Regulating device for the correction of image defects of an electron-optical system

Claim 1

A REGULATING DEVICE FOR CORRECTING AN ASTIGMATIC ELECTRON OPTICAL SYSTEM IN AN ELECTRON BEAM FOCUSSING APPARATUS IN WHICH CHARGE CARRIERS ARE ACCELERATED WITH THE AID OF AN ELECTROSTATIC STIGMATOR HAVING A PLURALITY OF ELECTRODES, THE VALUE AND THE DIRECTION OF THE ADDITIONAL ASTIGMATISM REQUIRED FOR THE CORRECTION OF THE IMAGE DEFECT OF THE ELECTRON-OPTICAL SYSTEM BEING SEPARATELY ADJUSTED BY VARIABLE RESISTORS, SAID ADVICE COMPRISING A CONTROL RESISTOR CONNECTED TO A SOURCE OF ADJUSTABLE VOLTAGE THROUGH TERMINALS AND FURTHER COMPRISING A PLURALITY OF EQUAL RESISTANCE ELEMENTS EACH CONSISTING OF A LAYER OF RESISTANCE MATERIAL ON AN INSULATED SUPPORT, THE CROSSSECTIONAL AREA OF EACH LAYER GRADUALLY DECREASING TO A MINIMUM BETWEEN THE VOLTAGE TERMINALS SO THAT THE RESISTANCE MEASURED BETWEEN ONE OF THE TERMINALS AND A POINT MOVING FROM ONE TERMINAL TO THE OTHER VARIES ACCORDING TO A NON-LINEAR FUNCTION FROM THE BEGINNING, SLIDING CONTACTS SIMULTANEOUSLY MOVABLE OVER A CONTACT AREA ON THE RESISTANCE ELEMENTS BETWEEN ADJACENT TERMINALS, SAID CONTACTS BEING SPACED FROM ONE ANOTHER BY ONE HALF OF THE LENGTH OF A RESISTANCE ELEMENT AND BEING CONNECTED EACH TO AT LEAST ONE ELECTRODE OF THE CORRECTING SYSTEM, THE RESISTANCE ELEMENTS CONSTITUTING A CONTINUOUS MEMBER, THE POTENTIAL GRADIENT IN EACH SECTION BEING OPPOSITE TO THE POTENTIAL GRADIENT IN THE ADJOINING SECTIONS.