The effects of gravity and bolt-joint force on the optical design of a projection lens assembly

The position of the projection lens in a lens mount needs to be fixed using bolts in the alignment process. The lens mount is need bolts to fix lens in correct position. The bolt joint stress causes the mount and lens to deform. The bolt's number increase can help the bolt joint force effect decline, thus this project is design 8 bolts to fixed mount. The weight of each projection lens is about 2.5 kg, and the total lens assembly weighs about 35 kg. Thus, the effect of the gravity force on the lens surface needs to be considered. The lens bolt joint and gravity force will introduce optical aberration to the wavefront error of the system. The optical aberrations include two parts: lens surface deformation and lens-stress Optical Path Difference (OPD). This study applied the Finite Element Method (FEM) to calculate the lens surface deformation and lens stress distribution by the bolt joint and gravity force. The lens deformation in the optical axis direction can be fitted by a Zernike polynomial. The stress OPD is calculated using incident rays passing through the stress-affected glass parallel to the optical axis. The calculation results can be used to evaluate the effects of the bolt joint and gravity force on the projection lens assembly.