlled by a computer, which is meant to make life easier for us. None of the knobs on the console are actually connected to variable resistors: they just give out pulses on a data bus which are interpreted by the computer. The computer is interfaced into D to A boards which control the currents actually going through the different lenses and deflection coils. New machines are mouse driven.

The computer saves us an enormous amount of time. For example, changing magnification would be a real drag if we had to alter each projector lens individually in order to select a particular magnification and keep the image in focus. All this happens automatically as we turn the magnification knob.

However, the computer can also be a source of huge frustration to experienced microscopists, because it assumes the lowest denomination in our expertise, and so it occasionally attempts to stop us adjusting parameters which it thinks we don’t understand.

By now, you will have seen the demonstrator select various screens and adjustments on the console. It is now worth learning about the computer in more detail.

Ask the demonstrator: Show me how the computer screens of information are arranged. How do I select different modes? How do I select the alignment parameters (double-deflection coils), stigmators, etc. Show me how to check the settings of each lens (free lens control), shifts and tilts, etc.

Experiment: Jump between different modes – say from imaging mode to diffraction mode – and monitor the current settings of the lenses. Do they change as you expect them to? Try changing spot size, and see what happens to C1 and C2: is that what you expect?

Note that in each mode, the microscope computer recalls the last settings for all the lenses and deflection coils that were last used in that mode. This can be very confusing, especially since sometimes some modes are treated the same as others (e.g. on the Philips CM20, ‘nanoprobe’ has the some of the same settings as ‘scanning’). Settings can also mysteriously change between one session on the microscope and another (if somebody else is using the microscope in that particular mode). Even exactly how many settings the microscope can remember is not often made clear by the manufacturer. You just have to work it out for yourself.

Ask the demonstrator: Show me how the computer controls the vacuum system.

The demonstrator will now read another ‘riot act’. Just about the most dangerous thing you can do (as far as annoying the demonstrator and your other colleagues are concerned) is to press buttons when the computer is in vacuum mode: say to accidentally let the column up to air pressure. However, vacuum mode is useful to monitor the pressure inside the column – say when you are loading a specimen, so you do need to know about it.

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Copyright J M Rodenburg