A method for reducing apparatus performance variation. The method includes obtaining (i) a reference performance (e.g., CD) of a reference apparatus (e.g., a reference scanner), (ii) a set of initial leading degrees of freedom selected from a plurality of degrees of freedom of a plurality of pupil facet mirrors of an apparatus (e.g., to be matched scanner) that is selected to reproduce the reference performance, and (iii) exposure data related to one or more parameters (e.g., CD, overlay, focus, etc.) of the patterning process indicating a performance of the apparatus based on the set of initial leading degrees of freedom; and determining a matching pupil of the apparatus based on the set of initial leading degrees of freedom and the exposure data such that the matching pupil reduces a difference between the performance of the apparatus and the reference performance.