| 摘要 |
PURPOSE:To prevent the distortion of a scanned surface formed by a central electron beam and both outer electron beams, by providing an upper and a lower rectangular openings each having a width not smaller than the distance between both outer apertures. CONSTITUTION:A cylindrical portion 29 having a height h1 has an upper and a lower rectangular openings 25, each of which has a height h2 and a width W not smaller than the distance between both outer electron beam apertures 23, 24 and is symmetric to each other as to three apertures provided in the bottom 21 of a converging magnetic pole means 20. Most of the magnetic flux in a horizontal deflecting magnetic field extending through the cylindrical portion 29 in the direction of an axis Y-Y perpendicular to a faceplace is caused to pass through the rectangular openings 25, by the converging magnetic pole means 20 having the apertures. This results in preventing the generation of a magnetic flux which would hinder the change in the magnetic flux in the horizontal deflecting magnetic field. A scanned surface formed by a central electron beam and both outer electron beams is thus kept from being distorted due to coma aberration. |
