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A tubular shaped item designed to maintain a predetermined distance between optical elements, such as lenses, reticles, and the like. The bore may be straight, tapered or counterbored, and may have annular serrations to converge or diverge light rays. The surfaces, which mate with the optics, are usually chamfered, concave, and/or have a specified finish designation in the range of 1 to 125 microinches arithmetical average (AA). See also SPACER, SLEEVE and CELL, OPTICAL ELEMENT.
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