Abstract:
A color cathode ray tube having an electron gun including an electron beam generating portion arrayed in a horizontal direction for generating three electron beams, and a main lens for focusing the three electron beams from the electron beam generating portion upon a fluorescent face. A final stage of the main lens is formed between a focusing electrode and an accelerating electrode. The focusing electrode is divided into at least two focusing electrode parts. A quadrupole electron lens is formed for each of the electron beams between a first focusing electrode part and a second focusing electrode part, and the strength of the quadrupole electron lens for the central electron beam is different from the strength of the quadrupole electron lens for the side electron beams. The second focusing electrode part has an aperture for the central electron beam and apertures for the side electron beams with a vertical dimension of the aperture for central electron beam being different from the vertical dimension of the apertures for the side electron beams. The focusing electrode which together with the acceleration electrode has the final stage of the main lens formed therebetween has a single aperture having a diameter which is larger in a horizontal direction than a diameter thereof in a vertical direction, and the focusing electrode has an electrode plate with an central electron beam aperture.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. application Ser. No. 09/015,791, filed Jan. 29, 1998, now U.S. Pat. No. 6,051,919, which is a continuation of U.S. application Ser. No. 08/499,927, filed Jul. 10, 1995, now U.S. Pat. No. 5,739,630. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a color cathode ray tube to be used in a direct viewing type color TV receiver or a terminal color display and, more particularly, to a color cathode ray tube which has its resolution improved all over its screen area by improving the structure of a main lens for controlling the shape of an electron beam deflected to the peripheral portion of the screen. 
     2. Description of the Prior Art 
     In a color cathode ray tube, generally speaking, there are mounted in a vacuum enclosure made of glass or the like a fluorescent face formed of fluorescent films of fluorescent materials of three colors of red (R), green (G) and blue (B) colors, a shadow mask acting as electrodes for selecting color selecting electrodes elements, and an electron gun for emitting three electron beams, so that a predetermined color image is reproduced on the fluorescent face by modulating the aforementioned three electron beams with image signals of R, G and B colors. 
     FIG. 1 is a section for explaining the construction of a shadow mask type color cathode ray tube as the color cathode ray tube of this kind. Reference numeral  1  designates a panel portion; numeral  2  a neck portion; numeral  3  a funnel portion; numeral  4  a fluorescent film; numeral  5  a shadow mask; numeral  6  a mask frame; numeral  7  a magnetic shield; numeral  8  a shadow mask suspending mechanism; numeral  9  an in-line type electron gun; numeral  10  a deflection yoke; and numeral  11  an external magnetic device for centering and purity corrections. 
     In FIG. 1, the three electron beams (i.e., a central electron beam Bc and side electron beams Bs×2) emitted horizontally on one line (in-line) from the electron gun  9  are deflected by the horizontal and vertical magnetic fields, which are generated by the deflection yoke  10  mounted on the transitional region between the funnel portion  3  and the neck portion  2 , and have their colors selected by the apertures of the shadow mask  5  until they impinge upon the predetermined fluorescent materials. 
     The shadow mask  5  is supported by the mask frame  6  and is suspended and held on the inner wall of the skirt portion of the panel portion through the suspending mechanism fixed on that mask frame. 
     On the mask frame  6 , there is mounted the magnetic shield  7  which has a function to shield the electron beams from the external magnetic fields (e.g., the terrestrial magnetism) thereby to prevent the impinging positions of the electron beams from being displaced by the external magnetic fields. 
     In this color cathode ray tube, the resolution at the screen periphery is deteriorated due deflection defocusing caused by the self convergence deflection yoke. With the self convergence deflection yoke, the center and side beams can converge all over the screen. However, the yoke has the strong astigmatism that overfocuses the electron beams in the vertical cross section and extends the vertical spot size. 
     In order to reduce the deterioration of the resolution, the structure of the focusing lens system of the electron gun has been improved. 
     FIG. 2 a  is a schematic diagram, as taken in section along the tube axis, for explaining the construction of an electron gum according to the prior art for improving the resolution; FIG. 2 b  is a section as taken along line  101 — 101  of FIG. 2 a ; and FIG. 2 c  is a front elevation of an electrode plate. Reference numeral  21  designates a cathode; numeral  22  a G 1  electrode; numeral  23  a G 2  electrode; numeral  24  a focusing electrode; numeral  25  an accelerating electrode; and numeral  26  a shielding cup. 
     In these Figures, the cathode  21 , the G 1  electrode  22  and the G 2  electrode  23  constitute an electron beam generating portion, from which the electron beams are emitted along the initial passages arranged generally in parallel with a horizontal plane until they impinge upon the main lens portion. 
     This main lens portion is constructed of the focusing electrode  24  acting as the main lens electrode, the accelerating electrode  25  and the shielding cup  26 . 
     The focusing electrode  24  is divided into a first kind of focusing electrode  241  and a second kind of focusing electrode  242 , the former of which is formed with a single horizontally elongated aperture and equipped therein with an electrode plate  245  having three circular electron beam passing holes. 
     On the other hand, the second kind of focusing electrode  242  is formed with three circular electron beam passing holes at the end face confronting the first kind of focusing electrode  241 . To the second kind of focusing electrode  242 , there are attached plate-shaped correcting electrodes  243  (as will also be shortly called the “plate electrodes”) which are extended toward the first kind of focusing electrode  241  in parallel with the array direction of those electron beam passing holes. 
     The electron beam passing holes of the electrode plate  245  and the focusing electrode  242  are given common axes and diameters for the individual electron beams. 
     The plate-shaped correcting electrode and the electrode plate  245  have their electron beam passing holes confronting each other to form the electrostatic quadrupole lens. 
     Moreover, the first kind of focusing electrode  241  is supplied with a constant focusing voltage Vf at 5 to 10 kV, and the second kind of focusing electrode  242  is supplied with a dynamic voltage Vd in superposition over the constant focusing voltage Vf. On the other hand, the accelerating electrode  25  is supplied with a final accelerating voltage at 20 to 35 kV. 
     The aforementioned dynamic voltage Vd has a waveform in which a parabolic waveform having a period of the horizontal deflection period 1H and a parabolic waveform having a period of the vertical deflection period 1V of the electron beams are synthesized. 
     When the electron beams are not deflected at the central portion of the screen, the dynamic voltage drops to 0 so that not only the potential difference between the first kind of focusing electrode  241  but also the second kind of focusing electrode  242  but also the electrostatic quadrupole lens action substantially disappear. When the electron beams are deflected toward the screen corner portions (i.e., the peripheral portions), on the other hand, the dynamic voltage is maximized to maximize not only the potential difference between the first kind of focusing electrode  241  and the second kind of focusing electrode  242  but also the electrostatic quadrupole lens action. 
     When the electron beams are thus deflected, the dynamic voltage Vd is raised according to the increase in the deflection. As this dynamic voltage Vd rises, the quadrupole lens to be formed in the confronting portion between the first kind focusing electrode  241  and the second kind of focusing electrode  242  is intensified to correct the astigmatism resulting from the electron beam deflection. 
     At the same time, the voltage difference between an accelerating voltage Eb of the accelerating electrode  25  and the voltage applied to the second kind of focusing electrode  242  can be reduced to elongate the distance between the main lens and the electron beam focal point to focus the electron beams even on the screen peripheral portion. 
     By employing such electron gun, the resolution of the screen peripheral portion of the color cathode ray tube is drastically improved. 
     Specifically, the astigmatism to horizontally extend the electron beams deflected to the screen periphery by the self-converging magnetic field is corrected by the astigmatism to vertically extend the electron beams by the electrostatic quadrupole lens. At the same time, the corrections are also made upon the field curvature aberrations. 
     This field curvature aberration is an aberration which will deteriorate the resolution because the focusing conditions go out of the optimum ones in the screen periphery when the electron beam is focused in optimum at the screen center due to the difference between the distance to the screen center and the distance to the screen periphery from the main lens. 
     The intensity of the main lens final stage lens to be formed between the accelerating electrode and the second kind of focusing electrode when the dynamic voltage is applied is reduced so that the deflected electron beams can be focused in optimum in the screen periphery to correct not only the astigmatism but also the field curvature aberration. 
     Incidentally, if the electron gun having that electrostatic quadrupole lens is used, the action (i.e., the so-called “STC: Static Convergence”) to converge the three electron beams upon the screen by the main lens final stage lens fluctuates with the fluctuation of the dynamic voltage Vf, to raise a problem of the convergence misalignment. 
     In the electrode structure of the type described with reference to FIG. 2 a , this problem of convergence misalignment is solved by fluctuating the STC in the opposite direction at the electrostatic quadrupole lens portion to mutually cancel the STC fluctuations at the main lens final stage lens. 
     In the color cathode ray tube using the electron gun of the aforementioned type, however, the following problems arise due to the electrode construction of the electron gun. 
     Specifically, in order to fluctuate the STC by the electrostatic quadrupole lens, the horizontal electric field is applied to only the side electron beams so that these side electron beams are horizontally moved. 
     FIG. 3 is a section of an electrostatic quadrupole lens portion of the electron gun shown in FIG. 2 a  for explaining the operations of the same. 
     In FIG. 3, the plate electrodes  243  are fitted in the first kind of focusing electrode  241  and connected with the second kind of focusing electrode. Reference numeral  201  designates equipotential lines indicating the potential distribution which is established in the section of the plate electrodes  243 , and numerals  202 ,  203  and  204  designate the same electric fields. 
     The electric field  202  to be established in the sections of the plate electrodes  243  contains not only the horizontal component  203  but also a small quantity of the vertical component  204  to be established by the quadrupole lens effect, so that the electrostatic quadrupole lens is intensified against the side electron beams to cause an unbalance from the astigmatism correction sensitivity for the central electron beam. 
     As a result, if the dynamic voltage is set to such a proper value as to correct the astigmatism of the side electron beams in the screen periphery, the astigmatism cannot be corrected for the central electron beam. If, on the other hand, the dynamic voltage is set to a proper value for the central electron beam, the astigmatism in the quadrupole lens becomes excessive for the side electron beams. In either case, there arises a problem that the resolution in the screen peripheral portions is deteriorated. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the aforementioned various problems of the prior art and to provide a color cathode ray tube which has its resolution improved at the central portion and peripheral portions of its screen. 
     The above-specified object is achieved by elongating or narrowing the plates of plate electrodes forming an electrostatic quadrupole lens, at the upper and lower portions of a passage for a central electron beam, or by making the shape of a central electron beam passing hole of such an electrode of a first kind of focusing electrode as is formed with electron beam passing holes, longer than the shape of electron beam passing holes for side electron beams, that is, by enlarging the ratio of the vertical diameter to the horizontal diameter. 
     The object is achieved by the following constructions 1 to 5, for example. 
     1. The plate electrode pair is shaped such that its lens intensity acts more upon the vertically upper and lower portions of the passage for a central one of said three electron beams than upon the vertically upper and lower portions of the side electron beam passages. 
     2. The plate electrode pair is made longer in the axial direction of said electron gun at the vertically upper and lower portions of the central electron beam passage of said three electron beams than at the vertically upper and lower portions of said side electron beam passages. 
     3. The plate electrode pair is more spaced at the vertically upper and lower portions of the central electron beam passage of said three electron beams than at the vertically upper and lower portions of said side electron beam passages. 
     4. The ratio of the horizontal diameter to the vertical diameter of a central electron beam passing hole, which is formed in such an end face of the electrodes belonging to said first kind of focusing electrode group forming said axially asymmetric electronic lens as confronts the electrodes belonging to said second kind of focusing electrode group for passing the central one of said three electron beams therethrough, is made larger than the ratio of the vertical diameter to the horizontal diameter of the side electron beam passing holes for passing the side electron beams therethrough. 
     5. The ratio of the horizontal diameter to the vertical diameter of a central electron beam passing hole, which is formed in such an end face of the electrodes belonging to said second kind of focusing electrode group forming said axially asymmetric electronic lens as confronts the electrodes belonging to said first kind of focusing electrode group for passing the central one of said three electron beams therethrough, is made smaller than the ratio of the vertical diameter to the horizontal diameter of the side electron beam passing holes for passing the side electron beams therethrough. 
     Thanks to the above-enumerated constructions of the present invention, the astigmatism correction sensitivity for the central electron beam can be increased to eliminate the unbalance from the astigmatism correction sensitivity for the side electron beams so that a proper dynamic voltage can be set for both the central electron beam and the side electron beams to provide an image display of high resolution all over the screen by eliminating the deterioration of the resolution in the screen peripheral portions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a section for explaining the construction of a shadow mask type color cathode ray tube; 
     FIG. 2 a  is a schematic diagram, as taken in section along the tube axis, for explaining the construction of an electron gum according to the prior art for improving the resolution; FIG. 2 b  is a section as taken along line  101 — 101  of FIG. 2 a ; and FIG. 2 c  is a front elevation of an electrode plate constructing a focusing electrode; 
     FIG. 3 is a section of an electrostatic four-pole portion of the electron gun shown in FIG. 2 a  for explaining the operations of the same; 
     FIG. 4 is a broken diagram showing an essential portion of the focusing electrode portion of the electron gun for explaining a first embodiment of the color cathode ray tube according to the present invention; 
     FIG. 5 is a perspective view showing an essential portion of the electron gun for explaining a second embodiment of the color cathode ray tube according to the present invention; 
     FIG. 6 is a perspective view showing an essential portion of the electron gun or explaining a third embodiment of the color cathode ray tube according to the present invention; 
     FIG. 7 is a section for explaining the structure of the electron gun which has an electrostatic four-pole lens equipped with plate electrodes at each of its divided focusing electrodes; 
     FIG. 8 is a perspective view showing an essential portion of the electron gun for explaining a fourth embodiment of the color cathode ray tube according to the present invention; 
     FIG. 9 is an exploded section taken along line  102 — 102  of FIG. 8; 
     FIG. 10 is a perspective view showing an essential portion of the electron gun for explaining a fifth embodiment of the color cathode ray tube according to the present invention; 
     FIG. 11 is a perspective view showing an essential portion of the electron gun for explaining a sixth embodiment of the color cathode ray tube according to the present invention; 
     FIG. 12 is a perspective view showing an essential portion of the electron gun for explaining a seventh embodiment of the color cathode ray tube according to the present invention; and 
     FIG. 13 is a perspective view showing an essential portion of the electron gun for explaining an eighth embodiment of the color cathode ray tube according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention will be described in detail in the following with reference to the accompanying drawings. 
     First Embodiment 
     FIG. 4 is a broken diagram showing an essential portion of the focusing electrode portion of the electron gun for explaining a first embodiment of the color cathode ray tube according to the present invention. Reference numeral  24  designates a focusing electrode; numeral  241  a first kind of focusing electrode; numeral  242  a second kind of focusing electrode; numeral  243  plate electrodes; numeral  245  an electrode plate having a central electron beam passage  16  and side electron beam passages  17  and  17 ; and numeral  25  designates an accelerating electrode. 
     The main lens is constructed of the first kind of focusing electrode  241  and the second kind of focusing electrode  242  constituting the focusing electrode  24 , and the accelerating electrode  25 . 
     The first kind of focusing electrode  241  is supplied with a first kind of focusing voltage Vf 1  at a constant level, and the second kind of focusing electrode  242  is supplied with a second kind of focusing voltage in which a dynamic voltage dvf fluctuating in synchronism with the deflection of the electron beam is superposed on a constant voltage Vf 2 . Incidentally, the accelerating electrode  25  is supplied with a final accelerating voltage Eb at 20 to 30 kv, to form the final stage lens of the main lens between itself and the second kind of focusing electrode  242 . 
     In FIG. 4, the main lens has its final stage lens constructed of an electrode plate  2421  which is formed with a single aperture having a large aperture in the electrode confronting face and with elliptical electron beam passing holes arranged in electrode, as disclosed in Japanese Patent Laid-Open No. 103752/1983. 
     This final stage lens structure is enabled to reduce the lens aberration and the beam spot diameter on the screen by making the lens aperture substantially larger than the ordinary cylindrical lens. 
     Between the first kind of focusing electrode  241  and the second kind of focusing electrode, there are arranged portions above and below (or vertically of) the central and side electron beam passages  16  and  17  and  17 , to form the electrostatic quadrupole lens. 
     The electrostatic quadrupole lens structure thus made has portions  2430  which are formed above and below the central electron beam passage  16  of the plate electrodes  243  and made axially longer than the side electron beam passages  17 . 
     Thanks to the presence of that portion  2430 , the lens intensity against the central electron beam passage  16  is higher than that against the side electron beam passages  17 . 
     According to this embodiment, more specifically, the lens intensity to act upon the central electron beam can be selectively increased to eliminate the unbalance in the astigmatism correction sensitivity. 
     Second Embodiment 
     FIG. 5 is a perspective view showing an essential portion of the electron gun for explaining a second embodiment of the color cathode ray tube according to the present invention. Reference numerals  301 ,  302  and  303  designate electron beam passing holes. 
     In FIG. 5, the plate electrodes  243  forming the electrostatic quadrupole lens are connected with the second kind of focusing electrode and are inserted into the first kind of focusing electrode to confront the electrode plate  245 . 
     Of the electron beam passing holes  301 ,  302  and  303  formed in the electrode plate  245 , the central * electron beam passing hole  302  has its vertical diameter made larger than its horizontal diameter. The central electron beam passing hole  302  of the present embodiment is formed by vertically shortening a circular hole similar to the side electron beam passing holes  3 Q 1  and  303 . 
     Thanks to this hole shape, the action to vertically diverge and horizontally focus the electron beam can be intensified to increase the quadrupole lens action thereby to eliminate the unbalance in the astigmatism correction sensitivity of the side electron beams. 
     According to this embodiment, more specifically, the lens intensity to act upon the central electron beam can be selectively increased to eliminate the unbalance in the astigmatism correction sensitivity. 
     Third Embodiment 
     FIG. 6 is a perspective view showing an essential portion of the electron gun or explaining a third embodiment of the color cathode ray tube according to the present invention. 
     In this embodiment, the electrode construction is similar to that of the foregoing embodiment of FIG.  5 . However, all the electron beam passing holes  301 ,  302  and  303  to be formed in the electrode plate  245  are given the same shape, and the central electron beam passing hole  302  has its vertical diameter made larger than that or the side electron beam passing holes  301  and  303 . 
     Thanks to this hole shape, the action to vertically diverge and horizontally focus the electron beam can be intensified to increase the quadrupole lens action thereby to eliminate the unbalance in the astigmatism correction sensitivity of the side electron beams. 
     According to this embodiment, too, the lens intensity to act upon the central electron beam can be selectively increased to eliminate the unbalance which is caused in the astigmatism correction sensitivity. 
     The electron beam passing holes  301 ,  302  and  303  to be formed in the electrode plate  245  should not be limited to the shapes of the foregoing embodiments of FIGS. 5 and 6 but may be shaped to intensify the action to vertically diverge and horizontally focus the electron beam which has passed through the central electron beam passing hole, as in the known electron beam passing hole shapes such as elliptical or rectangular shapes or in their combinations. 
     Fourth Embodiment 
     Here will be described an embodiment in which the present invention is applied to an electron gun of a type different from those of the foregoing embodiments. 
     FIG. 7 is a section for explaining the structure of the electron gun which has an electrostatic quadrupole lens equipped with plate electrodes at each of its halved focusing electrodes. Reference numerals  21 ,  21 ′ and  21 ″ designate cathodes; numeral  22  a first grid electrode; numeral  23  designate a second grid electrode; numeral  24  a focusing electrode composed of a first kind of focusing electrode  241  and a second kind of focusing electrode  242 ; and numeral  25  an accelerating electrode. 
     On an electrode plate  245  of the first kind of focusing electrode  241  constituting the focusing electrode  24 , as located at the side of the second kind of focusing electrode, there are so embedded first plate electrodes  244  in the direction of the second kind of focusing electrode as to horizontally interpose the individual electron beam passages. On the second kind of focusing electrode  242  as located at the side of the first kind of focusing electrode, on the other hand, there are embedded second plate electrodes  243  which are composed of a pair of plate members. The first plate electrodes  244  so vertically intersect the second plate electrodes  243  as to vertically interpose them to form the electrostatic quadrupole lens. 
     FIG. 8 is a perspective view showing an essential portion of the electron gun for explaining a fourth embodiment of the color cathode ray tube according to the present invention, and the present invention is applied to the electron gun of the type which has been described with reference to FIG.  7 . 
     In FIG.  8 : reference numerals  301 ,  302  and  303  designate electron beam passing holes which are formed in the electrode plate  245 ; numerals  244   a ,  244   b ,  244   c  and  244   d  first plate electrodes at the side of the first kind of focusing electrode; and numerals  409   a  and  409   b  and  409   c  electron beam passing holes which are formed in the second plate electrodes  243  at the side of the second kind of focusing electrode. 
     With the construction described above, in order to solve the fluctuation of the aforementioned STC, the second plate electrodes  243  are formed at their portions corresponding to the central electron beam with projecting portions  2430  which project toward the first kind of focusing electrode  241 , as in the foregoing embodiment of FIG.  4 . At the same time, the first plate electrodes  244   a ,  244   b ,  244   c  and  244   d  at the side of the first kind of focusing electrode are made shorter at H 1  for the central electron beam, as taken in the direction of the electron gun, than at H 2  for the site electron beams. 
     FIG. 9 is an exploded section taken along line  102 — 102  of FIG.  8 . As to the first plate electrodes  244   a ,  244   b ,  244   c  and  244   d  embedded on the electrode plate  245 , the axial length H 1  of the plate electrodes  244   b  and  244   c  interposing the central electron beam passing hole  302  is made shorter than the axial length H 2  of the plate electrodes  244   a  and  244   d  located at the outer sides of the side electron beam passing holes  301  and  303 . 
     Thanks to this construction, there can be established an electric field for deflecting the side electron beams toward the central electron beam to cancel the STC fluctuation by the main lens. 
     However, the mere shortening of the axial length of the aforementioned plate electrodes  244   b  and  244   c  will lower the intensity of the electrostatic quadrupole lens against the central electron beam. As a result, there arises a problem of an unbalance in the astigmatism correction effect for the central electron beam and the side electron beams, as has been described in connection with the embodiment of FIG.  4 . 
     Therefore, the portions of the second plate electrodes  243  for the central electron beam are formed with the projecting portions  2430  projecting toward the first kind of focusing electrode  241  so that the reduction of the intensity of the electrostatic four-pole lens against the central electron beam is corrected to eliminate the unbalance in the astigmatism correction sensitivity from the side electron beams. 
     Incidentally, the present embodiment can be combined with the electron guns of the types shown in FIGS. 5 and 6, and the electrostatic quadrupole lens intensity against the central electron beam can be selectively increased by making the vertical diameter of the central electron beam passing hole larger than that of the side electron beam passing holes, so that the unbalance of the astigmatism correction sensitivity from the side electron beams can be eliminated. 
     On the other hand, the unbalance of the astigmatism correction sensitivity can be corrected by changing the shape of the central electron beam passing hole  409   b  at the side of the plate electrodes  243 . In this case, the vertical diameter of the central electron beam passing hole  409   b  is made smaller than that of the horizontal diameter. 
     This is because the second plate electrodes  243  are connected with the second kind of focusing electrode so that their potential are inverted from that of the first plate electrodes  244 . Specifically, the electrostatic quadrupole lens intensity is increased when the electron beam passing hole of the electrode supplied with a higher potential is horizontally elongated to the contrary of the lower-potential electrode. 
     Fifth Embodiment 
     FIG. 10 is a perspective view showing an essential portion of the electron gun for explaining a fifth embodiment of the color cathode ray tube according to the present invention. This embodiment is different from that of FIG. 8 in that the second plate electrodes  243  connected with the second kind of focusing electrode are formed, at its portion corresponding to the central electron beam, with protruding portions  2430 ′ which are folded toward said central electron beam. 
     Thanks to this construction, too, there can be attained effects similar to the aforementioned ones of FIG.  8 . 
     Sixth Embodiment 
     FIG. 11 is a perspective view showing an essential portion of the electron gun for explaining a sixth embodiment of the color cathode ray tube according to the present invention. What is different from the foregoing embodiment of FIG. 8 is that the second plate electrodes connected with the second kind of focusing electrode are formed, at its portion corresponding to the central electron beam, with step portions  2430 ″ which are stepped toward said central electron beam. 
     Specifically, for the aforementioned paired plate electrodes, the central one of the aforementioned three electron beam passages has its vertical gap made smaller than that of the side electron beam passages. 
     This construction can also achieve effects similar to the aforementioned ones of FIGS. 8 and 10. 
     Incidentally, the constructions of FIGS. 10 and 11 can be applied to the electron guns of the types similar to those of FIGS. 5 and 6 as in the foregoing embodiments. 
     Seventh Embodiment 
     FIG. 12 is a perspective view showing an essential portion of the electron gun for explaining a seventh embodiment of the color cathode ray tube according to the present invention. The second plate electrodes  243  are divided for the individual electron beam passing holes into side plate electrodes  2431  and  2433  for the side electron beam passing holes and central plate electrodes  2432  for the central electron beam passing hole. 
     Moreover, the central plate electrodes  2432  of the second plate electrodes  243  thus divided have a larger axial length than that of the side plate electrodes  2431  and  2433 . Still moreover, the paired central plate electrodes may be either folded toward the central electron beam or formed such that the vertical gap of the central one of the three electron beam passages is made smaller than the vertical one of the side electron beam passages. 
     Thanks to this construction, there can be attained effects similar to those of the aforementioned fourth embodiment. 
     In case, moreover, the second plate electrodes  243  are thus divided, the present embodiment may be combined with the elongated central aperture, as shown in FIGS. 5 and 6. 
     Eighth Embodiment 
     FIG. 13 is a perspective view showing an essential portion of the electron gun for explaining an eighth embodiment of the color cathode ray tube according to the present invention. The present invention is applied to an electron gun which has an electrostatic quadrupole lens different from those of the individual foregoing embodiments. 
     In FIG.  13 : reference numeral  511  designates a first kind of focusing electrode constituting the focusing electrode; numeral  512  a second kind of focusing electrode constituting the same; numerals  501 ,  502  and  503  electron beam passing holes formed in the first kind of focusing electrode  511 ; numerals  504 ,  505  and  506  electron beam passing holes formed in the second kind of focusing electrode  512 ; numerals  507  and  508  the center axes of the side electron beam passing holes  501  and  503  of the first kind of focusing electrode  511 ; and numerals  509  and  510  the center axes of the side electron beam passing holes  504  and  506  of the second kind of focusing electrode  512 . 
     The vertically longer electron beam passing holes  501 ,  502  and  503  of the first kind of focusing electrode  511  of the halved focusing electrode and the horizontally longer electron beam passing holes  504 ,  505  and  506  of the second kind of focusing electrode  512  are arranged to confront each other to form the electrostatic quadrupole lens. 
     Moreover, the center axes  507  and  508  of the side electron beam passing holes  501  and  503  formed in the first kind of focusing electrode  511  are slightly offset inward with respect to the center axes  509  and  510  of the side electron beam passing holes  504  and  506  formed in the second kind of focusing electrode  512 . 
     Thanks to this offset, the side electron beams can be deflected toward the central electron beam without passing through the sides of the center axis of the lens, to cancel the STC fluctuation by the main lens. 
     However, the offset reduces the areas of the confronting portions of the electron beam passing holes  501  and  503  of the first kind of focusing electrode  511  and the electron beam passing holes  504  and  506  of the second kind of focusing electrode  512 . As a result, the electrostatic quadrupole lens intensity against the side electron beams is increased. 
     As a result, there arises an unbalance in the astigmatism correction effect for the central electron beam and the side electron beams, as has been described in connection with the embodiment of FIG.  4 . In order to eliminate this, the ratio of the horizontal diameter of the central electron beam passing hole  505  of the second kind of focusing electrode  512  to the vertical diagram is made larger than that of the side electron beam passing holes to make a horizontally elongated shape. 
     As a result, the effect of the horizontally elongated hole shape corrects the electrostatic quadrupole lens intensity against the side electron beams, to eliminate the unbalance of the astigmatism correction sensitivity from the central electron beam. 
     Incidentally, in this embodiment, the unbalance in the astigmatism correction sensitivity between the side election beams and the central electron beam is corrected at the side of the second kind of focusing electrode, but a similar correction can be made at the side of the first kind of focusing electrode. 
     In this case, the ratio of the vertical diameter of the central electron beam passing hole  502  of the first kind of focusing electrode  511  to the horizontal diameter may be made larger than that of the side electron beam passing holes. 
     In the first to eighth embodiments thus far described, the plate electrode to be disposed at the side of the second kind of focusing electrode so as to construct the electrostatic quadrupole lens is composed of a pair of parallel plates with respect to the three electron beams. However, the present invention should not be limited to that construction but may be modified such that each electrode pair may be disposed for each electron beam. Moreover, the plate electrodes should not be limited to the flat plates, but similar effects car apparently be attained in case the quadrupole lens is composed of plate electrodes having a suitable shape such as curved plates, portions of cylinders, or partial cylindrical plates. 
     Moreover, the foregoing individual embodiments have been described in case the present invention is applied to the electron gun of the type in which the focusing electrode is halved. The present invention should not be limited thereto but can naturally be likewise applied to the construction in which the focusing electrode is composed of a plurality of electrode groups. 
     As has been described hereinbefore, according to the present invention, in the color cathode ray tube having the dynamic focus type electron gun which has its resolution improved all over the screen including the peripheral portions by having the electrostatic quadrupole lens mounted therein, the unbalance of the astigmatism correction sensitivity, which is caused due to the different intensities of the electrostatic quadrupole lens against the central electron beam and the side electron beams, can be corrected to further improve the resolution all over the screen including the peripheral portions to display an image of a high quality.