shadow mask 24 is preferably a slit mask as shown in fig2 a 2b and 2c or a circular aperture mask as shown in fig3 a and 3b an inline electron gun 26 shown schematically by dotted lines in fig1 is centrally mounted within the neck 14 to generate and direct a trio of electron beams 28 along spaced coplanar convergent paths through the mask 24 to the screen 22 the tube 10 is designed to be used with an external magnetic deflection yoke such as the yoke 30 schematically shown surrounding the neck 14 and funnel 16 in the neighborhood of their junction when activated the yoke 30 subjects the three beams 28 to vertical and horizontal magnetic flux which cause the beams to scan horizontally and vertically respectively in a rectangular raster over the screen 22 the initial plane of deflection at zero deflection is shown by the line p p in fig1 at about the middle of the yoke 30 for simplicity the actual curvature of the deflected beam paths in the deflection zone is not shown in fig1 the shadow mask 24 is made of an improved iron nickel alloy sheet which exhibits improved formability and oxidation characteristics compared to conventional invar invar is a trademark with registration number 63 970 table i compares the compositions in weight percent wt of the improved alloy used in the present invention with a.