A method of determining a configuration of a projection system for a lithographic apparatus as an implementation of a quadratic programming problem with a penalty function. The method includes: receiving dependencies of one or more optical properties of the projection system on a configuration of a plurality of manipulators of the projection system; receiving a plurality of constraints which correspond to physical constraints of the manipulators; finding an initial configuration of the manipulators; and iteratively finding an output configuration of the manipulators. The iteration includes repeating the following steps: determining a set of the plurality of constraints that are violated; determining an updated configuration of the manipulators, the updated configuration of the manipulators being dependent on the set of the plurality of constraints that are violated and a penalty strength; and increasing the penalty strength. These steps are repeated until a convergence criterion is met.