Stigmator
A Stigmator is a variable strength Cylinder lens used to correct Astigmatism in Electron Microscopes, proposed by Otto Scherzer and first implemented in the AEG / Zeiss EM8 by O. Rang in 1948. This device greatly increased the resolving power of the Electron Microscope and reduced the necessity for frequent cleaning of the optics. The device corrected the inevitable asymmetric shape of the lens field caused by manufacturing the optics, and the charging based parasitic lenses caused by contamination buildup in the optics and beampipes. There are many ways a Stigmator can be created, these include Magnetic and Electric types, the first practical Stigmator, as designed by Rang was Electrostatic.
At the time of its first implementation, the EM8 was the highest resolution Electron Microscope world wide, the Stigmator came standard in the subsequent EM8 versions sold. Microscopes like the ÜM 100 from Siemens where retrofitted with Magnetic Stigmators of the Helical Moving Iron type, and where standard in later Iteration of the ÜM 100. Ever since, every Electron Microscope came standard with at least 1 Stigmator for the Objective Lens.
The Stigmator field has polar coordinates, and controls for such where historically in such, these being Amplitude and Angle. This was historically done by use of a Sin / Cos Potentiometer for the angle, and a linear potentiometer for the amplitude. Other machines like the Zeiss EM 10 featured 2 linear potentiometers controlling the strength of each Quadrupole separately. Most modern machines feature the XY implementation of the Stigmator control.
Types
Electrostatic Stigmator
Used in AEG / Zeiss EM8.
Moving Iron Stigmator
2 Pol Piece
Example Siemens ÜM 100 (Objective), Siemens Elmiskop I (Condenser)
4 Pol Piece
Example Siemens Elmiskop I
Quadrupole Stigmator
In a Quadrupole Stigmator, a single Quadrupole lens is used as the correcting element, this lens has 2 north and 2 south poles each opposing one another and oriented at 90° to each other, forming a cross shaped structure. This lens Focuses in 1 axis, and defocuses in the 90° opposing axis, thus allowing correction of astigmatism. Since this Aberration is rarely in the exact axis of the Quadrupole, means of rotating this lens are needed. Machines with such a Stigmator have a mechanical Angle control, usually allowing 90° of Rotation, and an electrical control for the field strength. A notable Example of this Stigmator type is the Tesla BS 242.
Dual Quadrupole Stigmator
Example Zeiss EM 10