Cathode ray tube for achieving small electron beam landing deviation

In a magnetic shield for use within a cathode ray tube, the cathode ray tube having at least one source of an electron beam, wherein

• the magnetic shield is in the shape of a hollow rectangular frustum, the first and third sides of the hollow rectangular frustum being formed of two opposite long sides, the second and fourth sides of the hollow rectangular frustum being formed of two opposite short sides, the interface between each adjacent long side and short side forming corner joints that are continuous from the substantially wider base to the substantially narrower top of the hollow rectangular frustum, the magnetic shield being disposed to surround the path of the electron beam with the top of the hollow rectangular frustum positioned near the source of the electron beam, the electron beam defining an axis from the top to the base of the rectangular frustum, the improvement comprising:
• each top edge of each long side of the hollow rectangular frustum has a long side extension of height H1 forming a planar continuation of each long side in the direction of the top of the hollow rectangular frustum, each long side extension being centered along the top edge of each long side, each long side extension having lateral edges separated from the nearest corner joint by a width W1 along the top edge of each long side, to enable the magnetic shield to reduce the deviation of an electron beam within the magnetic shield caused by any external magnetic field.