| 摘要 |
PURPOSE:To surely correct horizontal astigmatism while being able to set an interval of plates small so as to effectively check invasion of electric potential by providing plates connecting with a screening cup top and bottom of its opening and projecting into the inside of G4 electrode. CONSTITUTION:Plates 26 and 26' for correcting potential are provided top and bottom of the central beam outlet of G4 electrode 12. Namely, a screening cup 13 consists of cup-shaped electrodes while the base thereof, that is to say, the opposing surface to G4 electrode 12 is provided with the openings 30, 31 and 32 which three beams pass through respectively. And G4 electrode, the screening cup 13 and the plates 26 and 26' are electrically connected while being equipotentially impressed. The curves 27, 27' and 28, 28' in the figure are both showing the equipotential lines of the equipotential invading the inside of G4 electrode. The invasion of the equipotential lines in the vertical direction can strongly be checked by providing the plates 26, 26' while strengthening a divergent lens in the vertical direction thus extending beams in the vertical detection. Accordingly, horizontal astigmatism can surely be corrected. |
