Mike - hard to work out what the hell you're on about. The raster is the scanned electron beam trace within which the video picture sits. With video i/p blanked at black, tweaking up your brightness should illuminate your raster on the tube faces....a gently glowing area usually a little bigger than your active video piccy.
The image is of the green tube in My GE Imager 901 when it is being sent a black field from an HTPC. The brightness is cranked up so you can see the raster and active image area. The raster is defined by the larger trapezium. The active image area is defined by the brighter inner trapezium. The borders you can see are the porches (blacker than black). It is important that the video image never reaches the edge of the tubes especially in a nec projector as that could overheat and even crack the tube. The raster should be centred on the tube surface and the video image should be centred within the raster.