Simulation of large-angle scattered light propagation in non-rotationally symmetric optical systems

Large-angle scattered light propagation in non-rotationally symmetric optical systems is difficult to simulate because a wide angular spectrum can impose conflicting requirements on spatial sampling and computational-window size. We propose a sectorized angular domain synthesis method (SASM) to address this problem. The target scattering angular domain is determined by downstream aperture acceptance and divided into overlapping local sectors using partition-of-unity weighting to preserve the prescribed angular power distribution. In each sector, the large deflection carrier is removed, the residual field is propagated as a slowly varying envelope in a moving window, and the carrier is restored after registration on a common image plane grid. Sector fields are coherently synthesized within each random-phase realization, followed by Monte Carlo ensemble averaging. The method is evaluated in a tilted two-mirror system under the first-order scattering approximation. Partition-of-unity consistency tests show that the BSDF-weighted sector allocation is independent of the sector number and the overlap ratio within numerical precision. Comparison with an independent direct full-field calculation gives a relative two-dimensional intensity error of 0.849% and an intensity correlation coefficient of 0.999964. Additional sampling tests demonstrate that sectorization suppresses propagation-induced spatial truncation and enables accurate calculation when a single local angular representation exceeds the Nyquist limit. Monte Carlo convergence further confirms a stable ensemble-averaged power estimate. These results demonstrate the numerical reliability of SASM for large-angle scattered light propagation in non-rotationally symmetric optical systems.