A coupled opto-thermo-mechanical simulation of an objective lens system in a casing was conducted. Its optical performance due to thermal effects and a mechanical deformation induced by the lens casing was evaluated. The optics and thermomechanics of the domain have been considered separately as well as in a coupled simulation using the Robust Design Optimization approach. The results indicate that the optimization in separate domains can be totally misleading concerning the optimization potential and it is therefore crucial to couple the physical domains in order to obtain optimized results for the real (=coupled) system. With this methodology an overall insight of the important parameters within the design process can be obtained. This can be used to reduce the thermo-mechanical effects on the optical performance.