A method of determining a configuration of a projection system for a lithographic apparatus, wherein manipulators of the projection system manipulate optical elements so as to adjust its optical properties, the method comprising: receiving dependencies of the optical properties of the projection system on a configuration of the manipulators, receiving a plurality of constraints of the manipulators, formulating a cost function, wherein the cost function represents a difference between the optical properties of the projection system for a given configuration of the manipulators and desired optical properties, wherein the cost function is formulated using the dependency of the optical properties on the configuration of the manipulators, scaling the cost function into a scaled variable space, wherein the scaling is performed by using the plurality of constraints and finding a solution configuration of the manipulators which substantially minimizes the scaled cost function subject to satisfying the plurality of constraints.