| 摘要 |
PURPOSE:To simplify the operation of correcting the astigmatism when it is performed with an astigmatism correction tetrode lens by providing an optimum astigmatism correction state of an image displayed in the central division among many images of the same visual field. CONSTITUTION:While electron rays from an electron gun 2 are irradiated upon a sample 5, a signal is supplied from a scanning power supply 7 through a multiplying-factor-controlling circuit 9 to perform two dimensional scanning thereby producing secondary electrons. The thus produced secondary electrons are detected by a detector 10 before a sample image is displayed on a cathode-ray tube 12. An astigmatism-correcting tetrode lens 14 is installed which is controlled by adding different d.c. level signals 32 and 34 to the output of an astigmatism correction power supply, which is obtained synchronously with the scanning signal of the scanning power supply 7 by the turning of switches 15 and 16, in adder circuits 31 and 33. Accordingly, the controlling can be performed only by producing an optimum astigmatism correction state of the image for the central area of the picture screen. Consequently, the procedure for the controlling is simplified and the comparison of the image can be easily performed. |
