Active compensation of ambient AC magnetic fields for high-resolution electron microscopy

Electron microscopes are often badly sited in rooms with ambient magnetic fields high enough to blur high-resolution images, particularly at low magnification. The method of compensation presented here deals with magnetic fields at power supply frequencies, which are often difficult to eliminate. Sensors located near the microscope provide signals which are amplified and fed to room-sized compensating coils. These are connected to give feedback which nulls the field at the sensors themselves. Typically, the ambient field at the microscope is reduced by a factor of around 20. The system has been in operation with several EMs for many years.