A further check is necessary to insure that the masks will not vignette for all blend line positions. With the plane mirrors returned to proper match position the location of the overlap at top and bottom of the screen should be noted or identified with tapes. Then when infinity and 3 foot unvignetted targets are projected, the two overlap positions should fall well within the tapes. Alternatively, perfect gray sky infinity and 3 foot vignetted strips may be prepared and used. Any asymmetry in appearances will be due to panel unbalance or to projector vignetting.

    A further, very serious effect is matchline "wedging" due to improper alignment of the flat mirrors. To achieve proper alignment, a special target film has been prepared but it is usually used in such a manner that it is impossible to align to the requisite accuracy. In this target film, a vertical line appears in panels in the overlap area. The mirrors are to be aligned so that the two lines are absolutely parallel. This cannot be done if the lines are superimposed on the screen. Instead, proper technique calls for first separating the lines and then slowly bringing them back to coincidence, noting whether the top or bottom touches first, adjusting the mirror and repeating as often as necessary until both top and bottom touch at the same time. This is the only approved technique. The target lines are then brought into coincidence. At this time, after carefully rechecking focus and framing in the center of the matchline, the magnification should be checked and adjusted. If magnification is incorrect, adjustment should be made by changing the path-length (mirror position) or the lens itself. If any adjustment is made, the procedure is of course repeated. As a handy guide, a magnification error of one inch on the screen calls for a path length change of about one focal length.


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