It's known as an AKB (Auto Klein Bias) line and is used by the machine for detecting contrast or something. I believe there is a mod available (dealer, not DIY) to move it out of view. Seleco gurus or your local dealer will know more.
The AKB line monitors the projected image and makes changes to the bias voltage of each coloured gun over the life of the projector. This is to compensate for aging of the tubes and to try and keep greyscale tracking as accurate as possible.
Speak to Owl Video Systems about possible mods. If you want to make it disappear you can line the area behind the screen with black velvet or felt in the meantime and that should soak it up.
Gordon
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