Beam-index tube apparatus having deflection field correcting elements

Claim 1

A beam-index tube apparatus comprising: (a) a beam-index pucture tube having an electron gun generating an electron beam of an oval cross-section; and (b) a beam deflecting device comprising, a deflecting yoke provided with at least one pair of horizontal deflecting coils and at least one pair of vertical deflecting coils for deflecting said electron beam in the corresponding directions, the magnetic field created by the deflecting yoke acting upon the electron beam so as to overconverge the same when the beam is deflected along the horizontal and vertical axes; and first and second groups of correcting elements, each made of a permanent magnet, arranged in the proximity of an end of said deflecting yoke facing toward the phosphor surface of the picture tube at positions corresponding to the horizontal axis and the diagonal axes of the same tube, said correcting elements acting upon the electron beam so as to provide positive isotropic astigmatism when the electron beam is deflected along the horizontal axis, and to provide negative anisotropic astigmatism when the electron beam is positioned on the diagonal axes, thus correcting a widened and rotated electron beam.