Patent ID: 6429607
Filing Date: 2002-08-06
Classification: H01J

Abstract:
A magnetic focus adjustment element for operation at constant power, mounted in proximity to a magnetic lens such that the field of said element adds to the field of the lens and comprising a set of focus coils, each with the same coil resistance and consuming the same power;at least one first coil of said set of focus coils having a first number of turns, whereby in operation said first coil alters a focal plane of said lens from a first value to a second value greater than said first value by a coil offset when said first coil is driven with first focus coil current of a first polarity and said first coil alters said focal plane of said lens from a first value to a second value less than said first value by a coil offset when said first coil is driven with second focus coil current of a second polarity and of the same magnitude as said first focus coil current.