Chief stacker

Colloquial term for a three-tiered projector mount from Chief Manufacturing.

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From Adjusta-Set Slide Stackers - ASU Series (discontinued):

Chief’s ASU Adjusta-Set Slide Stackers accommodate Kodak Ektagraphic III projectors. For Ektapro projectors, see model ASE.

• Available in one-tier, two-tier and three-tier models, with optional additional levels

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For multi-image projection, it is imperative that all projectors on the same portion of the screen be aligned (overlapped) precisely. The stacker provides precise pitch and roll adjustments so that the images from all three projectors exactly overlapped, allowing dissolves with no up/down or left/right movement.

Discussion at http://www.controlbooth.com/forums/question-day/21386-what-69-a.html .