Apparatus and method for positioning a lens to expand an optical beam of an imaging system

An optical beam expander mechanism includes a plurality of lenses movably secured to a carriage of an imaging system for varying the magnification and adjusting the focus of an optical beam. A scanning means is also secured to the carriage for reflecting the optical beam from or to a media disposed within an exposure chamber. The carriage is slidably mounted to a spar for scanning the media as the carriage moves along the length of the spar. The carriage includes a plurality of independently controlled magnetically preloaded air bearings to provide an air cushion for frictionless movement of the lens along the carriage. The beam expander mechanism further includes an arm member which sequentially engages each lens and maintains the lens in fixed relationship to the spar for positioning the lens on the carriage. To move each lens to its proper position on the carriage, the arm member engages the lens. The corresponding air bearing is actuated to permit movement of the lens while the carriage is commanded to move a predetermined distance for the required magnification and focus of the optical beam. The air bearing is then deactuated to magnetically couple the lens to the carriage at its proper position. The arm member then disengages the lens.