Stigmator

For early electron microscopes - between the 1940s and 1960s[1] - astigmatism was one of the main performance limiting factors.

[7] By adjusting the magnitude and azimuth of the stigmator field, asymmetric astigmatization can be compensated for.

It can consist of metal rods which induce a magnetic field, which are inserted with their long axis towards the beam center.

[9][12] In most cases, the astigmatism can be corrected using a constant stigmator field which is adjusted by the microscope operator.

The main cause of astigmatism, the non-uniform magnetic field produced by the lenses, usually does not change noticeable during a TEM session.

Quadrupole field created by four wires. The principle of a stigmator is that the current through each of the wires would be adjusted to change the shape of the beam.