More about gun tilt
Form a sharp image of the source (by focussing C2), with
the filament slightly under-saturated; that means, the filament heating current
is a few clicks lower that usual. Correct the condenser
stigmators so you can see the image clearly. As a function
of filament current, you will see structure in the filament.
Ask the demonstrator: Is the filament properly aligned? If not,
please adjust the gun tilt and let me see what happens.
Different types of filament appear differently. Ask to be
shown what the under-saturated image should look like and
how to use the image to the adjust gun tilt. Ask to be
shown how to judge that the filament is 'saturated'. If a
filament is run above saturation, virtually no extra current
comes out of it, but its lifetime is severely reduced.
Do not adjust the gun tilt unless you have been explicitly
told that you are allowed to do so. Never adjust the
filament limit setting, that is, the saturation limit,
unless you have the research funds available to replace the
filament, and the lost operating time of the microscope.
Copyright J M Rodenburg