Magnetic deflection apparatus positioned behind target

In an electron beam optical system of the type having an electron gun means for focussing a collimated electron beam upon one of a plurality of lenslets, and a planar target spaced from the electron gun and having a surface disposed essentially transverse to the direction of travel of the electron beam prior to deflection, which target surface is to be selectably illuminated by said electron beam, and improved fine deflection apparatus comprising:

• first means for producing a magnetic field parallel to said target surface and having an equimagnetic surface of selectively variable magnitude in a plane parallel to said target surface and disposed between said plurality of lenslets and said target surface;
• second means for producing another magnetic field parallel to said target surface and having an equimagnetic surface of magnitude selectively variable independent of the magnitude of the equimagnetic surface produced by said first means, the selectively variable field of said second means being produced in said plane;
• said first and second magnetic fields being substantially perpendicular to each other and to a normal to said target surface;
• said first and second means both being positioned entirely beyond the plane of said target surface with respect to said electron gun means and said plurality of lenslets.