When viewed on screen, the leftmost portion of the raster is drawn first after the electron beam completes horizontal retrace. Looking into the projection lens the orientation is backwards. During the first portion of the horizontal movement the beam has not settled completely and one may see some waviness in the image if the extreme left edge of the raster is used. Some installers will intentionally displace the raster slightly leftward so the active image is displayed on the later, more stable portion of the raster.
Centering the raster is performed by use of centering magnets on the CRT necks just behind the deflection yokes. The centering magnets are a pair of rings with small tab handles. By rotating the rings relative to each other and also around the neck of the tube one can shift the raster about the phosphor surface. Most projectors also have electronic static position controls for fine adjustment of the raster centering. It is best to use minimal electronic correction to reduce strain on the convergence circuitry. One can do so by centering the electronic controls prior to centering the raster with the centering magnets. On some projectors (such as NEC XG's), there are no centering magnets and raster centering is carried out purely with electronic controls for raster centering hidden in a service menu. Even on such machines it is reasonable to first center the user centering controls prior to setting the service menu raster centering controls.
Image Area - The portion of the raster which is actually used to display the video image
Within the raster, the active video image is displayed. The projector often has "position" or "image shift" controls which allow movement of the image within the raster. The name of this control varies from brand to brand. You can verify you have the correct control by making the raster visible and seeing if the image is moving about within the raster but the raster is not moving as you use the control. If raster moves as well as the picture, you are adjusting the raster position and not the image position within the raster.
Ideally, the image area is centered both vertically and horizontally on the phosphor surface. You can achieve this by first neutralizing the linearity controls and then centering the raster relative to the phosphor edges. Then center the image area within the raster edges. Once both are done, display a white field pattern from a calibration disc (AVIA, S&V Home Theater Tune-Up) and verify that the active is centered in the phosphor. We'll cover this in greater detail later.
BTW, Don't use an internal test pattern for checking centering as they are often not themselves centered relative to external signals unless you have also taken the steps described next in this note.
