an annular top flange 314 fits in yoke assembly 212 so that top flange 314 rests on surface 304 of yoke ring 306 scan lens 30 contained in the interior of air bushing 214 forms the inner race of air bearing assembly 202 and an inner surface 316 of air bushing 214 forms the outer race of air bearing assembly 202 the implementation of air bearing assembly 202 increases the rigidity of scan lens 30 in the x y plane but allows scan lens 30 to move along the z axis in a very smooth controlled manner flexible steel member 276 has a free end 320 that fits in a recess 322 in an upper surface 324 of yoke mount 310 to move it along z axis 280 and thereby move scan lens 30 along beam axis 206 an encoder head mount 326 holding an encoder 328 and attached to voice coil bridge 236 cooperates with an encoder body mount 330 holding an encoder scale and attached to guiding mount 256 to measure using light diffraction principles the displacement of guiding mount 256 relative to voice coil bridge 236 in response to the movement of annular coil 232 because flexible tubular steel member 276 is attached to annular coil 232 the displacement measured represents the position of scan lens 30 along beam axis 206 a quarter waveplate 340 secured in place on a mounting ring 342 is positioned between a lower surface 344 of rectangular yoke mount 310 and top flange 314 of scan lens.