Abstract: High performance infrared optical designs require careful tolerancing to ensure that the as built performance meets the system requirements. Using modern optical design programs a wide vareity of single element and group tolerances can be defined and statistical analysis employed to formulate a prediction of the as built performance. When the number of units being produced is small or near diffraction limited performance is required, then a worst case analysis is often employed to generate a more accurate prediction. However, in some situations, such as low Fumber lenses, certain effects that may be important are not always adequately addressed by first order tolerance analysis. This paper reviews one such effect; zonal irregularity or slope errors. To quantify the impact of zonal irregularity on the MTF performance of an infrared lens, detailed comparisons are made between measured and predicted performance. Interferometric analysis of the lens element irregularity is incorporated within the design to predict the as built lens line spread function. !0
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